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Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future (2019)

Chapter:Appendix G: Potential Impacts of Climate Change on the Interstate Highway System

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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Suggested Citation:"Appendix G: Potential Impacts of Climate Change on the Interstate Highway System." National Academies of Sciences, Engineering, and Medicine. 2019. Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future. Washington, DC: The National Academies Press. doi: 10.17226/25334.
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Appendix G Potential Impacts of Climate Change on the Interstate Highway System Donald J. Wuebbles and Jennifer M. Jacobs INTRODUCTION The Interstate Highway System in the United States is vital to the transport of people and goods across our country. As required in Section 6021 of the Fixing America’s Surface Transportation Act of 2016, the Future Interstate Study will address the actions needed to upgrade and restore the Interstate Highway System as a premier system that meets the demands of the 21st century. In particular, Congress has asked that the Future Interstates Study consider a 50-year planning horizon. Over a time period of such duration, key trends critical to the analyses for this study need to be estimated. Among the many stresses on the Interstate Highway System over the coming decades, one of the most important is the change occurring in Earth’s climate system and how the associated changes in temperature, precipitation, and other climate-related parameters are likely to affect the United States. Earth’s climate system includes the land surface, atmosphere, oceans, and ice. The world, including the United States, has warmed over the past 150 years, especially over the past six decades, and that warming has triggered many other changes in Earth’s climate. Evidence for a chang- ing climate abounds, from the top of the atmosphere to the depths of the oceans. Many thousands of studies conducted by thousands of scientists around the world have documented changes in surface, atmospheric, and oceanic temperatures; melting glaciers; disappearing snow cover; shrinking sea ice; rising sea levels; and increasing levels of atmospheric water vapor. Rainfall patterns and storms are changing, and the occurrence of droughts is shifting. 389

390 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Earth’s climate is changing at a pace and in a pattern not explainable by natural influences. Many different lines of evidence demonstrate that human activities, especially emissions of greenhouse gases, are primarily responsible for the observed climate changes in the industrial era, especially over the past six decades. Over the past century, there do not appear to be any plausible alternative explanations supported by the evidence that are either credible or that can contribute more than marginally to the ob- served patterns. Solar flux variations over the past six decades have been too small to explain the observed changes in climate (Bindoff et al. 2013). The observational record also does not indicate any natural cycles that can explain the recent changes in climate (e.g., Marcott et al. 2013; PAGES 2K Consortium 2013). Natural cycles within Earth’s climate system can only redistribute heat; they cannot be responsible for the observed increase in the overall heat content of the climate system (Church et al. 2011). Any explanations for the observed changes in climate must be grounded in understood physical mechanisms, appropriate in scale, and consistent in timing and direction with the long-term observed trends. Known human activities quite reasonably explain what has happened without the need for other factors. Internal variability and natural forcing factors cannot explain what is happening and there are no suggested factors, even speculative ones, that can explain the timing or magnitude, and that would somehow cancel out the role of human factors (Anderson et al. 2012). People throughout the world are already feeling the effects of climate change—going well beyond an increasing temperature—especially from increasing intensity of certain types of extreme weather and from sea level rise that are fueled by the changing climate. Prolonged periods of heat and heavy downpours, and in some regions, floods and in others, drought, are affecting human health, agriculture, water resources, energy, transportation infrastructure, and much more. Almost every facet of our lives is being or likely will be affected by the changes occurring in climate, including po- tential effects on the types and quantity of food we eat, where we live, the types of available jobs, and, critically, how people and goods move. Like other sectors of our society, the transportation sector and the Interstate System are vulnerable to the changes occurring in climate. The consensus finding from the U.S. transportation sector is that the nation’s transportation systems and networks will be affected by changes in tem- perature, precipitation, and sea levels and that these changes may threaten or enhance transportation performance at the facility, system, and national levels. The most relevant potential climate change impacts to transporta- tion infrastructure are increases in intense precipitation events, increases in Arctic temperatures (leading to permafrost melting), rising sea levels, increases in very hot days and heat waves, and increases in hurricane intensity (Burbank 2012; Caltrans 2013; CNA Military Advisory Board

APPENDIX G 391 2014; MacArthur et al. 2012; U.S. DOT 2014). Climate stressors affect Interstate System activities including operations and maintenance, design, and long-term planning. Our understanding of how climate change affects the nation’s trans- portation systems is largely informed by studies conducted over just the past 10 years (Savonis et al. 2014). These assessments primarily use na- tional climate reports (e.g., IPCC 2014; Melillo et al. 2014; TRB 2014) and sea level rise assessments, but also use climate change information generated by regional and local climate scientists (Douglas et al. 2017). In that brief period, significant progress has been made in understanding the vulnerability of transportation assets and impacts to system performance. Furthermore, new tools, methods, and frameworks are emerging that can improve the consistency of exposure and sensitivity assessments and, to a lesser extent, inform adaptation strategies and guide resource allocation decisions. To date, climate change assessments and adaptation are rarely included in transportation agencies’ decision-making processes. Given the long lifetime of Interstate System assets, effective resource investment and strategies would be well served by considering the likely effects on the Interstate Highway System from climate change. Additionally, because the nation’s transport system is highly interdependent, there are also second- ary impacts to the Interstate System from climate change—performance impacts to local roadways, public transportation, rail, air, pipelines, and maritime and port facilities (Caltrans 2013; CNA Military Advisory Board 2014; Johnson 2012; TRB 2014; U.S. DOT 2015). Beyond transportation, essential products and services such as energy, food, manufacturing, and trade all depend in interrelated ways on the reliable functioning of our nation’s transportation system, including the Interstate Highway System. OUR CHANGING CLIMATE Climate is defined as long-term averages and variations in weather mea- sured over multiple decades. Predicting how climate will change in future decades is a different scientific issue from predicting weather a few weeks from now. Local weather is short term, with limited predictability, and is determined by the complicated movement and interaction of high-pressure and low-pressure systems in the atmosphere; thus, it is difficult to fore- cast day-to-day changes beyond a week currently or up to about 2 weeks eventually. Climate is the statistics of weather—meaning not just average values but also the prevalence and intensity of extremes—as observed over a period of decades. There are clear effects of physical factors (e.g., latitude, mountains, distance to the coast) on the statistical character of the weather. As a result, the statistical properties are a result of the physical processes and conditions present and are readily predicted. Climate emerges from the

392 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM interaction, over time, of rapidly changing local weather and more slowly changing regional and global influences, such as the distribution of heat in the oceans, the amount of energy reaching Earth from the Sun, and the composition of the atmosphere. Observed Trends in Temperature Highly diverse types of measurements made on land, sea, and in the atmo- sphere over many decades have allowed scientists to conclude with confi- dence that global mean temperature is increasing. Global annual average temperature, as measured over both land and oceans (used interchangeably with global average temperature in the discussion below), has increased by more than 1.7°F (0.9°C) over the entire period (see Figure G-1); see Vose and colleagues (2012) for discussion on how global annual average tem- perature is derived by scientists. Global average temperature is not expected to increase smoothly over time in response to the human warming influ- ences, because the warming trend is superimposed on natural variability associated with, for example, the El Niño/La Niña ocean-heat oscillations and the cooling effects of particles emitted by volcanic eruptions. Even so, 16 of the top 17 warmest years in the instrumental record (since the late 1800s) occurred in the period from 2001 to 2016 (1998 was the exception). Global average temperature for 2016 surpassed 2015 by a small amount as the warmest year on record. The year 2015 far surpassed 2014 by 0.29°F (0.16°C), four times greater than the difference between 2014 and the next warmest year, 2010 (NCEI 2016a). In addition, looking at longer time scales, every decade since 1956–1965 has been warmer than the previous decade (see Figure G-2). Although there has been widespread warming over the past century, not every region has warmed at the same pace (see Figure G-3). Warming during the first half of the 1900s occurred mostly in the Northern Hemi- sphere (Delworth and Knutson 2000). Recent decades have seen greater warming, particularly at high northern latitudes, and over land compared to the ocean. In general, winter is warming faster than summer (especially in northern latitudes). Also, nights are warming faster than days (Alexander et al. 2006; Davy et al. 2016). There is also some evidence of faster warm- ing at higher elevations (Mountain Research Initiative 2015). Even in the absence of significant ice melt, the ocean is expected to warm more slowly given its larger heat capacity, leading to land–ocean differences in warming. As a result, the climate for land areas often responds more rapidly than the ocean areas, even though the forcing driving a change in climate occurs equally over land and the oceans (IPCC 2013). A few regions, such as the North Atlantic Ocean, have experienced cooling over the past century, though these areas have warmed over recent

APPENDIX G 393 decades. Regional climate variability is important (e.g., Hoegh-Guldberg et al. 2014; Hurrell and Deser 2009), as are the effects of the increasing freshwater in the North Atlantic from melting of sea and land ice (Rahmstorf et al. 2015). The average annual temperature of the contiguous United States has risen since the start of the 20th century. In general, temperature increased until about 1940, decreased until about 1970, and increased rapidly through 2016. Because the increase was not constant over time, multiple methods were evaluated to quantify the trend. All methods yielded rates of warming that were significant at the 95 percent level. The lowest estimate FIGURE G-1 Global annual average temperatures (as measured over both land and oceans) for 1880–2016 relative to the reference period of 1901–2000. NOTES: Red bars indicate temperatures above the average over 1901–2000 and blue bars indicate temperatures below the average. Global annual average tempera- ture has increased by more than 1.7°F (0.9°C) over the entire period. Although there is a clear long-term global warming trend, some years do not show a temperature increase relative to the previous year, and some years show greater changes than oth- ers. These year-to-year fluctuations in temperature are mainly due to natural sources of variability, such as the effects of El Niño, La Niña, and volcanic eruptions. SOURCE: Based on the National Centers for Environmental Information (NOAA- GlobalTemp) data set (updated from Vose et al. 2012).

394 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM of 1.2°F (0.7°C) was obtained by computing the difference between the average for 1986–2016 (i.e., present day) and the average for 1901–1960 (i.e., the first half of the last century). The highest estimate of 1.8°F (1.0°C) was obtained by fitting a linear (least-squares) regression line through the period 1895–2016. More than 95 percent of the land surface of the contiguous United States has had an increase in average annual temperature (see Figure G-4). In con- trast, only small (and somewhat dispersed) parts of the Southeast and South- ern Great Plains experienced cooling. From a seasonal perspective, warming was greatest and most widespread in winter, with increases of more than 1.5°F (0.8°C) in most areas. In summer, warming was less extensive (mainly along the East Coast and in the western third of the nation), while cooling was evident in parts of the Southeast, Midwest, and Great Plains. FIGURE G-2 Global average temperature averaged over decadal periods (1886– 1895, 1896–1905, …, 1996–2005, except for the 11 years in the last period, 2006–2016) relative to the reference period of 1901–2000. NOTES: Red bars indicate temperatures above the average over 1901–2000 and blue bars indicate temperatures below the average. Horizontal label indicates mid- point year of decadal period. Every decade since 1956–1965 has been warmer than the previous decade. SOURCE: Based on the National Centers for Environmental Information (NOAA- GlobalTemp) data set (NCEI 2016b).

APPENDIX G 395 Other Indicators of Climate Change Observational data sets for many other climate variables support the con- clusion with high confidence that the global climate (including that of the United States) is changing (Blunden and Arndt 2016; EPA 2016a; Meehl et al. 2016). Not only have temperatures in the lower atmosphere increased, but so have ocean temperatures. Basic physics tells us that a warmer at- mosphere can hold more water vapor; increasing atmospheric humidity is exactly what is measured from satellite data. At the same time, the warmer world should result in higher evaporation rates and major changes to the hydrological cycle, including observed increases in the prevalence of tor- rential downpours. Multiple observational data sets show that the heat content of the oceans is increasing and that sea levels are rising. Arctic sea FIGURE G-3 Surface temperature change for the period 1986–2015 relative to 1901–1960. NOTES: For visual clarity, statistical significance is not depicted on this map. Changes are generally significant (at the 90 percent level) over most land and ocean areas. Changes are not significant in parts of the North Atlantic Ocean, the South Pacific Ocean, and the southeastern United States. There are insufficient data on the Arctic Ocean and Antarctica for computing long-term changes. The relatively coarse resolution (5.0° × 5.0°) of these maps does not capture the finer details associated with mountains, coastlines, and other small-scale effects. SOURCE: Based on the National Centers for Environmental Information (NOAA- GlobalTemp) data set (updated from Vose et al. 2012).

