zones already vulnerable to storm surges and other causes of temporary coastal flooding, along with sea-level rise.¹⁰⁰ In other areas, prolonged dry periods and droughts are increasing the risk of destructive wildfires and water shortages.

If greenhouse gas emissions continue to rise in the 21st century, Earth is expected to warm by an additional 4.7°F to 8.6°F (2.6°C to 4.8°C) by 2100 (relative to 1986-2005).¹⁰¹ The greater the warming, the greater the impacts will be. In the United States, each degree of warming (Celsius) is projected to result in a 3 to 10 percent increase in the amount of rainfall during the heaviest rain events, a 5 to 15 percent reduction in the yields of crops as currently grown, and a 200 to 400 percent increase in the area burned by wildfire in western states.¹⁰² Similar types of changes are expected in many other parts of the world, which could be most devastating to low-income countries that do not have the resources to respond or adapt.¹⁰³

Warming of about 5.4°F (3°C) or more could push Earth past several “tipping points.” For example, this amount of warming could melt the Greenland ice sheet, which would raise global average sea level an additional 20 feet (6 meters).¹⁰⁴ It could also accelerate the thawing of permafrost, which would accelerate the release of CO₂ and methane stored in frozen soil, exacerbating warming.¹⁰⁵ While projections such as these are useful in planning for the changes ahead, it is also important to recognize that a great deal remains unknown, particularly when it comes to the complex feedbacks among human activities, ecosystems, and the atmosphere.

For decades, scientists have led the efforts to understand and predict climate change effects, but engineers are now recognizing that their efforts are needed to help develop and implement solutions. Conceptually, climate solutions are divided into two areas of focus: mitigation and adaptation. Mitigation refers to efforts to reduce the magnitude or rate of climate change by reducing emissions of carbon dioxide and other greenhouse gases or removing them from the atmosphere. Adaptation refers to solutions that avoid or lessen the impacts of climate change on people, ecosystems, resources, and

infrastructure. Environmental engineers have an opportunity to be leaders in developing technologies and systems that provide solutions on both of these fronts. Given that future climate changes likely hold surprises, it will be important to remain nimble, incorporate new knowledge, and work to address uncertainty as environmental engineers develop, test, and implement solutions.

Reducing the Rate and Magnitude of Climate Change

A sharp reduction in emissions of greenhouse gases to the atmosphere is needed to slow climate change and prevent some of the most severe impacts. For the past few decades, international climate talks have focused on establishing goals to minimize the planet’s warming, with the most recent goals set at limiting future warming to 3.6°F (2°C) above preindustrial levels. The 2016 Paris Agreement set an aspirational target of limiting warming to 2.7°F (1.5°C). Since the planet already has warmed about 1.8°F (1°C), scientists have calculated that, in order to stay within the 3.6°F (2°C) limit, atmospheric CO₂ concentrations must not rise beyond 450 ppm, which in turn requires 40 to 70 percent reductions in global anthropogenic greenhouse gas emission by 2050 compared to 2010, and emissions levels near zero or below in 2100.¹⁰⁶

A special report issued in 2018 by the Intergovernmental Panel on Climate Change urged world leaders to work toward limiting warming to 2.7°F (1.5°C) to avoid the severe impacts on weather extremes, ecosystems, human health, and infrastructure that are expected to occur at 3.6°F (2°C) warming.¹⁰⁷ Meeting that tougher goal will require global emissions to be reduced by about 45 percent from 2010 levels by 2030, reaching net zero emissions by 2050. Meeting those emissions targets will require dramatic reductions in global CO₂ emissions combined with the active removal of CO₂.¹⁰⁸

Greenhouse gas emissions are driven by the use of energy for electricity generation, transportation, industry uses, commercial and residential needs, and agriculture. Figure 2-2 shows the U.S. breakdown for greenhouse gas emissions sources. Emissions can be reduced by using energy more efficiently, switching to fuels that produce less (or no) greenhouse gases, and capturing the emissions before they enter the atmosphere.

In general, reducing emissions will require that existing and planned transportation, building, and industrial infrastructure be converted to electricity that is generated with substantially lower carbon intensity. Doing so will have the added co-benefit of reducing the environmental and human health impacts associated with coal, oil, and natural gas extraction and fossil-fuel-generated electricity (see Challenge 3).¹⁰⁹

Using Energy More Efficiently

High-income countries have already substantially reduced their energy use per capita and per unit of economic output. These improvements have resulted from significant technological changes, such as the advent of LED lighting, energy-efficient appliances, and other efficiency intelligence in buildings; industrial restructuring to enhance productivity; and investment in fuel-efficient transportation technologies. Lower- and middle-income countries are beginning to make similar gains.

