America's Climate Choices (2011) / Chapter Skim
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2 Causes and Consequences of Climate Change
2 Causes and Consequences of Climate Change
Pages 15-28
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From page 15... ...
4 Additional, indirect indications of warming include widespread reductions in glaciers and Arctic sea ice, 5 rising sea levels, 6 and changes in plant and animal species.7 The preponderance of the scientific evidence points to human activities -- especially the release of CO2 and other heat-trapping greenhouse gases (GHGs) into the atmosphere -- as the most likely cause for most of the global warming that has occurred over the last 50 years or so.8 This finding is supported by numerous lines of evidence, including: 15
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From page 16... ...
Consequences also vary in terms of perception and significance to those who face the risks -- for example, in the degree to which the consequence is understood or evokes dread (unknown risks can sometimes concern people more than other risks) , and even in the trust in the organizations that manage such risks (mistrust in the managing organizations tends to increase perceived risk)
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From page 17... ...
. Global warming has been accompanied by a number of other global and regional environmental changes, which are broadly consistent with the changes expected in a warming world.
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From page 18... ...
global warming and these other changes is difficult: often the regional changes remain within the range of past observed variability, the data are not extensive enough, or the models not sufficiently developed to clearly identify an anthropogenic signal. As a result, only a few changes have been directly linked to human activities using formal scientific attribution methods.13 Among the ongoing changes in the physical climate system14 that can be linked, at least in part, to increasing temperatures at the Earth's surface are widespread melting of glaciers and ice sheets,15 rising global average sea levels,16 and decreases in Northern Hemisphere snow cover17 and Arctic sea ice.18 These changes have, in turn, been linked to a number of impacts on other physical and biological systems over the past several decades.19 For example, permafrost (permanently frozen ground)
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From page 19... ...
coast, and sea level rise is al ready eroding shorelines, drowning wetlands, and threatening the built environment;25 • Permafrost temperatures have increased throughout Alaska since the late 1970s, damaging roads, runways, water and sewer systems, and other infrastructure;26 • There have been widespread temperature-related reductions in snowpack in the northeastern and western United States over the last 50 years, leading to changes in the seasonal timing of river runoff;27 • Precipitation patterns have changed: heavy downpours have become more frequent and more intense;28 the frequency of drought has increased over the past 50 years in the southeastern and western United States, while the Mid west and Great Plains have seen a reduction in drought frequency;29 and • The frequency of large wildfires and the length of the fire season have in creased substantially in both the western United States and Alaska.30 FUTURE CLIMATE CHANGE Projections of future climate change impacts are developed in three steps: (i) Emission Scenarios: Scientists first develop different scenarios of how GHG emissions and other human drivers of climate change (such as land use change)
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From page 20... ...
The second source of uncertainty is the response of the climate system to the increased concentration of GHGs, or "climate sensitivity." Even if future emissions were known exactly -- that is, if a given emission scenario held true exactly -- the magnitude of future climate change and the severity of its impacts cannot be predicted with absolute certainty, due to incomplete knowledge of how the climate system will respond. What is known with a high degree of certainty however, is the direction of the climate system's response to changes in GHG emissions: that is, reducing GHG emissions will lead to less warming and less severe impacts than if emissions are not reduced.
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From page 21... ...
for four illustrative scenarios of future emissions. SOURCE: USGCRP, Global Climate Change Impacts in the United States, eds.
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From page 22... ...
. The shading around each curve indicates the range of central values produced by 15 different models using the same emission scenario (exact models used are listed in footnote #93 in USGCRP, Global Climate Change Impacts in the United States, eds.
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From page 23... ...
may be less vulnerable to the impacts of sea level rise and storm surges. The physical and social impacts of climate change are expected to have substantial economic implications throughout the United States, but these effects will be unevenly distributed across regions, populations, and sectors.50 Quantitatively estimating economic impacts is controversial, due to the uncertainties in climate change impact projections themselves, and to the lack of sound methodologies for assigning economic value to many key impacts, especially nonmarket costs such as loss of ecosystem services and spillover costs occurring as a result of climate change impacts elsewhere in the world.
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From page 24... ...
A rough estimate can be obtained using the near-linear relationship that exists between the cumulative carbon emissions from human activities since the Industrial Revolution and the long-term rise in the Earth's average surface temperature.b Based on this relationship, it is estimated that keeping global temperature rise within 2°C requires limiting cumula tive emissions to approximately 4000 billion tons of CO2. There is considerable uncertainty in this result however, with the cumulative emissions likely ranging from about 2900 to 5800 billion tons.
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From page 25... ...
One study estimates that if the world were to build no further energy-using stock, while letting every existing fossil fuel-using device reach the end of its useful life without modification, somewhere in the range of 280-700 billion tons of CO2 would be emitted.c How much is emitted above that amount depends critically upon the types of energy infrastructure the world invests in during the coming decades. If much of the new capital stock is powered by fossil fuels, then the emissions headroom will be rapidly depleted.
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From page 26... ...
. SOURCE: USGCRP Global Climate Change Impacts, p.
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From page 27... ...
Causes and Consequences of Climate Change Thus in the judgment of the committee, the environmental, economic, and humanitarian risks of climate change indicate a pressing need for substantial actions to limit the magnitude of climate change and to prepare for adapting to its impacts. Undertaking such actions will require making choices in the face of incomplete and imperfect information about the future.
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