396 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM ice, mountain glaciers, and Northern Hemisphere spring snow cover have all decreased. The relatively small increase in Antarctic sea ice in the 15- year period from 2000 through early 2016 appears to be best explained as being due to localized natural variability (see, e.g., Meehl et al. 2016; Ramsayer 2014); while possibly also related to natural variability, the 2017 Antarctic sea ice minimum reached in early March was the lowest measured since reliable records began in 1979. The vast majority of the glaciers in the world are losing mass at significant rates. The two largest ice sheets on our planet—on the land masses of Greenland and Antarctica—are shrinking. There are a number of other climate indicators (e.g., see EPA [2016a] for a discussion of other indicators such as changes in the growing season and the allergy season). The observational data sets all paint a consistent and con- vincing picture that the climate of our planet is warming. FIGURE G-4 Observed changes in annual, winter, and summer temperature. NOTE: Changes are the difference between the average for present day (1986–2016) relative to 1901–1960 for the contiguous United States, and relative to 1925–1960 for Alaska and Hawaii. SOURCE: Adapted from Vose et al. 2014a, 2017.

APPENDIX G 397 Observed Trends in Precipitation Precipitation is perhaps the most societally relevant aspect of the hydro- logical cycle and has been observed over global land areas for more than a century. However, spatial scales of precipitation are small (e.g., it can rain several inches in Washington, DC, but not a drop in nearby Baltimore) and this makes interpretation of the point measurements difficult. Annual aver- age precipitation across global land areas (see Figure G-5) exhibits a slight rise (which is not statistically significant because of a lack of data coverage early in the record) over the past century along with ongoing increases in atmospheric moisture levels. Interannual and interdecadal variability is clearly found in all precipitation evaluations. There are strong geographic trends including a likely increase in precipitation in Northern Hemisphere mid-latitude regions taken as a whole. Stronger trends are generally found over the past four decades. FIGURE G-5 Surface annually averaged precipitation change for the period 1986– 2015 relative to 1901–1960. NOTE: The data are from long-term stations, and so precipitation changes over the ocean and Antarctica cannot be evaluated. The trends are not considered to be statistically significant because of a lack of data coverage early in the record. The relatively coarse resolution (0.5° × 0.5°) of these maps does not capture the finer details associated with mountains, coastlines, and other small-scale effects. SOURCES: NOAA National Centers for Environmental Information (NCEI) and Cooperative Institute for Climate and Satellites.

398 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Annual precipitation averaged across the United States has increased approximately 4 percent over the 1901–2015 period. There continue to be important regional and seasonal differences in precipitation changes (see Figure G-6). Regional differences are apparent, as the Northeast, Midwest, and Great Plains have had increases while parts of the Southwest and Southeast have had decreases. The lingering droughts in the western and southwestern United States were an important part of this (Barnston and Lyon 2016; NCEI 2016a). However, for now, the meteorological drought FIGURE G-6 Annual and seasonal changes in precipitation derived from observa- tions over the contiguous United States. NOTE: Changes are the average for present day (1986–2015) minus the average for the first half of the last century (1901–1960 for the contiguous United States, 1925–1960 for Alaska and Hawaii) divided by the average for the first half of the century. SOURCE: Data from NOAA National Centers for Environmental Information (NCEI) nCLIMDiv data set.

APPENDIX G 399 in California that began in late 2011 (NCEI 2016a; Seager et al. 2015) ap- pears to be largely over, due to the substantial precipitation and snowpack the state received in winter 2016–2017 that greatly increased reservoirs. For the United States, the year 2015 was the third wettest on record, just behind 1973 and 1983 (all of which were years marked by El Niño events). Interannual variability is substantial, as evidenced by large multiyear me- teorological and agricultural droughts in the 1930s and 1950s. Seasonally, national increases are largest in the fall, while little change is observed for winter (NCEI 2016a). For the contiguous United States, fall exhibits the largest (10 percent) and most widespread increase, exceeding 15 percent in much of the Northern Great Plains, Southeast, and Northeast. Winter has the smallest increase (2 percent), with drying over most of the western United States as well as parts of the Southeast. Changes in snow cover extent (SCE) in the Northern Hemisphere ex- hibit a strong seasonal dependence (Vaughan et al. 2013). There has been little change since the 1960s (when the first satellite records became avail- able) in the winter, while fall SCE has increased. However, the decline in spring SCE is larger than the increase in fall and is due in part to higher temperatures that shorten the time that snow spends on the ground in the spring. An analysis of seasonal maximum snow depth for 1961–2015 over North America indicates a statistically significant downward trend (of 0.11 standardized anomalies per decade) and a trend toward the seasonal maximum snow depth occurring earlier—approximately 1 week earlier on average since the 1960s (Kunkel et al. 2016). There has been a statistically significant decrease over the period of 1930–2007 in the frequency of years with a large number of snowfall days (years exceeding the 90th percentile) in the southern United States and the U.S. Pacific Northwest and an increase in the northern United States (Kluver and Leathers 2015). In the snow belts of the Great Lakes, lake-effect snowfall has increased overall since the early 20th century for Lakes Superior, Michigan, Huron, and Erie (Kunkel et al. 2010). However, individual studies for Lake Michigan (Bard and Kristovich 2012) and Lake Ontario (Hartnett et al. 2014) indicate that this increase has not been continuous. In both cases, upward trends were observed until the 1970s and early 1980s. However, since then lake-effect snowfall has decreased in these regions. Lake-effect snows along the Great Lakes are affected greatly by ice cover extent and lake water temperatures. As ice cover diminishes in winter, the expectation is for more lake-effect snow until temperatures increase enough that much of what now falls as snow instead falls as rain (Vavrus et al. 2013; Wright et al. 2013). End-of-season snow water equivalent (SWE)—especially important where water supply is dominated by spring snow melt (e.g., in much of the American West)—has declined since 1980 in the western United States,

400 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM based on analysis of in situ observations, and is associated with springtime warming (Pederson et al. 2013). Satellite measurements of SWE based on brightness temperature also show a decrease over this period (Gan et al. 2013). Observed Changes in Severe Weather Along with the overall changes in climate, there is strong evidence of an in- creasing trend over recent decades in some types of extreme weather events, including their frequency, intensity, and duration, with resulting impacts on our society. It is becoming clearer that the changing trends in severe weather are already affecting us greatly. A change in the frequency, duration, and/or magnitude of extreme weather events is one of the most important consequences of a warming cli- mate. A small shift in the mean of a weather variable, with or without this shift occurring in concert with a change in the shape of its probability distri- bution, can cause a large change in the probability of a value relative to an extreme threshold (Katz and Brown 1992; see also IPCC 2013, Figure 1.8). Examples include extreme high-temperature events and heavy-precipitation events. Additionally, extreme events such as intense tropical cyclones, mid- latitude cyclones, and hail and tornadoes associated with thunderstorms, can occur as isolated events that are not generally studied in terms of ex- tremes within a probability distribution. Detecting trends in the frequency and intensity of extreme weather events is challenging (Sardeshmukh et al. 2015). The most intense events are rare by definition, and observations may be incomplete and suffer from reporting biases. For some events, such as those relating to temperature and precipitation extremes, there is strong understanding of the trends and the underlying causes of the changes (e.g., IPCC 2012, 2013; Kunkel et al. 2013a, 2013b; Peterson et al. 2013; Stott 2016; Vose et al. 2014a, 2014b; Wuebbles et al. 2014b). Through 2016, the United States has sustained 203 weather/climate disasters due to severe weather events since 1980 for which damages/costs reached or exceeded $1 billion per event (including Consumer Price Index adjustment to 2016 to account for inflation), with an overall increasing trend (NCES 2018; see also Smith and Katz 2013). The total cost of these 203 events over the 36 years is more than $1.1 trillion. The year 2016 had 15 such events, costing the United States $46 billion in damages and result- ing in 138 fatalities. As of April 6, 2017, there were five events with losses exceeding $1 billion each across the United States. As seen in Figure G-7, the number of U.S. billion-dollar events has increased from about three such events per year in the decade of the 1980s to more than 10 events per year over the past decade; costs per event have also more than doubled. Every U.S. state has been affected by the billion-dollar events. The events in these analyses include major heat waves, severe storms, tornadoes,

APPENDIX G 401 droughts, floods, hurricanes, and wildfires. A portion of these increased costs can be attributed to the increase in population and infrastructure near coastal regions. However, even if hurricanes and their large, mostly coastal, impacts were excluded, there still would be an overall increase in the number of billion-dollar events over the past 34 years. Similar analyses by Munich Re1 and other organizations come to similar conclusions, finding that there are growing numbers of severe weather events worldwide caus- ing extensive damage and loss of lives. In summary, there is a clear trend in the impacts of severe weather events on human society in the United States and throughout the world. Changing trends in some types of extreme weather events have been observed in recent decades. Modeling studies indicate that these trends are consistent with the changing climate. Much of the world is being affected by changing trends in extreme events, including increases in the number of extremely hot days, fewer extreme cold days, more precipitation events coming as unusually large precipitation, and more floods in some regions and more drought in others (IPCC 2012, 2013; Melillo et al. 2014; Min et al. 2011, 2013; Wuebbles et al. 2014a, 2014b; Zwiers et al. 2013). High- impact, large-scale extreme events are complex phenomena involving vari- ous factors that come together to create a “perfect storm.” Such extreme 1 See https://www.munichre.com/topics-online/en/climate-change-and-natural-disasters/ natural-disasters/overview-natural-catastrophe-2016.html. FIGURE G-7 Increasing trend in number of severe loss events in the United States from natural catastrophes per year since 1980 through 2016 by type of event. NOTES: NOAA has tracked such billion-dollar events since 1980. The costs for each year and the 5-year running mean of costs are also shown. SOURCE: Adapted from NCEI 2018.

402 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM weather obviously does occur naturally. However, the influence of human activities on global climate is altering the frequency and/or severity of many of these events. Observed trends in extreme weather events, such as more hot days, fewer cold days, and more precipitation coming as extreme events, are expected to continue and to intensify over this century. The frequency of extreme high temperatures at both daytime and night- time hours and multiday heat waves is increasing over many of the global land areas (IPCC 2013). There are increasing areas of land throughout our planet experiencing an excess number of daily highs above given thresholds (e.g., the 90th percentile), with an approximate doubling since 1998 of the world’s land area with 30 extreme heat days per year (Seneviratne et al. 2014). At the same time, frequencies of cold waves and extremely low temperatures are decreasing over much of the world, including the United States. The number of record daily high temperatures in the United States has been about double the number of record daily low temperatures in the 2000s (Meehl et al. 2009), and much of the United States has experienced decreases of 5 to 20 percent per decade in cold-wave frequency (Easterling et al. 2016; IPCC 2013). The enhanced radiative forcing caused by green- house gases has a direct influence on heat extremes by shifting distributions of daily temperature (Min et al. 2013). The meteorological situations that cause heat waves are a natural part of the climate system. Thus, the timing and location of individual events may be largely a natural phenomenon, although even these may be affected by human-induced climate change (Trenberth and Fasullo 2012; Trenberth et al. 2015). However, there is emerging evidence that most of the increas- ing heat-wave severity over our planet is likely related to the changes in climate, with a detectable human influence for major recent heat waves in the United States (Duffy and Tebaldi 2012; Meehl et al. 2009; Rupp et al. 2012, 2013), Europe (Stott et al. 2010; Trenberth 2011), and Russia (Christidis et al. 2011). As an example, the summer 2011 heat wave and drought in Oklahoma and Texas, which cost Texas an estimated $8 billion in agricultural losses, was primarily driven by precipitation deficits, but the human contribution to climate change approximately doubled the prob- ability that the heat was record-breaking (Hoerling et al. 2013). So while an event such as this Texas heat wave and drought could be triggered by a naturally occurring event such as a deficit in precipitation, the chances for record-breaking temperature extremes have increased and will continue to increase as the global climate warms. Generally, the changes in climate are increasing the likelihood for these types of severe events. In most of the world, including the United States, over the past three decades, the heaviest rainfall events have become more frequent (e.g., Fig- ure G-8 gives percentage changes in the top 1 percent of rainfalls over vari- ous regions of the United States and Figure G-9 shows the changing trend

APPENDIX G 403 nationally in the 2-day precipitation events exceeding the station-specific threshold for a 5-year recurrence interval, i.e., the one-in-5-year events). When there is precipitation, the amount falling in very heavy precipitation events has been significantly above average. This increase has been greatest in the Northeast, Midwest, and upper Great Plains. Because basic physics tells us that a warmer atmosphere should generally hold more water vapor, this finding is not so surprising. Analyses indicate that these trends will continue (Janssen et al. 2014, 2016; Melillo et al. 2014; Wuebbles et al. 2014a, 2014b). Detection and attribution of trends in past tropical cyclone activity, referred to as hurricanes when they occur in the Atlantic Ocean, are ham- pered by uncertainties in the data collected prior to the satellite era and by uncertainty in the relative contributions of natural variability and anthro- pogenic influences. Whether global trends in high-intensity tropical cyclones are already observable is a topic of active debate. Some research suggests positive trends (Elsner et al. 2008; Kossin et al. 2013), but significant un- certainties remain (Kossin et al. 2013). There has been no significant trend in the global number of tropical cyclones (IPCC 2012, 2013) nor has any FIGURE G-8 Change in the amount of extreme precipitation falling in daily events that exceed the 99th percentile of all non-zero precipitation days, by region of the United States. NOTES: The numerical value is the percentage change over the entire period, 1958– 2016. The percentages are first calculated for individual stations, then averaged over 2° latitude by 2° longitude grid boxes, and finally averaged over each region. SOURCES: Cooperative Institute for Climate and Satellites and National Oceanic and Atmospheric Administration National Centers for Environmental Information.