Efficiency gains made to date, however, will not be sufficient to avoid a 3.6°F (2°C) average rise in global temperatures. More than 80 percent of vehicle miles traveled in 2050 need to be powered by something other than an internal combustion engine.¹¹⁰ Substantial efficiencies also are needed in industry and in the heating and cooling of buildings. In Germany, for example, a high-level commission calculated that German buildings would need to achieve a 54 percent improvement in efficiency by 2030 to meet stated emission reduction goals.¹¹¹ Effectively deploying new and emerging technologies can help advance these goals. It has been estimated that energy-efficient technologies for residential and commercial buildings, transportation, and industry that exist today or are expected to be developed soon could reduce U.S. energy use by 30 percent, slashing greenhouse gas emissions along with other air pollutants, while also saving money.¹¹²

Switching to Fuels That Produce Less (or No) CO₂

As discussed in Challenge 1, there are many sources of energy that produce little or no CO₂ emissions, including solar, wind, geothermal, and hydropower. Although low-emissions energy sources exist, there is still a long way to go toward their widespread adoption. As of 2017, U.S. electricity generation was composed of about 63 percent fossil fuels, 20 percent nuclear, and 17 percent hydropower and other renewables.¹¹³ A study by the Department of Energy’s National Renewable Energy Laboratory shows that it is feasible for the United States to generate most of its electricity from renewable energy by 2050, but a number of challenges remain.¹¹⁴ Cost has been a significant barrier, although costs are dropping for both solar and wind power technologies.¹¹⁵

Reducing U.S. emissions enough to stay within the 2.7°F (1.5°C) limit would require the current balance of energy production to shift substantially, such that 70-85 percent of electricity is generated from noncarbon-emitting sources.¹¹⁶ In China, maturation and economic restructuring of the industrial sector has already substantially reduced coal consumption per unit of output, a trend that is projected to continue and be further enhanced by their recently introduced carbon cap and trade system.¹¹⁷ In addition, China is leading the charge in developing renewable energy, for example, building 45 percent of the world’s solar installations in 2016.¹¹⁸

Advances are needed to improve the efficiency and reduce the costs of such energy sources to make them competitive with traditional fossil fuel–based sources. In addition, since many renewables produce energy intermittently, there is a need for energy storage systems with increased capacity, scalability, reliability, and affordability, as discussed in Challenge 1.

Nuclear power is one low-emission energy source that already comprises one-fifth of U.S. electricity generation. Increasing the use of nuclear power could help reduce carbon-emitting energy generation, but there are significant barriers, including cost, public concerns related to safety and waste disposal, the high business and regulatory risks involved in designing and building nuclear power plants, and the lack of progress in developing long-term waste repositories. Retiring existing nuclear plants will exacerbate the challenge of reducing CO₂ emissions from the power system, because large increases in renewable and other zero-emitting energy sources will be needed simply to replace zero-emitting nuclear energy. To support continued nuclear capacity, working in combination with renewables, research is needed on advanced nuclear technologies for next generation reactors designed to significantly improve performance and safety.¹¹⁹

Moving to electrically powered transportation with increased renewable energy generation would substantially reduce fossil fuel use, because more than 90 percent

of the transportation fuels are petroleum based.¹²⁰ Electric vehicle technology has advanced substantially in the past 5 years, with roughly 2 million all-electric and plug-in hybrid vehicles on the road worldwide today,¹²¹ and automobile companies are increasing investments in electric vehicle production. For example, Volvo announced a plan to transition all of the company’s car models to electric or hybrids by 2030, Ford has announced an $11 billion investment in electric vehicles, and GM plans to release 20 new models of electric vehicles by 2023.¹²² Several countries, including Britain, France, and Norway, cities such as Beijing, and several U.S. states have proposed banning gasoline- and diesel-powered cars as early as 2030.¹²³ Achieving the transition to electric-based transportation systems raises many engineering challenges beyond the need for low-carbon energy sources, including the need for charging infrastructure, better battery performance, and faster recharge times.

