while others, such as forests of the Southeast, are likely to experience major species shifts or break up. The goods and services lost through the disappearance or fragmentation of certain ecosystems are likely to be costly or impossible to replace.

4. Widespread water concerns. Water is an issue in every region, but the nature of the vulnerabilities varies, with different nuances in each. Drought is an important concern in every region. Floods and water quality are concerns in many regions. Snowpack changes are especially important in the West, Pacific Northwest, and Alaska.

5. Secure food supply. At the national level, the agriculture sector is likely to be able to adapt to climate change. Overall, U.S. crop productivity is very likely to increase over the next few decades, but the gains will not be uniform across the nation. Falling prices and competitive pressures are very likely to stress some farmers.

6. Near-term increase in forest growth. Forest productivity is likely to increase over the next several decades in some areas as trees respond to higher carbon dioxide levels. Over the longer term, changes in larger-scale processes such as fire, insects, droughts, and disease will possibly decrease forest productivity. In addition, climate change will cause long-term shifts in forest species, such as sugar maples moving north out of the U.S.

7. Increased damage in coastal and permafrost areas. Climate change and the resulting rise in sea level are likely to exacerbate threats to buildings, roads, power lines, and other infrastructure in climatically sensitive places, such as low-lying coastlines and the permafrost regions of Alaska.

8. Other stresses magnified by climate change. Climate change will very likely magnify the cumulative impacts of other stresses, such as air and water pollution and habitat destruction due to human development patterns. For some systems, such as coral reefs, the combined effects of climate change and other stresses are very likely to exceed a critical threshold, bringing large, possibly irreversible impacts.

9. Surprises expected. It is very likely that some aspects and impacts of climate change will be totally unanticipated as complex systems respond to ongoing climate change in unforeseeable ways.

10. Uncertainties remain. Significant uncertainties remain in the science underlying climate-change impacts. Further research would improve understanding and predictive ability about societal and ecosystem impacts, and provide the public with useful information about adaptation strategies.

Given these findings it is clear that climate impacts will vary widely across the Nation, as one would expect for a country as large and ecologically diverse as the U.S. Natural ecosystems appear to be highly vulnerable to climate changes of the magnitude and rate which appear to be likely; some ecosystems surprisingly so. The potential impacts on water resources are an important issue in every region examined, although the nature of the concern is very different for the mountainous West than for the East. The potential for drought is a concern across the country. The nation's food supply appears secure, but there are very likely to be regional gains and losses for farmers, leading to a more complex picture on a region-by-region basis. Forests are likely to grow more rapidly for a few decades because of increasing carbon dioxide concentrations in the atmosphere, but it is unclear whether those trends will be maintained as the climate system itself changes, leading to other disturbances such as fire and pest outbreaks. However, the climate change itself will, over time, lead to shifts in the tree species in each region of the country, some of them potentially quite profound. Coastal areas in many parts of the U.S. and the permafrost regions of Alaska are already experiencing disruptions from sea-level rise and recent regional warming; these trends are likely to accelerate. Climate change will very likely magnify the cumulative impacts of other environmental stresses about which people are already concerned, such as air and water pollution, and habitat destruction due to development patterns. There are clearly links between human health, current climate, and air pollution. The future vulnerability of the U.S. population to the health impacts of climate change depends on our capacity to adapt to potential adverse changes. Many of these adaptive responses are desirable from a public health perspective irrespective of climate change. Future assessments need to consider climate change in the context of the suite of environmental stresses that we all face. Perhaps most importantly, the report acknowledges very clearly that scientific uncertainties remain, and that we can expect surprises as this

uncontrolled experiment with the Earth's geochemistry plays out over the coming decades.

We hope that the public comment period will indeed result in a broad discussion of this draft report. This is, after all, a topic of immense importance and broad significance for Americans. We invite those with the interest to do so to participate by obtaining the current draft (www.usgcrp.gov), and to submit their comments, concerns, and criticisms. Our interest is in being as open and transparent as possible about what we have concluded, the scientific integrity of the results, and why we think they are important for us all.

Independent Review Board of the President's Committee of Advisers on

Peter Raven, Co-chair

Science and Technology (PCAST)

University Corporation for Atmospheric Research, and Washington, Advisory Group

Simulation of decadal average changes in temperature from leading climate models based on historic and projected changes in CO2 and sulfate atmospheric concentrations. The heavy red and black lines indicate the primary models chosen for use by the National Assessment. For the 21st century the projected global temperature increase for the Hadley model is 4.9°F and 7.4°F for the Canadian model. The model with the smallest projected increase of global temperature is the Climate System Model at 3.6°F. By comparison, the projected increase in temperature for the 21st century over the contiguous U.S. is: Canadian, 9.4°F; Hadley, 5.5°F; and the Climate System Model, 4.0°F.

Attachment 4

Observed and Projected Changes of Precipitation for various categories of daily precipitation events Annual Precipitation

These graphs of precipitation for the contiguous U.S. show both observed changes during the 20th Century and projected changes for the 21st Century based on the Canadian Global Climate Model (Version 1) and the Hadley Climate Model (Version 2). As the charts demonstrate, the largest increases have been and are projected to be in the heaviest precipitation events, the days already receiving large amounts of precipitation.

Summary

Attachment 5

Large impacts in some places. The impacts of climate change will be significant for Americans. The nature and intensity of impacts will depend on the location, activity, time period, and geographic scale considered. For the nation as a whole, direct economic impacts are likely to be modest. However, the range of both beneficial and harmful impacts grows wider as the focus shifts to smaller regions, individual communities, and specific activities or resources. For example, while wheat yields are likely to increase at the national level, yields in western Kansas, a key U.S. breadbasket region, are projected to decrease substantially under the Canadian climate model scenario. For resources and activities that are not generally assigned an economic value (such as natural ecosystems), substantial disruptions are likely.

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