404 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM trend been identified in the number of U.S. landfalling hurricanes (Melillo et al. 2014). Recent evidence indicates that the locations where tropical cyclones reach their peak intensity have migrated poleward in both the Northern and Southern Hemispheres, in concert with the independently measured expansion of the tropics (Kossin et al. 2014). A number of re- cent studies suggest that hurricane intensities are expected to increase with climate change, both on average and at the high end of the scale, as the range of achievable intensities expands, so that the most intense storms will exceed the intensity of any in the historical record (Sobel et al. 2016). Trends remain uncertain in some types of severe weather, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds, but such events are under scrutiny to determine if there is a climate change influence. Increasing air temperature and moisture increase the risk of extreme convection, and there is evidence for a global increase in severe thunderstorm conditions (Sander et al. 2013). Strong convection along with wind shear represent favorable conditions for tornadoes. Initial studies FIGURE G-9 Index of the number of 2-day precipitation events exceeding the station-specific threshold for a 5-year recurrence interval, expressed as a percentage difference from the 1901–1960 mean. NOTES: The annual values are averaged over 5-year periods, with the pentad label indicating the ending year of the period. Annual time series of the number of events are first calculated at individual stations. Next, the grid box time series are calcu- lated as the average of all stations in the grid box. Finally, a national time series is calculated as the average of the grid box time series. SOURCES: Cooperative Institute for Climate and Satellites and National Oceanic and Atmospheric Administration National Centers for Environmental Information; data from Global Historical Climatology Network-Daily.

APPENDIX G 405 do suggest that tornadoes could get more intense in the coming decades (Diffenbaugh et al. 2013). Observed Trends in Sea Level Sea level rise is closely linked to increasing global temperatures. Thus, even as uncertainties remain about just how much sea level may rise this century, it is virtually certain that sea level rise this century and beyond will pose a growing challenge to coastal communities, infrastructure, and ecosys- tems from increased (permanent) inundation, more frequent and extreme coastal flooding, erosion of coastal landforms, and saltwater intrusion within coastal rivers and aquifers. Sea level change is affected by a variety of mechanisms operating at different spatial and temporal scales (e.g., see Kopp et al. 2015). Global mean sea level (GMSL) rise is primarily driven by two factors: (1) increased volume of seawater due to thermal expansion of the ocean as it warms and (2) increased mass of water in the ocean due to melting ice from mountain glaciers and the Antarctic and Greenland ice sheets (Church et al. 2013). The overall amount (mass) of ocean water, and thus sea level, is also af- fected to a lesser extent by changes in global land-water storage, which reflects changes in the impoundment of water in dams and reservoirs and river runoff from groundwater extraction, inland sea and wetland drain- age, and global precipitation patterns, such as occurs during phases of the El Niño–Southern Oscillation (ENSO) (Church et al. 2013; Reager et al. 2016; Rietbroek et al. 2016; Wada et al. 2016, 2017). Sea level and its changes are not uniform globally for several reasons. First, atmosphere–ocean dynamics—driven by ocean circulation, winds, and other factors—are associated with differences in the height of the sea surface, as are differences in density arising from the distribution of heat and salinity in the ocean. Changes in any of these factors will affect sea sur- face height. For example, a weakening of the Gulf Stream transport in the mid- to late 2000s may have contributed to enhanced sea level rise in the ocean environment extending to the northeastern U.S. coast (Boon 2012; Ezer 2013; Sallenger et al. 2012), a trend that many models project will continue into the future (Yin and Goddard 2013; also see later discussion on the projections of sea level rise). Second, the locations of land ice melting and land water reservoir changes impart distinct regional “static-equilibrium fingerprints” on sea level, based on gravitational, rotational, and crustal deformation effects (Mitrovica et al. 2011). For example, sea level falls near a melting ice sheet because of the reduced gravitational attraction of the ocean toward the ice sheet; reciprocally, it rises by greater than the global average far from the melting ice sheet.

406 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM A variety of other factors can cause local vertical land movement. These include natural sediment compaction, compaction caused by local extrac- tion of groundwater and fossil fuels, and processes related to plate tecton- ics, such as earthquakes and more gradual seismic creep (Wöppelmann and Marcos 2016; Zervas et al. 2013). After at least 2,000 years of little change, the world’s average sea level rose by about 0.2 meters (8 inches) over the past century, and satellite data provide evidence that the rate of rise since 1993 has roughly doubled. Three inches (about 7 centimeters) of the increase has occurred since 1993. The world’s oceans are currently absorbing more than a quarter of the atmospheric carbon dioxide (CO2) emitted to the atmosphere annually (Le Quéré et al. 2016) from human activities, largely from fossil fuel burning, making them more acidic, with potential detrimental impacts on marine ecosystems (Melillo et al. 2014). In particular, higher-latitude systems typi- cally have a lower buffering capacity against pH change, exhibiting season- ally corrosive conditions sooner than low-latitude systems. Acidification is regionally increasing along U.S. coastal systems as a result of upwelling (e.g., in the Pacific Northwest), changes in freshwater inputs (e.g., in the Gulf of Maine), and nutrient input (e.g., in urbanized estuaries). The rate of acidification is unparalleled in at least the past 66 million years (Hönisch et al. 2012; Zeebe et al. 2016). THE BASIS FOR PROJECTING FUTURE CHANGES IN CLIMATE Earth’s climate has long been known to change in response to natural exter- nal factors, termed climate forcings. These include variations in the energy received from the Sun, volcanic eruptions, and changes in the Earth’s orbit, which affects the distribution of sunlight across the world. Earth’s climate is also affected by factors that are internal to the climate system, which are the result of complex interactions among the atmosphere, ocean, land surface, and living things. These internal factors include natural modes of climate system variability, such as those that form El Niño events in the Pacific Ocean. The temperature of the Earth system is determined by the amounts of incoming (short-wavelength) and outgoing (both short- and long- wavelength) radiation. Over recent decades, these fluxes have been well constrained from analyses of satellite measurements (IPCC 2013; Trenberth et al. 2009). About one-third (29.4 percent) of incoming, short-wavelength energy from the Sun is reflected back to space, and the remainder is absorbed by the Earth system. The fraction of sunlight scattered back to space is largely determined by the high reflectivity (albedo) of clouds, some land surfaces (especially those covered by snow and ice), oceans, and particles in the atmosphere.

APPENDIX G 407 In addition to reflected sunlight, Earth loses energy through infrared (long-wavelength) radiation from the surface and atmosphere. Greenhouse gases (GHGs) in the atmosphere absorb most of this radiation, much of which is radiated back toward the surface where it is absorbed, further heat- ing Earth; the remainder is emitted to space. The naturally occurring GHGs in Earth’s atmosphere—principally water vapor, carbon dioxide (CO2), and ozone—keep the near-surface air temperature about 60°F (33°C) warmer than it would be in their absence, assuming albedo is held constant (Lacis et al. 2010). Geothermal heat from Earth’s interior, direct heating from energy production, and frictional heating through tidal flows also contribute to the amount of energy available for heating Earth’s surface and atmosphere, but their total contribution is an extremely small fraction (<0.1 percent) of that due to net solar (short-wave) and infrared (long-wave) radiation (for these various forcings, see, e.g., Davies and Davies [2010]; Flanner [2009]; and Munk and Wunsch [1998]). Natural changes in external forcings and internal factors have been responsible for past climate changes. At the global scale, over multiple decades, the impact of external forcings on temperature far exceeds that of internal variability (which is less than 0.5°F (Swanson et al. 2009). At the regional scale, and over shorter time periods, internal variability can be responsible for much larger changes in temperature and other aspects of climate. Today, however, the picture is very different. Although natural factors still affect climate, human activities are now the primary cause of the current warming: specifically, human activities that increase atmo- spheric levels of CO2 and other heat-trapping gases and various particles that, depending on the type of particle, can have either a heating or cooling influence on climate. The greenhouse effect is key to understanding how human activities affect Earth’s climate. As the Sun shines on Earth, the planet heats up. Earth then radiates this heat back to space. Some gases, including H2O, CO2, ozone (O3), methane (CH4), and nitrous oxide (N2O), absorb some of the heat given off by Earth’s surface and lower atmosphere. These heat- trapping gases then radiate energy back toward the surface, effectively trap- ping some of the heat inside the climate system. This greenhouse effect is a natural process, first recognized in 1824 by the French mathematician and physicist Joseph Fourier and confirmed by British scientist John Tyndall in a series of experiments starting in 1859. Of all the GHGs, CO2 has been undergoing the largest changes in concentration and is the gas of most concern to climate change. Measure- ments of CO2 concentration in air trapped in ice cores indicate that the preindustrial concentration of CO2 was approximately 280 ppm (parts per million). These data show that CO2 concentrations fluctuated by ±10 ppm around 280 ppm for well over 1,000 years until the recent increase to the

408 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM current concentration of more than 400 ppm, an increase that is greater than 40 percent. CO2 emissions have grown in the industrial era primarily from fossil fuel combustion (i.e., coal, gas, oil), cement manufacturing, and land use change, for example, from deforestation (Ciais et al. 2013). This 400-ppm level of CO2 has not been seen on Earth for more than 1 million years, well before the appearance of humans. Although methane’s atmospheric abundance is less than 0.5 percent that of CO2 on a molecule-for-molecule basis, a molecule of methane is approximately 50 times more effective as a GHG in the current atmo- sphere than CO2 (this largely results from the center of the important infrared absorption features already being saturated for CO2). When this is combined with the large increase in its atmospheric concentration, methane becomes the second most important GHG of concern for climate change. Based on analyses of ice cores, the concentration of methane has more than doubled since preindustrial times. The current globally aver- aged atmospheric concentration of methane is about 1.8 ppm. Methane concentrations have primarily increased due to human activities, including agriculture, with livestock producing methane in their digestive tracts, and rice farming producing it via bacteria that live in the flooded fields; mining coal, extraction and transport of natural gas, and other fossil fuel–related activities; and waste disposal including sewage and decomposing garbage in landfills. In 2014, transportation accounted for 26 percent of the total emissions of GHGs in the United States, with 96 percent of these emissions being CO2 (EPA 2016b). Of these emissions, 61 percent are from light-duty vehicles (cars and light-duty trucks), 23 percent from medium- and heavy-duty trucks, and a few more percent from motorcycles and buses. Other important GHGs with changing concentrations are nitrous oxide (N2O) and various halocarbons. Nitrous oxide levels are increasing, primar- ily as a result of fertilizer use and fossil fuel burning. The concentration of nitrous oxide has increased by about 20 percent relative to preindustrial times. The major halocarbons are produced almost entirely by the chemical industry for a variety of uses (e.g., refrigeration). Human activities can also produce tiny atmospheric particles, includ- ing dust and soot. For example, coal burning produces sulfur gases that form particles in the atmosphere. These sulfur-containing particles reflect incoming sunlight away from Earth, exerting a cooling influence on Earth’s surface. Another type of particle, composed mainly of soot, or black car- bon, absorbs incoming sunlight and traps heat in the atmosphere, warming Earth. Changes in particle concentrations are also important in analyzing changes in climate. Particles both have a direct radiative effect on climate and an indirect effect through their effects in changing the properties of clouds. Overall, the net effect of these particles is to globally offset 20