Making progress toward reducing emissions will depend in large part on private-sector investments and on the behavioral and consumer choices of individual households, which are explored in more detail in Challenge 5. Governments at federal, state, and local levels can influence those choices through policies and incentives. Such policies can include setting a price on emissions, such as a carbon tax or cap-and-trade system; providing information and education on voluntary emission reductions; and mandates or regulations designed to control emissions, for example, the Clean Air Act, automobile fuel economy standards, appliance efficiency standards, building codes, and requirements for renewable or low-carbon energy sources in electricity generation.

Advancing Climate Intervention Strategies

Even if human-caused carbon dioxide emissions were to cease today, it would take millennia for natural processes to return Earth’s atmosphere to preindustrial carbon dioxide concentrations.¹²⁴ To avoid the worst impacts of warming, it is no longer enough to reduce emissions. Deploying negative-emission technologies that remove carbon dioxide from the atmosphere and reliably sequester it will also be needed.¹²⁵

Some carbon dioxide removal strategies focus on accelerating natural processes that take up carbon dioxide. Changes in agricultural practices can enhance soil carbon storage, for example, by planting fields year-round in crops or other cover crops.¹²⁶ Land use and management practices can be employed that increase the amount of carbon stored in terrestrial environments, such as forests and grasslands and in near-shore ecosystems, such as mangroves, tidal marshes, and seagrass beds.¹²⁷ One recent study estimates that nature-based approaches can deliver more than one-third of the carbon reductions needed by 2030 to stay within the 3.6°F

(2°C) limit at competitive costs,¹²⁸ but there are many unknowns. Further research is needed to determine what conditions and practices can maximize carbon uptake in plants over the long term. There can also be unintended effects. For example, planting more trees in northern boreal forests can contribute to warming, because in winter months the trees can obscure snow that reflects sunlight.

Other technologies being explored seek to actively remove CO₂ from the atmosphere and from point sources and sequester it. One technology involves growing plants such as switchgrass to be converted to fuel, coupled with capturing and storing any CO₂ emissions from biofuel burning (called bioenergy with carbon capture and sequestration, or BECCS). Another approach proposes using chemical processes to capture CO₂ directly from the air and concentrate it for storage (called direct air capture and sequestration, or DACS). These technologies will be needed around the world because many countries will still be using significant amounts of fossil-fuel-generated electricity by 2050. They will also be needed to mitigate emissions where electrification is not possible and for industrial processes that produce carbon dioxide.

Engineering challenges in carbon removal strategies include the need to reduce costs, increase the scale of the technologies, and store or reuse the carbon in ways that keep it from being released back into the atmosphere. Available land is a key limiting factor for the potential of removing CO₂ through reforestation or growing fuel crops; removing 10 gigatons CO₂ per year (about one quarter of global yearly emissions) by 2050 would require the use of hundreds of millions of hectares of arable land.¹²⁹ Land use at that scale could threaten food security, given that food demands are expected to increase by 25 to 70 percent over the same time period.¹³⁰ Breakthroughs in agriculture discussed in Challenge 1, including advances in crop productivity, alternative methods of growing food, food waste reduction, and changes in diet, will be needed.

A different set of climate intervention strategies seeks to reduce warming by reflecting sunlight off of specially treated clouds and aerosols. In general, such technologies are not as developed as carbon dioxide removal strategies and carry greater risks of unintended consequences that are not well understood.¹³¹

Reducing Other Greenhouse Gases

Methane, nitrous oxide, and some industrial gases (e.g., hydrofluorocarbons) comprise about 18 percent of U.S. greenhouse gas emissions in terms of CO₂ equivalents.¹³² Molecule for molecule, those gases are much stronger climate warming agents than CO₂, although they are less abundant, and some do not last as long in the atmosphere. Methane, for example, is about 28 times more potent as a greenhouse gas compared to CO₂, making it particularly important to prevent or capture methane leaks from oil and gas systems, coal mines, shale gas extraction, and landfills.¹³³ To that end, there is a need for better systems and methodologies to measure and track methane leakage throughout those systems.¹³⁴

Agriculture is one of the largest sources of non-CO₂ greenhouse gases. Methane is produced when livestock digest their food and also is emitted in large quantities

from rice paddies. Nitrous oxide arises from the use of nitrogen fertilizers. Precision agriculture techniques can help farmers minimize fertilizer use and reduce nitrous oxide emissions (see also Challenge 1). Feeding livestock easier-to-digest foods and strategically managing livestock waste—through proper storage, reuse as fertilizer, and recovery of methane—also can help reduce emissions.¹³⁵ Efforts to curb agricultural methane emissions can benefit from new insights and biotechnology tools that offer new ways to study the complex microbial ecosystems involved in soils, manure management, and livestock digestion.

