Key National Findings
Adapted from
the U.S. National Assessment

Increased warming is projected for the 21st century -- Assuming continued growth in world greenhouse gas emissions, the primary climate models drawn upon for the analyses carried out in the U.S. National Assessment projected that temperatures in the contiguous United States will rise 3 -- 5°C (5 -- 9°F) on average during the 21st century. A wider range of outcomes, including a smaller warming, is also possible. 

Impacts will differ across regions -- Climate change and its potential impacts are likely to vary widely across the country. Temperature increases are likely to vary somewhat among regions. Heavy precipitation events are projected to become more frequent, yet some regions are likely to become drier.

Ecosystems are especially vulnerable -- Many ecosystems are highly sensitive to the projected rate and magnitude of climate change, although more efficient water use will help some ecosystems. A few ecosystems, such as alpine meadows in the Rocky Mountains and some barrier islands, are likely to disappear entirely in some areas. Other ecosystems, such as southeastern forests, are likely to experience major species shifts or break up into a mosaic of grasslands, woodlands, and forests. Some of the goods and services lost through the disappearance or fragmentation of natural ecosystems are likely to be costly or impossible to replace.

Widespread water concerns arise -- Water is an issue in every region, but the nature of the vulnerabilities varies. Drought is an important concern virtually everywhere. Floods and water quality are concerns in many regions. Snowpack changes are likely to be especially important in the West, Pacific Northwest, and Alaska.

Food supply is secure -- At the national level, the agriculture sector is likely to be able to adapt to climate change. Mainly because of the beneficial effects of the rising carbon dioxide levels on crops, overall U.S. crop productivity, relative to what is projected in the absence of climate change, is very likely to increase over the next few decades. However, the gains are not likely to be uniform across the nation. Falling prices are likely to cause difficulty for some farmers, while benefiting consumers.

Near-term forest growth increases -- Forest productivity is likely to increase over the next several decades in some areas as trees respond to higher carbon dioxide levels by increasing water-use efficiency. Such changes could result in ecological benefits and additional storage of carbon. Over the longer term, changes in larger-scale processes, such as fire, insects, droughts, and disease, could decrease forest productivity. In addition, climate change is likely to cause long-term shifts in forest species, such as sugar maples moving north out of the country.

Increased damage occurs 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 climate-sensitive areas. For example, infrastructure damage is expected to result from permafrost melting in Alaska and from sea level rise and storm surges in low-lying coastal areas.

Adaptation determines health outcomes -- A range of negative health impacts is possible from climate change. However, as in the past, adaptation is likely to help protect much of the U.S. population. Maintaining our nation's public health and community infrastructure, from water treatment systems to emergency shelters, will be important for minimizing the impacts of water-borne diseases, heat stress, air pollution, extreme weather events, and diseases transmitted by insects, ticks, and rodents.

Other stresses are magnified by climate change -- Climate change is very likely to modify the cumulative impacts of other stresses. While it may magnify the impacts of some stresses, such as air and water pollution and conversion of habitat due to human development patterns, it may increase agricultural and forest productivity in some areas. For coral reefs, the combined effects of increased CO2 concentration, climate change, and other stresses are very likely to exceed a critical threshold, causing large, possibly irreversible impacts. 

Uncertainties remain and surprises are expected -- Significant uncertainties remain in the science underlying regional changes in climate and their impacts. Further research would improve understanding and capabilities for projecting societal and ecosystem impacts. Increased knowledge would also provide the public with additional useful information about options for adaptation. However, it is likely that some aspects and impacts of climate change, both positive and negative, will be totally unanticipated as complex systems respond to ongoing climate change in unforeseeable ways.

Sources: NAST 2000, 2001.