APPENDIX G 409 percent to 35 percent of the warming caused by the increasing concentra- tions of GHGs. It is not only the direct effects from human emissions that affect cli- mate. These direct effects also trigger a cascading set of feedbacks that cause indirect effects on climate—acting to increase or dampen an initial change (Melillo et al. 2014). For example, water vapor is the single most important gas responsible for the natural greenhouse effect. Together, water vapor and clouds account for between 66 percent and 80 percent of the natural greenhouse effect (Schmidt et al. 2010). However, the amount of water vapor in the atmosphere depends on temperature; increasing temper- atures increase the amount of water vapor. This means that the response of water vapor is an internal feedback, not an external forcing of the climate. Some of the other important feedbacks include effects of changes in clouds, changes in albedo, and changes in CO2 absorption by the oceans and the biosphere as the planet warms. Feedbacks are particularly impor- tant in the Arctic, where rising temperatures melt ice and snow, exposing relatively dark land and ocean that absorb more of the Sun’s energy, heating the region even further. Rising temperatures also thaw permafrost, releas- ing CO2 and methane trapped in the previously frozen ground into the atmosphere, where they further amplify the greenhouse effect. Both of these feedbacks act to further amplify the initial warming effects from GHGs. Together, these and other feedbacks determine the long-term response of Earth’s temperature to an increase in CO2 and other emissions from human activities. Scientific analyses largely indicate a significant overall amplifica- tion of the warming effect as a result of the feedbacks (IPCC 2013; Melillo et al. 2014). The conclusion that human influences are the primary driver of recent climate change is based on multiple lines of independent evidence. The first line of evidence is our fundamental understanding of how certain gases trap heat (these so-called GHGs include H2O, CO2, CH4, N2O, and some other gases and particles that can all absorb the infrared radiation emitted from Earth that otherwise would go to space), how the feedbacks within the climate system respond to increases in these gases, and how other human and natural factors influence climate. Evidence also comes from using climate models to simulate the climate of the past century, separating the human and natural factors that influence climate. As shown in Figure G-10, when the human factors are removed, these models show that solar and volcanic activity would have tended to slightly cool Earth, and other natural variations are too small to explain the amount of warming. The range of values accounted for the range of results from the different models from around the world that were used in these analyses for the international climate assessment (IPCC 2013). Only when the human influences are included do the models reproduce the warming

410 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM observed over the past 50 years. Over the past five decades, natural drivers of climate such as solar forcing and volcanoes would actually have led to a slight cooling. Accurate observations of the Sun from satellites show that the solar output has actually decreased slightly since 1978 (IPCC 2013). In another type of analysis, attribution assessment results for different forcings on climate for global mean temperature for the period 1951–2010 from IPCC (2013) are summarized in Figure G-11, which shows assessed likely ranges and midpoint estimates for several factors contributing to in- creases in global mean temperature. The majority of the observed warming can only be explained by the combined effects of the anthropogenic forcing of the warming influence from GHGs and the net cooling influence from par- ticles. Many other studies of past trends in temperature have come to similar conclusions (e.g., Gillett et al. 2012; Santer et al. 2013; Stott et al. 2010). FIGURE G-10 Comparison of observed global mean temperature anomalies from three observational data sets to CMIP5 climate model historical experiments using (a) anthropogenic and natural forcings combined or (b) natural forcings only. NOTES: In (a) the thick orange curve is the CMIP5 grand ensemble mean across 36 models while the orange shading and outer dashed lines depict the ±2 standard deviations and absolute ranges of annual anomalies across all individual simulations of the 36 models. Model data are a masked blend of surface air temperature over land regions and sea surface temperature over ice-free ocean regions to be more consistent with observations than using surface air temperature alone. All time se- ries (°F) are referenced to a 1901–1960 baseline value. The simulations in (a) have been extended from 2006 through 2016 using the RCP8.5 scenario projections. (b) As in (a), but the blue curves and shading are based on 18 CMIP5 models using natural forcings only. See legends to identify observational data sets. Observations after about 1980 are shown to be inconsistent with the natural forcing-only models (indicating detectable warming) and also consistent with the models that include both anthropogenic and natural forcing, implying that the warming is attributable in part to anthropogenic forcing according to the models. SOURCE: Adapted from Knutson et al. 2016.

APPENDIX G 411 Another line of evidence is from reconstructions of past climates us- ing evidence such as tree rings, ice cores, and corals. These show that the change in global surface temperatures over the past five decades are clearly unusual and outside the range of natural variability. These analyses show that the past decade (2000–2009) was warmer than any time in at least the past 1,300 years and perhaps much longer (IPCC 2013; Mann et al. 2008; PAGES 2K Consortium 2013). Through 2016, it appears that this decade will be much warmer than the previous decade. FIGURE G-11 Observed global mean temperature trend (black bar) and attribut- able warming or cooling influences of anthropogenic and natural forcings over 1951–2010. NOTES: Observations are from HadCRUT4, along with observational uncertainty (5 percent to 95 percent) error bars (Morice et al. 2012). Likely ranges (bar-whisker plots) and midpoint values (colored bars) for attributable forcings are from IPCC AR5 (Bindoff et al. 2013). GHG refers to well-mixed greenhouse gases, OA to other anthropogenic forcings, NAT to natural forcings, and ANT to all anthropogenic forcings combined. Likely ranges are broader for contributions from well-mixed GHGs and for other anthropogenic forcings, assessed separately, than for the con- tributions from all anthropogenic forcings combined, because it is more difficult to quantitatively constrain the separate contributions of the various anthropogenic forcing agents. SOURCE: Redrawn from Bindoff et al. 2013 (used with permission).

412 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM PROJECTIONS OF FUTURE CHANGES IN CLIMATE Choices made now and in the next few decades about emissions from fossil fuel use and land use change will determine the amount of additional future warming over this century and beyond. Global emissions of CO2 and other heat-trapping gases continue to rise. How much climate will change over this century and beyond depends primarily on two factors: (1) human ac- tivities and resulting emissions and (2) the sensitivity of the climate to those changes (i.e., the effects of the feedbacks on climate, discussed earlier). Uncertainties in how the economy will evolve, what types of energy will be used, or what our cities, buildings, or cars will look like in the future are all important and limit the ability to project future changes in climate. Scientists can, however, develop scenarios—plausible projections of what might happen under a given set of assumptions. These scenarios describe possible futures in terms of population, energy sources, technol- ogy, heat-trapping gas emissions, atmospheric levels of CO2, and/or global temperature change. The most recent set of time-dependent scenarios, called representative concentration pathways (RCPs) (Moss et al. 2010), are based on a given radiative forcing from which emissions are then evaluated (thus they fit well with prior emission-based scenarios); each scenario is tied to one value, the change in radiative forcing at the tropopause by 2100 rela- tive to preindustrial levels. The four RCPs are numbered according to the change in radiative forcing by 2100: +2.6, +4.5, +6.0, and +8.5 watts per square meter (W/m2) (Masui et al. 2011; Riahi et al. 2011; Thomson et al. 2011; van Vuuren et al. 2011). The three lower RCP scenarios (2.6, 4.5, and 6.0) are climate policy scenarios, in which future emissions are based on societal decisions to move away from the use of fossil fuels at different rates. At the higher end of the range, the RCP 8.5 scenario corresponds to a future in which carbon and methane emissions continue to rise as a result of fossil fuel use, albeit with significant declines in emission growth rates over the second half of the century. RCP 8.5 reflects the upper range of the open literature on emissions, but is not intended to serve as an upper limit on possible emissions. Note that the RCP 2.6 scenario is much lower than the other scenarios examined because it not only assumes significant mitigation to reduce emissions, but it also assumes that technologies are developed that can achieve net negative CO2 emissions (removal of CO2 from the atmosphere) before the end of the century. A certain amount of climate change is inevitable due to the buildup of CO2 and other GHGs in the atmosphere (although there is a rapid exchange of CO2 with the biosphere, the eventual lifetime for atmospheric CO2 is dependent on removal to the deep ocean). Earth’s climate system, particu- larly the oceans, tends to lag behind changes in atmospheric composition by decades, and even centuries, due to the large heat capacity of the oceans

APPENDIX G 413 and other factors. Another ~0.5°F (0.2°–0.3°C) increase is expected over the next few decades (Matthews and Zickfeld 2012), although natural vari- ability could still play an important role over this time period (Hawkins and Sutton 2011). The higher the human-related emissions of CO2 and other heat-trapping gases over the coming decades, the higher the resulting changes expected by midcentury and beyond. By the second half of the cen- tury, however, scenario uncertainty (i.e., uncertainty about what will be the level of emissions from human activities) becomes increasingly dominant in determining the magnitude and patterns of future change, particularly for temperature-related aspects (Hawkins and Sutton 2009, 2011). On the global scale, climate model simulations show consistent projec- tions of future conditions under a range of emission scenarios that depend on assumptions of population change, economic development, our contin- ued use of fossil fuels, changes in other human activities, and other factors. For temperature, all models show warming by late this century that is much larger than historical variations nearly everywhere. Figure G-12 shows the projected changes in globally averaged temperature for a range of future pathways that vary from assuming strong continued dependence on fossil fuels in energy and transportation systems over the 21st century (the high scenario is RCP 8.5) to assuming major emission-reduction actions (the very low scenario, RCP 2.6). Globally and annually averaged temperature changes as large as 6°–10°F (3.3°–5.5°C) are possible by the end of the century if we continue the current pathway of extensively relying on fossil fuels. This would be a very large change relative to past human history (the last ice age was about 12°F (7°C) colder than now). These analyses also suggest that global surface temperature increases for the end of the 21st century are very likely to exceed 2.7°F (1.5°C) relative to the 1850–1900 average for all projections, except for the very lowest part of the uncertainty range for RCP 2.6 (IPCC 2013). Average annual temperature over the contiguous United States is also projected to rise (see Figure G-13). Increases of about 2.5°F (1.4°C) are predicted for the next few decades (i.e., by roughly 2030) in all emission scenarios, implying recent record-setting years may be common in the near future. Much larger rises are projected by late century: 2.8°–7.3°F (1.6°–4.1°C) in a lower-emissions scenario (RCP 4.5) and 5.8°–11.9°F (3.2°–6.6°C) in a higher-emissions scenario (RCP 8.5). Projections of future changes in precipitation show small increases in the global average but substantial shifts in where and how precipita- tion falls. Models show decreases in precipitation in the subtropics and increases in precipitation at higher latitudes. Generally, areas closest to the poles are projected to receive more precipitation, while the dry subtropics (the region just outside the tropics, between 23° and 35° on either side of the equator) will generally expand toward the poles and receives less rain.

414 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Increases in tropical precipitation are projected during rainy seasons (such as monsoons), especially over the tropical Pacific. Extreme weather events associated with extremes in temperature and precipitation are likely to continue and to intensify. For the United States, future changes in seasonal average precipitation will include a mix of increases, decreases, or little change, depending on location and season (see Figure G-14). High-latitude regions are gener- ally projected to become wetter while the subtropical zone is projected to become drier. Because the contiguous United States lies between these two regions, there is significant uncertainty about the sign and magnitude of fu- ture anthropogenic changes to seasonal precipitation in much of the region, FIGURE G-12 Multimodel simulated time series from 1900 to 2100 for the change in global annual mean surface temperature relative to 1976–2005 for a range of the representative concentration pathway (RCP) scenarios. NOTES: These scenarios account for the uncertainty in future emissions from hu- man activities (IPCC 2013). The mean and associated uncertainties (1.64 standard deviations [5–95 percent] across the distribution of individual models [shading]) based on the average over 2081–2100 are given for all of the RCP scenarios as colored vertical bars. SOURCE: Adapted from Walsh et al. 2014.

APPENDIX G 415 particularly in the middle latitudes. Certain regions, including the western United States (especially the Southwest; Melillo et al. 2014), are pres- ently dry and are expected to become drier. The patterns of the projected changes of precipitation do not contain the spatial details that characterize observed precipitation, especially in mountainous terrain, because of model uncertainties and their current spatial resolution (IPCC 2013). Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States. FIGURE G-13 Projected changes in average annual temperature for North America. NOTES: Changes are the difference between the average for midcentury (2036– 2065, top) or late century (2071–2100, bottom) and the average for near present (1976–2005). Each map depicts the weighted multimodel mean. Increases are sta- tistically significant in all areas (i.e., more than 50 percent of the models show a statistically significant change, and more than 67 percent agree on the sign of the change; Sun et al. 2015). SOURCES: Cooperative Institute for Climate and Satellites and National Oceanic and Atmospheric Administration National Centers for Environmental Information.

416 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM A number of research studies have examined the potential criteria for dangerous human interferences in climate for which it will be difficult to adapt to the changes in climate without major effects on our society (e.g., Hansen et al. 2016; Kopp et al. 2016). Most of these studies have concluded that an increase in global average temperature of roughly 2.7°F (1.5°C) is an approximate threshold for dangerous human interferences with the cli- mate system (see IPCC [2013, 2014] for further discussion; earlier studies had proposed 2°C), but that this threshold is not exact and the changes in FIGURE G-14 Projected change in total seasonal precipitation from CMIP5 simula- tions for 2070–2099. NOTES: The values are weighted multimodel means and expressed as the percent- age change relative to the 1976–2005 average. These are results for the RCP 8.5 pathway. Stippling indicates that changes are assessed to be large compared to natu- ral variations. Hatching indicates that changes are assessed to be small compared to natural variations. Blank regions (if any) are where projections are assessed to be inconclusive. SOURCES: Data from World Climate Research Program (WCRP) Coupled Model Intercomparison Project; figure from National Oceanic and Atmospheric Adminis- tration National Centers for Environmental Information.