Some short-lived pollutants that are not greenhouse gases also contribute to warming. One example is black carbon, commonly called soot, which absorbs sunlight and traps heat in the atmosphere. Black carbon is produced by incomplete fuel combustion and burning of biomass (e.g., the dung used in cookstoves). Black carbon also can amplify regional warming by leaving a heat-absorbing black coating on otherwise reflective surfaces, such as snow in mountainous regions. Although North America and western Europe were the major sources of soot emissions until about the 1950s, low-income nations in the tropics and East Asia are the major source regions today. Identifying and targeting the largest sources of black carbon could be crucial to curbing warming in the short term.

What Can Environmental Engineers Do to Curb Climate Change?

Environmental engineers have an opportunity to be leaders in developing technologies that will help slow warming through alternative energy development, green infrastructure, carbon capture and sequestration, and monitoring and measurement, as summarized in Box 2-1. Although the challenge to curb climate change will stretch environmental engineering beyond its traditional boundaries, many of the skills typical of environmental engineers can be applicable for advancing these goals. For example, the design of technologies to capture and store carbon underground, in soils, and in coastal ecosystems can take advantage of environmental engineers’ expertise in water chemistry, environmental

microbiology, groundwater and surface water hydrology, and atmospheric chemistry. Environmental engineers can also bring large-scale perspectives to illuminate how proposed technologies will interact with multiple systems. Specific applications of those skills might include

• Using the tools of geochemistry to engineer accelerated mineralization processes that would transform carbon into a stable carbonate, while avoiding water quality impacts.
• Using the emerging tools of synthetic biology and microbial ecology to abate greenhouse gas emissions and generate chemicals, materials, and fuels.
• Using the tools of life-cycle assessment to explore efficiencies for producing low-carbon liquid fuels from biomass feedstocks without increasing overall water use.
• Using the tools of life-cycle assessment to assess and optimize the energy return on investment (the ratio of the amount of usable energy delivered from a particular energy resource to the amount of energy used to obtain that energy resource).

Adapting to Climate Change Impacts

Many of the things people use or do every day—from roads to farms, buildings to subways, jobs to recreational activities—were optimized for the climate of the 19th and 20th centuries. They were built with the assumption of certain temperature ranges, precipitation patterns, frequency of extreme events, and other manifestations of climate, which are now shifting. Even if humankind were to succeed in limiting global climate change in accordance with current goals, adaptation will be needed to protect people, ecosystems, infrastructure, and cultural resources from the impacts of climate change, many of which are already evident.

Sea level is one area in which those impacts are already being felt. Since 1900, global mean sea level has risen about 8 inches, driven by expansion of the warming ocean, melting of mountain glaciers, and losses from the Greenland and Antarctic ice sheets.¹³⁶ This rise has caused coastal cities to see an uptick in flooding, both during storms and as “sunny-day” flooding from tides alone. These flooding events disrupt economies, make it difficult to deliver emergency services, and disproportionately affect older, infirm, and low socioeconomic status populations.

Global sea level is expected to rise by an additional 0.5 to 1.2 feet by 2050 and 1 to 4.3 feet by 2100, which will increase the frequency and severity of flooding (see Figure 2-3). Even at the low end of that estimate, up to 200 million people could be affected worldwide and 4 million people could be permanently displaced as frequent or permanent flooding makes low-lying developed areas uninhabitable.¹³⁷ Some communities already are being forced to relocate as a result of sea-level rise, including Native American communities in Alaska, communities south of New Orleans in the Louisiana Delta and island communities in the Pacific and Indian oceans. In addition to flooding, sea-level rise causes erosion and saltwater encroachment, which kills forests near the coasts, reshapes marshes and wetlands, and renders aquifers along the coast unusable for human consumption without desalination technology.

Climate change is also expected to intensify regional contrasts in precipitation that already exist: Dry areas are expected to get drier and wet areas to become even wetter. Changes in precipitation patterns have resulted in heavier rainfalls, reduced snow cover, and glacial extent, and doubled the amount of land area classified as “very dry.”¹³⁸ Warmer temperatures tend to increase evaporation from oceans, lakes, plants, and soil, exacerbating the impacts in areas of reduced precipitation.

