Conclusion

A Synthesis of Potential Climate Change Impacts on the U.S.

In spite of the uncertainties about climate change, we can, based on the Pew Center on Global Climate Change report series and other literature, draw some conclusions about the relative vulnerability of sectors and regions. As noted above, we are unable to predict the exact effects of climate change, but we are improving our understanding of the sensitivity of various sectors to climate change. Thus, these conclusions should be treated as preliminary.

1) Natural ecosystems appear to be quite vulnerable to climate change. Many natural resources are currently under stress, and climate change could impose additional stress. Climate change threatens to result in the loss of many coral reefs, coastal wetlands, endangered species (particularly those with limited range and mobility), cool- and cold-water fish, boreal species, and alpine species. This threat to natural ecosystems is distinctly more severe because development has reduced species populations, fragmented ecosystems and placed them under stress from pollution, and introduced barriers to migration, such as communities, farms, roads, and dams.

2) A number of sectors in the United States have a high sensitivity to climate change. Climate change could inundate many low-lying coastal areas, put urban areas at risk from increased storms and hurricanes, substantially change runoff in many basins, significantly change crop yields, and result in large geographic shifts and changes in the productivity of terrestrial and aquatic species.

3) The capacity of the U.S. economy as a whole to adapt to a limited amount of climate change, with generally small impacts, appears to be quite high. The country’s high per capita income, relatively low population density, research base, institutions, and health care system give the United States a strong capacity to adapt to climate change. There will be costs for adaptation, but relative to the U.S. economy, these costs appear to be small and can most likely be absorbed. Finally, the country’s large size and the population’s mobility give it advantages in adapting to climate change.

4) Although the nation as a whole has a high capacity to adapt, sectors differ in their vulnerability. Sectors that can change the fastest, such as agriculture, are likely to be best able to adapt to climate change. Sectors with long-lived infrastructure and investments, such as water resources and coastal resources, may have more difficulty adapting and could experience some adverse impacts. However, their ability to adapt to climate change in the long run appears to be high. In contrast, natural ecosystems have a much more limited capacity to adapt to climate change compared to societal sectors.

5) The southern United States is, on the whole, more vulnerable to climate change than the northern United States. Regions such as the Southeast and Southern Great Plains appear to be more vulnerable to climate change than the nation as a whole. In some regions, specific sectors, such as water resources in the Southwest, are at particular risk from climate change.

6) Even within regions that may have net economic benefits, individual communities and people could be adversely affected. Those with limited financial resources and mobility may be at greatest risk to climate change. The urban poor appear to have the highest risk from increased heat stress. Poor farmers may be most vulnerable to changes in agricultural conditions. Poor and isolated populations, such as Native Americans, may be at risk should climate change substantially affect the natural resources on which they depend.

7) Studies of the economic impacts of climate change indicate that impacts for a few degrees of warming will be less than ±1 percent of gross domestic product (GDP). These studies tend to incorporate both market and nonmarket (e.g., biodiversity and quality of life) impacts and, as noted above, assume a gradual change in climate and no change in variability. Economic studies based on impact assessments conducted during the late 1980s and early 1990s tend to show damages of about 1 percent of GDP. More recent studies that consider new findings on the biophysical impacts of climate change and fully account for the potential for adaptation yield different results. These economic studies suggest that for up to 2-4°C (4-7°F) of warming, there could be net economic benefits of less than 1 percent of GDP. It is possible that because of factors not considered, economic impacts could be more negative than these studies estimate.

8) Economic impacts studies indicate that while there could be benefits from climate change, which peak at a few degrees of warming, there would be damages at higher levels of warming. Economic studies indicate that even in those sectors, such as agriculture, estimated to benefit from a small magnitude of warming, these benefits peak and subsequently decline. This is because beyond certain increases in temperatures, crop yields decline or the “carbon fertilization” effect, which enables plants to grow more and use less water, saturates at higher CO2 concentrations. In addition, other transient benefits such as reduced energy demand eventually become reversed as costs for cooling rise and savings from less heating are reduced. This is even true for regions such as the northern United States, which may experience economic benefits from a warming of less than several degrees, but losses beyond that. Economic studies suggest that benefits peak at approximately a 1-2°C (2-4°F) increase in mean temperature. Beyond this, benefits decline until net economic damages occur at a warming of approximately 2-4°C (4-7°F) and become progressively worse with further increases in temperature. Significant uncertainty exists about the level of increased temperature that leads to damages and the magnitude of damages beyond that point.

9) The rate and path of climate change matter. A gradual and monotonic change in climate (e.g., steady increases or decreases in precipitation) will be much easier to adapt to than rapid changes in climate or increased interannual or interdecadal climate variability. In a slowly and steadily changing climate, such adaptations as replacing infrastructure and introducing new technologies can be made gradually. A more rapid change in climate may necessitate more rapid than normal investments in infrastructure, technology, and other adaptations. These investments could be costly.

10) Increased warming heightens the risk of triggering large-scale changes to the climate system. Substantial increases in global mean temperature could set off large-scale changes to the earth’s system such as shutdown of the thermohaline circulation (i.e., the Gulf Stream) or melting of the West Antarctic ice sheet. The thresholds are uncertain (and for some of these events may be quite high), the time frames of the consequences of such events may take centuries to be fully realized, and the consequences are not currently well understood. However, it is possible that warming in the 21st century could trigger such events. Once started, they may be extremely difficult, if not impossible, to reverse.