APPENDIX G 417 climate are geographically diverse and impacts are sector dependent, and so there really is no defined threshold at which dangerous interferences are actually reached. The warming and other changes in the climate system will continue beyond 2100 under all RCP scenarios, except for a leveling of temperature under RCP 2.6. In addition, it is fully expected that the warming will con- tinue to exhibit interannual-to-decadal variability and will not be regionally uniform. PROJECTIONS FOR SEVERE WEATHER, SEA LEVEL, AND LAND SURFACE EFFECTS The observed trends for extreme weather events related to climate change are likely to continue and further amplify throughout this century and per- haps beyond (depending on the actions we take). Existing research indicates the following trends over the coming decades (see Melillo et al. [2014] or IPCC [2013] for more details): • It is likely that over the coming decades the frequency of warm days and warm nights will increase in most land regions, while the frequency of cold days and cold nights will decrease. As a result, an increasing tendency for heat waves is likely in many regions of the world. • Some regions are likely to see an increasing tendency for droughts (especially the Southwest and the Southeast) while others are likely to see an increasing tendency for floods (e.g., the Northeast and the Midwest). • It is likely that the frequency and intensity of heavy precipitation events will increase over land. These changes are primarily driven by increases in atmospheric water vapor content, but also are af- fected by changes in atmospheric circulation. • Tropical storm (hurricane)-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm. • Initial studies also suggest that tornadoes are likely to become more intense, but there are conflicting processes that could affect the resulting trends. • For some types of extreme events, such as windstorms, ice storms, and hailstorms, there is too little understanding currently of how they will be affected by the changes in climate. Daily extreme temperatures are projected to increase substantially in the contiguous United States, particularly under the high scenario, RCP 8.5. For instance, the coldest and warmest daily temperatures of the year are expected

418 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM to increase at least 5°F (2.8°C) in most areas by mid-century (Fischer et al. 2013), rising to 10°F (5.5°C) or more by late century (Sillmann et al. 2013). In general, there will be larger increases in the coldest temperatures of the year, especially in the northern half of the nation, whereas the warmest temperatures will exhibit somewhat more uniform changes geographically (see Figure G-15). On a regional basis, annual extremes are consistently projected to rise faster than annual averages. Future changes in very rare FIGURE G-15 Projected changes in the number of days per year with a maximum temperature above 90°F and a minimum temperature below 32°F in the contiguous United States. NOTES: Changes are the difference between the average for midcentury (2036– 2065) and the average for near present (1976–2005) under RCP 8.5. Maps in the top row depict the weighted multimodel mean whereas maps on the bottom row depict the mean of the three warmest models (i.e., the models with the largest tem- perature increase). Maps are derived from 32 climate model projections that were statistically downscaled using the localized constructed analogs technique (Pierce et al. 2014). Changes are statistically significant in all areas (i.e., more than 50 percent of the models show a statistically significant change, and more than 67 percent agree on the sign of the change [Sun et al. 2015]). SOURCES: Cooperative Institute for Climate and Satellites and National Oceanic and Atmospheric Administration National Centers for Environmental Information.

APPENDIX G 419 extremes are also striking; by late century, current 1-in-20-year maximums are predicted to occur every year, while current 1-in-20-year minimums are not expected to occur at all (Wuebbles et al. 2014b). The frequency and intensity of cold waves is projected to decrease while the frequency and intensity of heat waves is projected to increase throughout the century. The frequency of cold waves (6-day periods with a minimum temperature below the 10th percentile) will decrease the most in Alaska and the least in the Northeast, while the frequency of heat waves (6-day periods with a maximum temperature above the 90th percentile) will increase in all regions, particularly the Southeast, Southwest, and Alaska. By midcentury, decreases in the frequency of cold waves are similar across RCPs, whereas increases in the frequency of heat waves are about 50 percent greater in RCP 8.5 than RCP 4.5 (Sun et al. 2015). The intensity of cold waves is projected to decrease while the intensity of heat waves is projected to increase, dramatically so under RCP 8.5. By midcentury, both extreme cold waves and extreme heat waves (e.g., 5-day, 1-in-10- year events) are projected to have temperature increases of at least 11.0°F (6.1°C) nationwide, with larger increases in northern regions (the North- east, Midwest, Northern Great Plains, and Northwest). There are large projected changes in the number of days exceeding key temperature thresholds throughout the contiguous United States. For instance, there are about 20 to 30 more days per year with a maximum more than 90°F (32°C) in most areas by midcentury under RCP 8.5, with increases of 40–50 days in much of the Southeast. Consistent with wide- spread warming, there are 20–30 fewer days per year with a minimum temperature below freezing in the northern and eastern parts of the nation, with decreases of more than 40–50 days in much of the West. Atmospheric water vapor will increase with increasing temperature, with the result that confidence is high that projected future precipitation extremes will increase in frequency and intensity throughout the continental United States. The widespread trend of increasing heavy downpours is ex- pected to continue, with precipitation becoming more intense (e.g., Janssen et al. 2014, 2016; Sillmann et al. 2013). Similar to the observed changes, increases are expected in all regions, even those regions where total precipi- tation is projected to decline, such as the southwestern United States. Under the RCP 8.5 scenario the number of extreme events (exceeding a 5-year return period) increases by two to three times the historical average in every region (see Figure G-16) by the end of the 21st century, with the largest increases in the Northeast. Under the RCP 4.5 scenario, increases are 50 to 100 percent. Research shows that there is strong evidence, from both the observed record and modeling studies, that increased water vapor resulting from higher temperatures is the primary cause of the increases (Kunkel et al. 2013a, 2013b; Wehner 2013). Additional effects on extreme precipitation

420 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM

APPENDIX G 421 due to changes in dynamical processes are poorly understood. However, projected changes in atmospheric rivers, a narrow corridor of concentrated atmospheric moisture, have been found to increase in number and water vapor transport (Dettinger 2011), as well as resulting in more landfalling at lower latitudes (Shields and Kiehl 2016) as the climate changes—these events can result in significant rainfall on the West Coast. Around the world, many millions of people and many assets related to energy, transportation, commerce, and ecosystems are located in areas at risk of coastal flooding because of sea level rise and storm surge. Future projections show that by 2100, global mean sea level is very likely to rise by 1.6 to 4.3 feet (0.5 to 1.3 meters) under RCP 8.5, 1.1 to 3.1 feet (0.35 to 0.95 meters) under RCP 4.5, and 0.8 to 2.6 feet (0.24 to 0.79 meters) under RCP 2.6 (Kopp et al. 2014 [see Figure G-17]). Recent projections show that for even the lowest-emission scenarios, thermal expansion of ocean waters (Yin and Goddard 2013) and the melting of small mountain glaciers (Marzeion et al. 2012) will result in 11 inches of sea level rise by 2100, even without any contribution from the ice sheets in Greenland and Antarctica. This suggests that about 1 foot (0.3 meters) of global sea level rise by 2100 is probably a realistic low end. Recent analyses suggest that 4 feet (1.2 meters) may be a reasonable upper limit (IPCC 2013; Melillo et al. 2014; Rahmstorf et al. 2012). Although scientists cannot yet assign likelihood to any particular scenario, in general, higher emission scenarios would be expected to lead to higher amounts of sea level rise. The best estimates for the range of sea level rise projections for this century remain quite large; this may be due in part to what emission sce- nario we follow, but more importantly it depends on just how much melting occurs from the ice on large land masses, especially from Greenland and Antarctica. Emerging science suggests that the projections may be under- estimates, particularly for higher scenarios; a global mean sea level rise FIGURE G-16 (Facing Page) Regional extreme precipitation event frequency across the contiguous United States for RCP 4.5 (green) and RCP 8.5 (blue) for a 2-day duration and 5-year return. NOTES: Regions are based on those being used for the Third National Climate As- sessment except that the Great Plains are split into a Northern and a Southern Great Plains (Janssen et al. 2014). Frequency is calculated for 2006–2100 but decadal anomalies begin in 2011. Error bars are ±1 standard deviation; standard deviation is calculated from the 14 or 16 model values that represent the aggregated average over the regions, over the decades, and over the ensemble members of each model. The average frequency for the historical reference period is 0.2 by definition and the values in this graph should be interpreted with respect to a comparison with this historical average value. SOURCE: Janssen et al. 2014.

422 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM exceeding 8 feet (2.4 meters) by 2100 cannot be excluded, and even higher amounts are possible as a result of ice sheet instability. The U.S. Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force (henceforth referred to as Interagency Task Force) (Sweet et al. 2017) recently revised the GMSL rise scenarios for the United States, and now provides six scenarios that can be used for assessment and risk-framing purposes (also shown in Figure G-17). The low scenario of about 1-foot GMSL rise by 2100 is consistent with a continuation of the recent approximately 0.12 inches/year (3 millimeters/year) rate of rise through to 2100, while the five other scenarios span a range of GMSL rise between 1.6 and 8.2 feet (50 and 250 centimeters) in 2100 with cor- responding rise rates between 0.2 inches/year (5 millimeters/year) to 1.7 inches/year (44 millimeters/year) toward the end of this century. The highest scenario of 250 centimeters is consistent with several literature estimates of the maximum physically plausible level of 21st century sea level rise (e.g., Pfeffer et al. [2008], updated with Sriver et al. [2012] estimates of thermal expansion, Bamber and Aspinall [2013] estimates of Antarctic contribution, and incorporating land water storage, as discussed in Miller et al. [2013]; FIGURE G-17 Past and projected mean sea level for six future scenarios. NOTES: The six Interagency Task Force global mean sea level (GMSL) scenarios of Sweet et al. (2017) are shown over the 2000 to 2100 period relative to historic GMSL estimated by geological, tide gauge, and satellite altimeter reconstructions over 1800 to 2015 (black and magenta lines). The gray-shaded boxes are the central 90 percent conditional probability ranges of representative concentration pathways (RCPs)-based GMSL projections from several recent studies, which are augmented (dashed lines) by the difference between the median Antarctic contribution of Kopp et al. (2014) probabilistic sea level study and the median Antarctic projections of DeConto and Pollard (2016). The scenarios do not necessarily align with any par- ticular RCP-based GMSL solution; rather they span a range of future GMSL rise possibilities. Under the Kopp et al. (2014) framework, which was the basis for the scenario construction, the six scenarios align with a range of probabilistic RCP- based outcomes (e.g., the scenarios span the 2–99.95 percent range for RCP 4.5). SOURCE: Adaptation of Figure 8 in Sweet et al. 2017.

APPENDIX G 423 Kopp et al. [2014]). It is also consistent with the high end of recent projec- tions of Antarctic ice sheet melt (e.g., DeConto and Pollard 2016). Because of the warmer global temperatures, sea level rise will continue beyond this century. Sea levels will likely continue to rise for many centuries at rates equal to or higher than that of the current century. Many millions of people live within areas than can be affected by the effects of storm surge within a rising sea level. The Low Elevation Coastal Zone (less than 10-meter elevation) constitutes 2 percent of the world’s land area, yet contains 10 percent of the world’s population (more than 600 million people) (McGranahan et al. 2007; Neumann et al. 2015). Most of the world’s megacities are within the coastal zone. By 2030, with sea level rise, the area will expand and 800 million to 900 million people will be exposed (Güneralp et al. 2015; Neumann et al. 2015). Sea level will not rise uniformly around the coasts of the United States and its overseas territories. Local sea level rise is likely to be greater than the global average along the U.S. Atlantic and Gulf Coasts and less than the global average in most of the Pacific Northwest (Sweet et al. 2017). Based on the process-level projections of the Interagency Task Force GMSL scenarios, several key regional patterns are apparent in future U.S. regional sea level (RSL) rise as shown for the intermediate (3.3 feet [1 meter] GMSL rise by 2100) scenario in Figure G-18. • RSL rise due to Antarctic Ice Sheet melt is greater than GMSL rise along all U.S. coastlines due to static-equilibrium effects. • RSL rise due to Greenland Ice Sheet melt is less than GMSL rise in the continental United States due to static-equilibrium effects. This effect is especially strong in the Northeast. • RSL rise is additionally augmented in the Northeast by the effects of glacial isostatic adjustment. • The Northeast is also exposed to rise due to changes in the Gulf Stream and reductions in the Atlantic meridional overturning cir- culation (AMOC). • The western Gulf of Mexico and parts of the U.S. Atlantic Coast south of New York are currently experiencing significant RSL rise caused by the withdrawal of groundwater (along the Atlantic Coast) and of both fossil fuels and groundwater (along the Gulf Coast). Continuation of these practices will further amplify RSL rise. • The presence of glaciers in Alaska and their proximity to the Pacific Northwest reduces RSL rise in these regions, due to both the ongo- ing glacial isostatic adjustment to past glacier shrinkage and to the static-equilibrium effects of projected future losses.