Extreme precipitation events are becoming more frequent, leading to increased flooding as well as spikes in the release of some pollutants during heavy storms.¹³⁹ In August 2016, for example, more than 2 feet of rain fell in central Louisiana over 10 days, an event the National Weather Service called a “one in a thousand year” event. Scientists predict that climate change will cause an increase in the number of the most severe hurricanes, leading to stronger storm surges and more intense rainfall events.¹⁴⁰ In 2017, Hurricane Harvey dumped a staggering 50 inches of rain on Houston, which is as much rain as typically falls there over an entire year. Work is ongoing to assess the future probability of similar rainfall events.

As Earth’s climate warms, changing temperatures are expected to reduce agricultural productivity for some major crops and may exacerbate the impacts of agricultural pests and pathogens.¹⁴¹ Extreme heat waves will become more frequent, causing additional wildfires and further degrading air quality. Urban residents, especially those without access to air conditioning, are vulnerable to heat waves, as heat island effects make building and pavement surfaces 7°F to 22°F (4°C to 12°C) warmer than the surrounding natural environment.¹⁴²

Adapting Agricultural Practices

Technological advances during the 20th century’s green revolution dramatically improved agricultural yields, economic stability, and food security in many parts of the world.¹⁴⁸ However, climate change threatens to undercut some of these advances. Agricultural adaptations such as adjusting planting dates, seed or crop selection (for example, to develop more flood- and drought-tolerant crops), or altering irrigation practices have the potential to buffer the impacts of climate change.¹⁴⁹ In the long run, it may be necessary to shift the location of agricultural operations or even to shift human diets (see Challenge 1). Additional economic and institutional strategies will be necessary to maintain food security amid increased weather variability and climate extremes.¹⁵⁰

Adapting Infrastructure for Sea-Level Rise

Widespread adaptations in infrastructure are needed to adjust to climate change. Adaptation strategies include ensuring that critical infrastructure and systems such as water supply, wastewater, and solid waste management systems, electricity-generating facilities, hospitals, and transportation systems are resilient to expected heat, storm, and flooding stressors. With projections of 1 to 4 feet of sea-level rise by the end of the century,¹⁵¹ engineers are developing ways to hold back the sea where possible or to buy time until more transformative adaptation strategies, including managed retreat, are developed.

In the near term, the city of Miami, Florida, is spending $400 million to raise streets, build sea walls, and construct pumps to reduce frequent flooding.¹⁵² Natural areas, such as coastal wetlands and mangroves, are being protected or restored to maintain natural buffers against storm surge (see Box 2-2). In the Netherlands, engineers have designed long-term strategies to protect heavily developed areas and accommodate increased flooding in less-developed regions. Innovations include smart dikes with embedded sensors that relay real-time status reports to decision makers and ecologically enhanced dikes to provide habitat for marine organisms.¹⁵³

To inform decision making, cities need comprehensive analyses to understand the adaptation options, their potential impacts, and the benefits and costs of local, regional, national, and private-sector infrastructure investments to manage future risks. It will be particularly important to develop economic and institutional strategies to support low-income and vulnerable communities as these adaptation measures are implemented.

Anticipating and Responding to Health Threats

Climate change has a broad range of implications for human health.¹⁵⁵ Changes in temperature are expected to increase heat-related illnesses and deaths (see Figure 2-4), while increases in ozone and wildfires are expected to worsen air pollution, with major effects on human health. Temperature changes may directly affect the transmission of vectorborne and zoonotic diseases carried by rodents and insects, such as ticks and mosquitoes, by increasing the frequency and shifting the geographic areas at risk. Changes in temperature and precipitation patterns may also affect the prevalence or distribution of foodborne, waterborne, and water-related diseases.¹⁵⁶ Temperature changes can also affect wildlife migration patterns, potentially leading to more human-wildlife contact and increasing the risk of infectious diseases that originate in animal populations and spread to humans.

The risk of infectious disease outbreaks also can rise in mass displacement events, such as natural disasters. In the aftermath of Hurricane Maria in 2017, Puerto Rico grappled with many health issues including an outbreak of leptospirosis a bacterial disease.¹⁵⁷ Outbreaks in such settings pose enormous challenges for policy makers and medical, public health, and environmental health personnel, and such events can also contribute to food and water insecurity and malnutrition and cause stress to those who are displaced from their homes.