424 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Global sea level rise and its regional variability forced by climatic and ocean circulation patterns are contributing to significant increases in an- nual tidal-flood frequencies, which are measured by NOAA tide gauges. As seen in Figure G-19, some portions of the U.S. coast (including more than 25 East Coast and Gulf Coast cities) are seeing an accelerating frequency of the impacts from such events (Ezer and Atkinson 2014; Sweet and Park 2014). Trends in annual frequencies surpassing local emergency prepared- ness thresholds for minor tidal flooding (i.e., “nuisance” levels of about 1 to 2 feet [30 to 60 centimeters]) that begin to flood infrastructure and trigger coastal flood advisories by NOAA’s National Weather Service have increased 5- to 10-fold or more since the 1960s along the U.S. coastlines (Sweet et al. 2014). With rising sea levels, such flooding is expected to increase dramatically in the coming decades. The combination of a storm surge at high tide with additional dynamic effects from waves (Stockdon et al. 2006; Sweet et al. 2015) creates the most damaging coastal hydraulic conditions (Moritz et al. 2015). FIGURE G-18 Regional sea level rise in 2100 for the United States projected for the Interagency Task Force Intermediate Scenario (3.3 feet [1-meters] GMSL rise by 2100). NOTE: Much of the eastern and southern United States are projected to have higher sea level rise than the global average. SOURCE: Figure adopted from Sweet et al. 2017; based on Figure 12.4b in US- GCRP 2017.

APPENDIX G 425 Rising Alaskan permafrost temperatures are causing permafrost to thaw and become more discontinuous. Alaskan and Arctic permafrost characteristics have responded to increased temperatures and reduced snow cover in most regions since the 1980s (AMAP 2011). The permafrost warm- ing rate varies regionally; however, colder permafrost is warming faster than warmer permafrost (Romanovsky et al. 2015; Vaughan et al. 2013). This feature is most evident across Alaska, where permafrost on the North Slope is warming more rapidly than in the interior. This results in significant potential effects on buildings and roads in that region. OVERVIEW OF ENVIRONMENTAL VARIABLES AND EXTREME EVENTS AND THE INTERSTATE HIGHWAY SYSTEM Climate variability and change affect U.S. DOT’s strategic goals of safety, state of good repair, and environmental sustainability for all transportation modes including the U.S. Interstate Highway System (FHWA n.d.; U.S. FIGURE G-19 Annual occurrences of daily tidal flooding, also called sunny-day or nuisance flooding, have increased for a number of U.S. coastal cities. NOTES: Historical exceedances (yellow bars) are shown for two of the locations— Charleston, South Carolina, and San Francisco, California—and future projections through 2100 of the current trend (blue) and under median RCP 2.6 (green), 4.5 (teal), and 8.5 (red) conditions. SOURCE: Based on Sweet and Park 2014.

426 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM DOT 2014). For the Interstate System, climate variability and change may accelerate asset deterioration, increase operational disruptions, or cause catastrophic failure of structures. In some cases, such as projected winter weather moderation, climate change may positively affect the Interstate System. Notable impacts identified by U.S. DOT are wide-ranging and not limited to the Interstate Highway System (see Figure G-20). Some impacts may require changes in the planning, design, construction, and/or mainte- nance of infrastructure. At a national level, addressing potential climate impacts in planning and project development is one priority that will allow transportation systems to gradually become resilient to the future climate (U.S. DOT 2014). Another priority is incorporating climate change as a risk in risk-based asset management to allow assessment of climate risk consistently with the other risks that impact assets. Traditionally, infrastructure design standards and guidelines have used historical weather and climate observations to determine the environmental stress that an asset should be designed to withstand over its service life. However, in a nonstationary climate, past weather is not a reliable indicator of future weather and may not be appropriate for infrastructure design. The current trends for average climate and extreme weather events are likely to continue and further amplify throughout this century. Most Interstate System assets are designed to remain in service for decades or even longer. The decisions made today about future environmental risks will impact the costs, service, and design life of infrastructure assets. Potential climate impacts to the Interstate Highway System vary greatly depending on the climate stressor, the asset, and its location. Numerous infrastructure impacts are anticipated from projected temperature, pre- cipitation, and sea level rise change (TRB 2014). Strategies to mitigate future impacts include planning, design, and operations/maintenance ap- proaches (Caltrans 2013). Infrastructure impacts and strategies may also differ between existing and new infrastructure. The vulnerability of existing transportation infrastructure to future climate varies greatly due to its age, service life, location, and original design standards. As existing infrastruc- ture ages, decisions about repair, replacement, or abandonment should take into account the future climate. New infrastructure can be designed and built to handle future environmental risks. New strategies to incorporate future climate in infrastructure planning and design processes are now emerging. In 2014, FHWA released Hydraulic Engineering Circular 25—Volume 2: Highways in the Coastal Environment: Assessing Extreme Events. HEC 25 provides technical guidance and method- ologies for incorporating climate change considerations, including sea level rise, storm surge, and wave action, into planning and design analyses for highway projects in the coastal environment (Douglass et al. 2014). In 2016, FHWA released HEC 17—Highways in the River Environment: Floodplains,

APPENDIX G 427 FIGURE G-20 Notable potential impacts to the Interstate Highway System and other transportation modes. SOURCE: Adaptation of Figure 1 in U.S. DOT 2014. Notable Potential Impacts to the Interstate Highway System • More frequent/severe flooding of underground tunnels and low- lying infrastructure, requiring drainage and pumping, due to more intense precipitation, sea level rise, and storm surge. • Increased numbers and magnitude of storm surges and/or relative sea level rise potentially shorten infrastructure life. • Increased thermal expansion of paved surfaces, potentially causing degradation and reduced service life, due to higher temperatures and increased duration of heat waves. • Higher maintenance/construction costs for roads and bridges, due to increased temperatures, or exposure to storm surge. • Asphalt degradation and shorter replacement cycles; leading to limited access, congestion, and higher costs, due to higher temperatures. • Culvert and drainage infrastructure damage, due to changes in precipitation intensity, or snow melt timing. • Decreased driver/operator performance and decision-making skills, due to driver fatigue as a result of adverse weather. • Increased risk of vehicle crashes in severe weather. Notable Potential Impacts to Alternative Transport Modes • System downtime, derailments, and slower travel times, due to rail buckling during extremely hot days. • Reduced aircraft performance leading to limited range capabilities and reduced payloads. • Air traffic disruptions, due to severe weather and precipitation events that impact arrival and departure rates. • Reduced shipping access to docks and shore equipment and navigational aid damage. • Restricted access to local economies and public transportation. Extreme Events, Risk, and Resilience, 2nd ed. (Kilgore et al. 2016). HEC 17 provides technical guidance and methodologies for incorporating climate change considerations, with a focus on extreme flood events, into highway projects’ planning and design analyses in the riverine environments.

428 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Transportation systems are interdependent. When climate and weather compromise passenger and freight ability to reach their destination safely and efficiently using the Interstate Highway System, other modes of trans- portation are simultaneously affected and the overall system performance may be further compromised. However, because impacts on infrastructure and operations differ by mode, redundancies within the Interstate Highway System and other modes may allow the system to function efficiently. DIRECT EFFECTS OF PROJECTED FUTURE CLIMATE ON THE INTERSTATE HIGHWAY SYSTEM The Interstate Highway System and Temperature Changes Warming average temperatures, heat waves, and record-setting summer temperatures have immediate and long-term impacts on the Interstate High- way System. Sustained heat compromises pavement integrity by increasing rutting, cracking, and buckling while stressing bridge decks and joints. Vehicle mechanical failures including tire blowouts and overheating are linked to high temperatures. Elevated temperatures also affect transport capacity, safety, and construction. Under extremely hot temperatures, exces- sive expansion can cause roadway joints to buckle and, potentially, send vehicles airborne or cause the driver to lose control. Repairing pavement from heat waves and drought in addition to accelerated deterioration from higher average temperatures is already affecting state DOTs’ operations and maintenance activities. The Iowa DOT spends $400,000 annually to make temporary and permanent repairs to buckled pavement. Virginia DOT has crews available to quickly repair potholes or buckling pavement during extreme heat events (Gopalakrishna et al. 2013). For flexible pavements, higher average air temperatures will increase the maximum pavement temperature and the potential for rutting and shov- ing, particularly during extreme heat waves. For example, by midcentury in the Northeast, projected warming temperatures would increase asphalt concrete rutting 10 to 25 percent and reduce a typical Interstate pavement life span by 1 to 6 years as compared to current condition (see Figure G-21). These changes may be addressed through more rut-resistant asphalt mixtures and/or increased use of rut-resistant designs, but current binder grade selection guidelines may no longer be appropriate. Accelerated age hardening of asphalt binder is also a concern. For rigid pavement, there is an increased potential for concrete temperature-related curling that may require new design strategies. For existing pavements, attention to routine joint maintenance may be adequate to accommodate additional expansion, but in some cases new expansion joints may be required (Muench and Van Dam 2015).

APPENDIX G 429 FIGURE G-21 Percentage difference in asphalt concrete rutting between baseline and future periods. NOTES: Temperature values were obtained from the North American Regional Climate Change Assessment Program’s climate change simulations for the baseline period 1971–2000 and for the future period 2041–2070. Rutting was determined using the Mechanistic-Empirical Pavement Design Guide with typical pavement structures representing a secondary road and an Interstate. SOURCE: Adaptation of Figure 7 in Meagher et al. 2012.

430 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM High temperatures and rapid temperature changes can induce differential movement of joint and deck stress materials and may lead to premature failure (Meyer et al. 2013). Most in-service bridges’ bearings and expansion joints can accommodate the modest projected additional expansion (see Figure G-22). However, if the bridge joints cannot handle expansion extremes, then large forces could develop that severely affect the bridges (Niemeier et al. 2013; Peterson et al. 2008). In this case, the enhanced risk of structural damage may necessitate mandatory traffic diversion or load restrictions, particularly for freight, to more robust alternative routes (Gopalakrishna et al. 2013). Construction activities are sensitive to high temperatures. Heat com- promises worker and public safety. Higher extreme temperatures affect con- struction scheduling. Protocols governing worker safety limit construction during heat waves, reduce productivity, or require work to be conducted at night (Anderson et al. 2015b; Baglin 2012; Cambridge Systematics, Inc. 2015). There are also temperature limitations on some construction activi- ties including road painting, asphalt paving, and concrete pouring. Warming conditions will affect aspects of the Interstate Highway Sys- tem beyond bridges and pavements. In urban areas, temperature effects in tunnels may be amplified because there is additional heat generated by equipment and vehicles, and tunnels have reduced cooling efficiency. Elec- trical support equipment, including monitoring equipment, communication lines, and power lines, can be damaged by heat, overheat, or fail during rolling blackouts (Asam et al. 2015). Increased cooling costs for some assets will occur in the summer, but may be offset by decreased heating costs in the winter. Freight using the Interstate Highway System will need increased refrigeration to alleviate cargo overheating (Caltrans 2013; U.S. DOT 2015). Warming winters with less frequent extremely cold days and extreme cold waves are anticipated to extend the construction season and reduce winter road maintenance demand. In southern regions, fewer snow and ic- ing events will likely reduce vehicle accident risk (Cambridge Systematics, Inc. 2015; Tamerius et al. 2016) as well as decreased transport delays from grounded freight. Warming winters are also anticipated to reduce frost heaves and pavement degradation from freeze–thaw transitions in most regions. However, some locations that historically have had long, hard winters may experience increased road degradation due to more moderate freeze conditions, increased freeze–thaw conditions, and expanded use of deicing agents. For example, historically, Fairbanks, Alaska, and its interior roads would stay frozen from mid-fall to late spring. However, thawing and freezing rain events have now caused the Alaska Department of Transporta- tion & Public Facilities to perform anti-icing (Asam et al. 2015). Damage to bridges and roads caused by potholes and frost heaves cost hundreds of millions of dollars annually (Peterson et al. 2008), and changing winter

APPENDIX G 431 conditions will likely alleviate expenditures in some regions and amplify expenditures in other regions. In Alaska, the melting permafrost will have significant impacts on roads, bridges, and culverts and adjacent land due to ground settlement. This ground movement is particularly problematic for bridges and roads and also deforms the road surface (TRB 2014). The Interstate Highway System and Changes to Precipitation and Intense Rainfall The Interstate Highway System is highly vulnerable to flood events and mudslides/landslides from long-duration rainfall. From 2014 to 2016, the Dartmouth Flood Observatory reported 30 major floods in the United States. In the first 5 months of 2017, five major flooding and mudslide events shut down the Interstate System for days or weeks including Northern (I-80) FIGURE G-22 Percentage difference in bridge joint expansion and increase in expansion magnitude for wood, steel, and concrete spans between baseline (1971– 2000) and future temperatures in New England. NOTES: The future New England temperatures reflect those temperatures that occur when global mean temperatures increase by 5.4oF (3oC). It is likely that this warming will not be reached until 2065–2084, but these thresholds could be reached as early as 2050–2069 under a high-emission scenario (http://theicnet.org/ ?page_id=46). SOURCE: C. Noyes and J. M. Jacobs, unpublished study, 2015.

432 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM and Southern California (I-880) in January, North Central California (I-5) in February, Idaho (I-86) in March, and the Central United States including Missouri (I-44 and I-55) in May. Changing seasonal precipitation, increased rainfall intensity, and snow and rain transitions will affect the Interstate Highway System in a number of ways. As precipitation increases during winter months and changes from snow to rain due to warming winters, spring river flooding is anticipated to increase. More precipitation may cause groundwater tables to rise, thus increasing soil saturation and affecting the pavement strength of the In- terstates, foundation integrity, and tunnel function. The upper Midwest is increasingly vulnerable to spring floods from the changing climate (Hirsch and Ryberg 2012). The most dramatic impact from increased precipitation extremes is an elevated risk of flooded highways, tunnels, drainage systems, and sec- ondary roads. Flooded pavements are subject to washouts or accelerated deterioration. Intense storms can cause steep embankments to fail, leading to road closures. Severe storms with intense precipitation and high winds can damage signs, overhead cables, and other tall structures and topple trees. Heavier rainfall events and more intense storms also have serious driver safety implications. Crash rates increase by more than 50 percent with heavier rainfall intensities, and annual rainfall totals are linked to higher risk of accidents (Tamerius et al. 2016; Winguth et al. 2016). While all regions may see increased flooding impacts from climate change, the Northeast is particularly at risk from increasingly heavy rainfall while the Pacific Northwest faces increased slope stability challenges. Flooding and extreme precipitation are among the most studied climate change stressors in recent transportation vulnerability studies. For example, Iowa’s transportation infrastructure has repeat-flooded as recently as 2008 and 2016 (see Figure G-23). Multiple studies have found that many of the 4,100 bridges and structures in the state’s primary highway system are af- fected during flooding. The FHWA pilot study of Iowa found that all of the study’s Interstate and highway locations would be exposed to streamflow that exceeds current design standards and have increased vulnerability from more frequent episodes of highway overtopping. Additionally, bridges have higher vulnerability to potential bridge scour over the design lifetime of the bridge (Anderson et al. 2015a). Bridges are highly vulnerable to flooding events. The common bridge failure modes are scour, in which bridge foundations are compromised due to erosion, and failure during single-event floods of record (Flint et al. 2017). Scour failures can result from a series of strong storm events that collectively weaken the foundation. In some regions of the country, more than 75 percent of inland bridges are vulnerable due to climate change (see Figure G-24).

APPENDIX G 433 FIGURE G-23 Repeated floods in Iowa, including Cedar Rapids and Iowa City, closed I-80 in 2008 and threatened I-80 and I-380 again in 2016. SOURCE: U.S. Army Corps of Engineers. FIGURE G-24 Inland bridges identified as vulnerable in the second half of the 21st century due to climate change. SOURCE: Adaptation of Figure 1 in EPA 2015, 34.

434 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Pavements are affected by annual precipitation changes as well as ex- tremes. Wetter conditions will reduce the pavement structure load capacity, require improved surface and subsurface drainage, and likely negatively affect construction because of weather-related delays (Muench and Van Dam 2015). The trend to wetter winters and drier summers can amplify soil shrinking and swelling due to moisture changes. Impacts to roads from extreme rainfall include flooding and potential washout that close routes and cause travel delays. Flooding can also contribute to reduced pavement lifetime from decreased structural capacity when the base and subgrades are saturated. Additionally, increasing frequency and magnitude of moder- ate rainfall events will affect safety if the wet road surfaces do not have adequate friction or visible pavement markings. Sea Level Rise, Storm Surge, and Nuisance Flooding Changes The Interstate System’s infrastructure in the coastal zones is already vul- nerable to coastal extreme events; this vulnerability will increase with sea level rise, enhanced storm surge from tropical and nontropical storms, and land subsidence (Douglass et al. 2014; Peterson et al. 2008). Hurricanes Matthew (2016), Sandy (2012), Ike (2008), and Katrina (2005) caused billions of dollars in damage to coastal roadways and bridges. Significant economic losses also occurred due to transport disruption during and after these storms. During Hurricane Katrina, a 27-foot storm surge in coastal Mississippi washed out roads, bridges, and railways. Currently, 60,000 miles of highway are exposed to coastal storms (see Douglass et al. 2014). In the future, disruptions and damage will occur more frequently, and ris- ing seas will result in more severe events. Rising seas will cause storm surge impacts to extend farther inland and into new parts of the country. Infra- structure in these locations may be more susceptible to damage because they were not built to withstand damaging storm waves (U.S. DOT 2015). Coastal Interstates function as an evacuation route is anticipated to increase (GAO 2013) despite declining performance abilities due to sea level rise. Impacts on the Interstate Highway System during extreme events will delay evacuations and compromise the safety of the public and first responders. The primary natural processes that affect coastal Interstate infrastruc- ture are coastal water levels, waves, and high-velocity flows. Additional challenges may result from shoreline erosion and deposition, reduced drain- age capacity, roadway failures due to elevated groundwater tables, the loss of protective coastal wetlands, and saltwater intrusion and corrosion (Douglass et al. 2014). For bridges, accelerated corrosion will shorten the life expectancy and increase maintenance costs as well as increase the likeli- hood of structural failure (TRB 2014). Coastal and inland tidal zone flooding will affect the Interstate High- way System by flooding roadways, damaging pavements, and disrupting

APPENDIX G 435 transportation. For moderate storm events, rising seas present a flooding risk to low-lying roadways and underground infrastructure, such as road tunnels, during storm events. Culverts that are undersized for new flows may induce flooding in new locations or prolong flooding in existing vul- nerable areas. During tropical storms (e.g., Hurricane Sandy), flooded highways and tunnels are prolific. Waves on top of a storm surge promote flooding and can damage coastal bridge super- and substructures. Waves on storm surge in the Gulf Coast hurricanes of 2004–2005 caused billions of dollars in damage to bridges including moving bridge deck spans that weighed more than 340,000 pounds each (see Figure G-25). Sea level rise will also increase coastal erosion and cause damage to the substructure or a complete washout of vulnerable roadways. Periodic route closures and travel delays will result. Flowing water combined with waves and storm surge will damage embankments and pavements. In the Pacific Northwest, damage to roadways occurs on coastal bluffs due to waves and wave runup that creates erosion; portions of the Pacific Coast Highway have already been relocated due to bluff erosion (Hormann 2012). Erosion also causes scour on critical bridges in coastal zones and may compromise their integrity, leading to reduced capacity or even failure resulting in closure. The coastal storm impacts on the Interstates from future extreme events depend on a combination of sea level rise, whose magnitude varies by FIGURE G-25 Lake Pontchartrain Bridge damage from Hurricane Katrina. SOURCES: Adaptation of Figure 1.3 in Xu 2015, adopted from Sheppard and Marin 2009.

436 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM region, regional extreme events, and local conditions. In the Gulf of Mexico and South Atlantic coasts, tropical storms and hurricanes are key extreme events. Storm surge and wave impacts depend on the tide cycle, tropical storm position, wind stress, and inland rainfall-induced flooding. In the mid-Atlantic and New England, the timing and duration of tropical and nontropical storm surges relative to tide cycles are critical because tides range from 2 to 9 feet. On the Pacific Coast and in Alaska, El Niño and tsunamis combine with 6 feet or greater tides that enhance coastal flooding during storms. Additionally, major Pacific storms typically last multiple days, with the greatest impact occur during astronomical high tides. In some coastal areas, impacts on the Interstates will not be limited to storm events. Routine “sunny day flooding” is already occurring during moderate to extreme high tides. Phase I of the U.S. DOT Gulf Coast Study, completed in 2008, found that with 4 feet of sea level rise, 27 percent of the Gulf Coast region’s major highways as well as 9 percent of rail lines and 72 percent of ports would be inundated (FHWA n.d.). Emerging findings on rising groundwater due to sea level rise show that groundwater response occurs two to four times farther inland than surface water inundation, thus blurring the coastal and inland divide (Knott et al. 2018). Groundwater- induced impacts to roads occur where the groundwater is already high, with modest performance reduction for engineered Interstate pavements unless the road becomes inundated. A secondary impact from rising groundwater is wetland expansion that may affect permitting of new projects. Because sea level rise–induced coastal flooding decreases service and increases maintenance costs for existing facilities, mitigation strategies will be an increasingly critical aspect of transportation planning, design, and operations. Planning activities can identify those portions of the Interstate roadways and bridges that will likely be vulnerable to future flooding and erosion, address the future vulnerability in transportation plans, and, as needed, design redundancy into the system to account for declining perfor- mance. Design practices may protect infrastructure from future flooding and storm surges by strengthening and raising infrastructure and developing protective seawalls and drainage. In some cases, Interstate roadways may need to be relocated or abandoned. Damages and repair needs will rise for existing roadways. As weather-related delays become more frequent and expand in extent, operations and maintenance will need to ensure that drainage systems function efficiently. Advanced monitoring, forecasting, and communicating tools can provide system users timely information about route status and alternative routes.

APPENDIX G 437 INDIRECT EFFECTS OF PROJECTED FUTURE CLIMATE ON THE INTERSTATE HIGHWAY SYSTEM The Interstate Highway System and Inland Navigation Industries that need inland transport services can use barges, trains, or trucks. For the Interstate Highway System, changes in access to or reli- ability of another mode of transportation may change the shipment mode. If inland navigation via ships and barges is negatively impacted by climate change, then the demand for truck and rail transport may increase. Po- tential climate change impacts to inland waterways are winter conditions, water levels, and siltation. Winter conditions affect the waterway’s shipping season. Water levels and siltation control the number of days per year that waterways can be used without restrictions. Northern inland ports on the Great Lakes and the Saint Lawrence Seaway are closed and icebound in the winter. Milder winters will lengthen the shipping season on both the Great Lakes and the Seaway (Moser et al. 2008; U.S. DOT 2014). Water levels in the Mississippi and Ohio Rivers, the Saint Lawrence Seaway, and the Great Lakes are sensitive to changes in the long-term water balance driven by precipitation and temperature as well as flooding and drought. Lower water levels restrict some boat access to ports and shipping channels or require shipping companies to lighten loads in order to reduce draft. In recent years, water levels on the Great Lakes and the Seaway have been at historical lows. There is some evidence that climate change will drop the Great Lakes water levels (Angel and Kunkel 2010; Attavanich et al. 2013) and enhance seasonal variability (MacKay and Seglenieks 2013), but results vary among studies and climate models (see Figure G-26). Thus, the certainty about changing water levels is low (U.S. DOT 2014). Similar to water levels, siltation is already a significant chal- lenge in the Great Lakes waters with dredging needed to maintain channel depth. Increased intense storms could lead to more erosion in watersheds and siltation and temporary waterway closures that shift cargo to trucks. The Interstate Highway System and Disruption from Dust and Fire Dust storms and smoke from wildfires can lower visibility substantially. Limited visibility in transportation corridors disrupts operations and affects safety. In the Southwest, dust storms and wildfires have forced extended road closures and endangered drivers (Meyer et al. 2013). Projections of drier summer and fall seasons as well as extended heat waves and droughts in the central Contiguous United States could potentially increase blowing dust and wildfires and threaten portions of the Interstate Highway System (FHWA 2016; NRC 2008). Increased summer temperatures are projected

438 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM FIGURE G-26 Summary of projected lake level change from mean sea level (MSL) for Lakes Superior, Michigan, and Erie by month. NOTES: Gray lines are for the mid-21st century (2040–2059) and red lines are for late 21st century (2080–2099) as compared to the late 20th century (1948–2006) using dynamically downscaled model output from two regional climate models (a)–(c) RCM-MIROC5 and (d)–(f) RCM-CNRM. SOURCE: Adaptation of Figure 13 in Notaro et al. 2015.

APPENDIX G 439 to elevate the fire risk up to 30 percent by 2100 (IPCC 2007). In the Pacific Northwest, drier summer conditions, due to warmer temperatures combined with shorter winters, will increase wildfire risk (USGCRP 2009). Wildfires routinely close the Interstate System in Oregon during fires because of proximity of the fire or when the road is used as a staging area for firefight- ing crews (Hormann 2012). Regional assessments conducted through the FHWA Pilot Program generally found future dust and wildfire trends to be highly uncertain (FHWA 2016). Although these are not new events to many regions, changes in the frequency, duration, and intensity could affect the Interstate System performance (FHWA n.d.; Gopalakrishna et al. 2013). The Interstate Highway System and Arctic Sea Ice Opening Northern Transportation Routes Opening of the Arctic sea ice has recently generated significant consider- ation about the potential for Arctic shipping routes using the Northwest Passage (NWP) to ship goods between Asia and the eastern United States. Arctic shipping lanes offer vessels a shorter route and, for large vehicles that cannot use the Panama Canal, an alternative to shipping goods via rail or highways to the eastern United States (Niemeier et al. 2013). The use of the NWP could reduce cargo delivered to western U.S. ports and as- sociated trucking across the United States (Pharand 2007) and change the distribution of goods between the western and eastern U.S. ports (see Figure G-27). Increasingly, potential shipping lanes that are ice-free exist across the breadth of the Arctic during the summer. Canada’s International Policy Statement predicted in 2005 that the NWP would be sufficiently ice-free for regular use during summer as early as 2015 (Government of Canada 2005), but to date dramatic increases in commercial ship traffic remain unrealized. Under ideal conditions, the economics are somewhat balanced in favor of using the NWP to transport goods from northern Asia to eastern North America and Europe (Kiiski 2017). These conditions would at best result in tens of millions of tons of cargo, two orders of magnitude less than the Suez Canal. However, there are significant barriers to the NWP, including variable weather and ice conditions, the availability of ice breakers and ice- classed ships, and inadequate port infrastructure (Kiiski 2017; Lackenbauer and Lajeunesse 2014; Stephens 2016). Given this balance, Stephens (2016) indicates that the NWP appears unlikely to have a major impact in the near future (20 to 25 years or longer). The Interstate Highway System and Northward Changes in Agricultural Production Climate change will likely shift the location of production centers for agri- culture, forestry, and fisheries. For agriculture, climate change will increase

440 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM FIGURE G-27 Top 25 ports by tonnage, 2013 (top); top 25 water ports by con- tainerized cargo, 2014 (bottom). SOURCE: FHWA.

APPENDIX G 441 U.S. agricultural production overall (Niemeier et al. 2013). However, in- creases will not be uniform across the country because of the changes in temperature and/or precipitation, control-crop production location, and combination of crops that are planted. Climate changes from 1970 to 2010 explain up to 50 percent of the shift in crop production location for that pe- riod. For example, from 1990 to 2009, North Dakota wheat acres fell from 60 to 45 percent of cropland while corn acres doubled and soybean acres increased 10-fold. Under projected future temperature and precipitation values, almost all major crops in U.S. production regions will shift north and east (Cho and McCarl 2017). The production of corn and soybeans is expected to continue to increase in the northern regions and decrease in southern regions (Attavanich et al. 2013). As agriculture relocates or changes crops, transportation needs will be affected. For example, corn yields by volume and weight are much higher than wheat. Thus, increased grain transport is anticipated in the North. The transport mode will shift away from barges to trucks and rail, primarily due to increasing distance from the river system (see Figure G-28). Truck transport increases by up to 34 percent are predicted. Climate change is also expected to lengthen the navigation season and lower Great Lakes levels. These changes will likely shift the transport season to later in the year and add modest additional increases to truck transport (Attavanich et al. 2013). Regions identified for road expansion and improvement to accom- modate increased truck traffic include the Upper Mississippi River in Minnesota, the Ohio River, the Arkansas River, and the Lower Mississippi FIGURE G-28 Grain shipment modes of transportation due to climate-induced shifts in crop production patterns under baseline (2007–2008) conditions and future (2050) conditions using output from four different global climate models. Quanti- ties are in 1,000 metric tons. SOURCE: Figure S1 in Attavanich et al. 2013.

442 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM River in Kentucky. Truck routes in northern regions that connect highways with ports and rail terminals may also need to be improved. Target regions include roads in northern parts of Ohio leading toward ports on Lake Erie and roads in New York, Ohio, and Pennsylvania leading to Atlantic ports at Norfolk, Virginia (Attavanich et al. 2013). SYNTHESIS AND CONCLUDING REMARKS Climate change is happening; it is happening now, it is already affecting the Interstate Highway System, and it will do so increasingly in the future. We have high confidence that rising sea levels, increased storm surges, warming summers, and new temperature extremes will affect the Interstate System’s routine performance as well as the ability for the Interstate System to perform during extreme events. The Interstate Highway System is already highly vulnerable to extreme precipitation and inland flooding. In some regions, including the Northeast, Pacific Northwest, and Northern Great Plains, climate change–induced alterations to the magnitude, duration, and intensity of rainfall as well as changing antecedent conditions and precipita- tion transitions from snow to rain will likely challenge system performance. Beneficial aspects of climate change that result from moderating winter conditions include a longer construction season and reduced winter main- tenance and wear. Beyond direct impacts to the Interstate System, second- ary environmental impacts and impacts to other transportation modes and other sectors may further exacerbate challenges to the Interstate Highway System. Information regarding the Interstate Highway System and climate change impacts were largely drawn from several qualitative vulnerability studies conducted at a national level, numerous local and state vulner- ability studies that provide an uneven approach to assessing local assets and systems, and a relatively sparse academic literature. Despite the recent progress, there are significant gaps. Relatively little adaptation planning and implementation have occurred except where disasters provided the impetus for change. Nationally consistent, reliable indicators of vulnerability and societal impacts are needed to provide actionable, quantitative measures for Interstate System planning, design, and operations and maintenance activities (Savonis et al. 2014). REFERENCES Abbreviations AMAP Arctic Monitoring and Assessment Programme Caltrans California Department of Transportation EPA Environmental Protection Agency

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454 NATIONAL COMMITMENT TO THE INTERSTATE HIGHWAY SYSTEM Trenberth, K. E., J. T. Fusillo, and J. Kiehl. 2009. Earth’s Global Energy Budget. Bulletin of the American Meteorological Society, Vol. 90, pp. 311–323, doi: 10.1175/2008BAMS2634.1. Trenberth, K. E., J. T. Fasullo, and T. G. Shepherd. 2015. Attribution of Climate Extreme Events. Nature Climate Change, Vol. 5, pp. 725–730. http://dx.doi.org/10.1038/ nclimate2657. U.S. DOT. 2014. U.S. Department of Transportation Climate Adaptation Plan: Ensuring Transportation Infrastructure and System Resilience. Washington, D.C. https://www. transportation.gov/sites/dot.dev/files/docs/DOT%20Adaptation%20Plan.pdf. U.S. DOT. 2015. Beyond Traffic 2045: Final Report. Washington, D.C. https://www. transportation.gov/sites/dot.gov/files/docs/BeyondTraffic_tagged_508_final.pdf. USGCRP. 2009. Global Climate Change Impacts in the United States (T. R. Karl, J. M. Melillo, and T. C. Peterson, eds.). Cambridge University Press, New York. van Vuuren, D. P., S. Deetman, M. G. J. den Elzen, A. Hof, M. Isaac, K. Klein Goldewijk, T. Kram, A. Mendoza Beltran, E. Stehfest, and J. van Vliet, 2011. RCP 2.6: Exploring the Possibility to Keep Global Mean Temperature Increase Below 2°C. Climatic Change, Vol. 109, pp. 95–116. http://dx.doi.org/10.1007/s10584-011-0152-3. Vaughan, D. G., J. C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, T. Murray, F. Paul, J. Ren, E. Rignot, O. Solomina, K. Steffen, and T. Zhang. 2013. Observations: Cryosphere. In Climate Change 2013: The Physical Science Basis. Contribution of Work- ing Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, eds.). Cambridge University Press, Cambridge, UK, and New York, pp. 317–382. http://www.climatechange2013.org/report/full-report. Vavrus, S., M. Notaro, and A. Zarrin. 2013. The Role of Ice Cover in Heavy Lake-Effect Snowstorms Over the Great Lakes Basin as Simulated by RegCM4. Monthly Weather Review, Vol. 141, pp. 148–165. http://dx.doi.org/10.1175/mwr-d-12-00107.1. Vose, R. S., D. Arndt, V. F. Banzon, D. R. Easterling, B. Gleason, B. Huang, E. Kearns, J. H. Lawrimore, M. J. Menne, T. C. Peterson, R. W. Reynolds, T. M. Smith, C. N. Williams, and D. L. Wuertz. 2012. NOAA’s Merged Land-Ocean Surface Temperature Analysis. Bulletin of the American Meteorological Society, Vol. 93, pp. 1677–1685. http://dx.doi. org/10.1175/BAMS-D-11-00241.1. Vose, R. S., S. Applequist, M. Squires, I. Durre, M. J. Menne, C. N. Williams, Jr., C. Fenimore, K. Gleason, and D. Arndt. 2014a. Improved Historical Temperature and Precipitation Time Series for U.S. Climate Divisions. Journal of Applied Meteorology and Climatology, Vol. 53, pp. 1232–1251. http://dx.doi.org/10.1175/JAMC-D-13-0248.1. Vose, R. S., S. Applequist, M. A. Bourassa, S. C. Pryor, R. J. Barthelmie, B. Blanton, P. D. Bromirski, H. E. Brooks, A. T. DeGaetano, R. M. Dole, D. R. Easterling, R. E. Jensen, T. R. Karl, R. W. Katz, K. Klink, M. C. Kruk, K. E. Kunkel, M. C. MacCracken, T. C. Peterson, K. Shein, B. R. Thomas, J. E. Walsh, X. L. Wang, M. F. Wehner, D. J. Wuebbles, and R. S. Young. 2014b. Monitoring and Understanding Changes in Extremes: Extra- tropical Storms, Winds, and Waves. Bulletin of the American Meteorological Society, Vol. 95, No. 3, pp. 377–386. doi: 10.1175/BAMS-D-12-00162.1.d. Vose, R. S., S. Applequist, M. Squires, I. Durre, M. J. Menne, C. N. Williams, C. Fenimore, K. Gleason, and D. Arndt. 2017. Improved Historical Temperature and Precipitation Time Series for Alaska Climate Divisions. Journal of Service Climatology, Vol. 53, pp. 1232–1251, https://doi.org/10.1175/JAMC-D-13-0248.1. Wada, Y., M.-H. Lo, P. J. F. Yeh, J. T. Reager, J. S. Famiglietti, R.-J. Wu, and Y.-H. Tseng. 2016. Fate of Water Pumped from Underground and Contributions to Sea-Level Rise. Nature Climate Change, Vol. 6, pp. 777–780. http://dx.doi.org/10.1038/nclimate3001. Wada, Y., J. T. Reager, B. F. Chao, J. Wang, M.-H. Lo, C. Song, Y. Li, and A. S. Gardner. 2017. Recent Changes in Land Water Storage and Its Contribution to Sea Level Variations. Sur- veys in Geophysics, Vol. 38, pp. 131–152. http://dx.doi.org/10.1007/s10712-016-9399-6.

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TRB Special Report 329: Renewing the National Commitment to the Interstate Highway System: A Foundation for the Future explores pending and future federal investment and policy decisions concerning the federal Interstate Highway System. Congress asked the committee to make recommendations on the “features, standards, capacity needs, application of technologies, and intergovernmental roles to upgrade the Interstate System” and to advise on any changes in law and resources required to further the recommended actions. The report of the study committee suggests a path forward to meet the growing and shifting demands of the 21st century.

The prospect of an aging and worn Interstate System that operates unreliably is concerning in the face of a vehicle fleet that continues to transform as the 21st century progresses and the vulnerabilities due to climate change place new demands on the country’s transportation infrastructure. Recent combined state and federal capital spending on the Interstates has been about $20–$25 billion per year. The estimates in this study suggest this level of spending is too low and that $45–$70 billion annually over the next 20 years will be needed to undertake the long-deferred rebuilding of pavements and bridges and to accommodate and manage growing user demand. This estimated investment is incomplete because it omits the spending that will be required to meet other challenges such as boosting the system’s resilience and expanding its geographic coverage.

The committee recommends that Congress legislate an Interstate Highway System Renewal and Modernization Program (RAMP). This program should focus on reconstructing deteriorated pavements, including their foundations, and bridge infrastructure; adding physical capacity and operations and demand management capabilities where needed; and increasing the system’s resilience. The report explores ways to pay for this program, including lifting the ban on tolling of existing general-purpose Interstate highways and increasing the federal fuel tax to a level commensurate with the federal share of the required RAMP investment.

View the videos, recorded webcast, graphics, summary booklet, press release, and highlights page at interstate.trb.org.

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