USGCRP logo & link to home

Updated 12 October, 2003

Climate Action Report 2002
The United States of America's Third National Communication Under the United Nations Framework Convention on Climate Change
Chapter 6: Impacts and Adaptation
May 2002


Back to Table of Contents for Chapter 6

[next section]

Projected Changes in the Mean Climate

The model scenarios used in the National Assessment project that the continuing growth in greenhouse gas emissions is likely to lead to annual-average warming over the United States that could be as much as several degrees Celsius (roughly 3 -- 9F) during the 21st century. In addition, both precipitation and evaporation are projected to increase, and occurrences of unusual warmth and extreme wet and dry conditions are expected to become more frequent. For areas experiencing these changes, they would feel similar to an overall northern shift in weather systems and climate condition. For example, the central tier of states would experience climate conditions roughly equivalent to those now experienced in the southern tier, and the northern tier would experience conditions much like the central tier. Figure 6-1 illustrates how the summer climate of Illinois might change under the two scenarios. While the two models roughly agree on the amount of warming, the differences between them arise because of differences in projections of changing summertime precipitation.

Recent analyses indicate that, as a result of an uncertain combination of natural and human-induced factors, changes of the type that are projected for the 21st century were occurring to some degree during the 20th century. For example, over the last 100 years most areas in the contiguous United States warmed, although there was cooling in the Southeast. Also, during the 20th century, many areas experienced more periods of very wet or very dry conditions, and most areas experienced more intense rainfall events. While warming over the 48 contiguous states amounted to about 0.6C (about 1F), warming in interior Alaska was as much as 1.6C (about 3F), causing changes ranging from the thawing of permafrost to enhanced coastal erosion resulting from melting of sea ice.

Model simulations project that minimum temperatures are likely to continue to rise more rapidly than maximum temperatures, extending the trend that started during the 20th century. Although winter temperatures are projected to increase somewhat more rapidly than summer temperatures, the summertime heat index is projected to rise quite sharply because the rising absolute humidity will make summer conditions feel much more uncomfortable, particularly across the southern and eastern United States.

Although a 0.6C (1F) warming may not seem large compared to daily variations in temperature, it caused a decline of about two days per year in the number of days that minimum temperatures fell below freezing. Across the United States, this change was most apparent in winter and spring, with little change in autumn. The timing of the last spring frost changed similarly, with earlier cessation of spring frosts contributing to a lengthening of the frost-free season over the country. Even these seemingly small temperature-related changes have had some effects on the natural environment, including shorter duration of lake ice, a northward shift in the distributions of some species of butterflies, changes in the timing of bird migrations, and a longer growing season.

With respect to changes in precipitation, observations for the 20th century indicate that total annual precipitation has been increasing, both worldwide and over the country. For the contiguous United States, total annual precipitation increased by an estimated 5 -- 10 percent over the past 100 years. With the exception of localized decreases in parts of the upper Great Plains, the Rocky Mountains, and Alaska, most regions experienced greater precipitation (Figure 6-2). This increased precipitation is evident in daily precipitation rates and in the number of rain days. It has caused widespread increases in stream flow for all levels of flow conditions, particularly during times of low to moderate flow conditions -- changes that have generally improved water resource conditions and have reduced situations of hydrologic drought.

For the 21st century, models project a continuing increase in global precipitation, with much of the increase occurring in middle and high latitudes. The models also suggest that the increases are likely to be evident in rainfall events that, based on conditions in each region, would be considered heavy (Figure 6-3). However, estimates of the regional pattern of changes vary significantly. While there are some indications that wintertime precipitation in the southwestern United States is likely to increase due to warming of the Pacific Ocean, changes across key U.S. forest and agricultural regions remain uncertain.

Soil moisture is critical for agriculture, vegetation, and water resources. Projections of changes in soil moisture depend on precipitation and runoff; changes in the timing and form of the precipitation (i.e., rain or snow); and changes in water loss by evaporation, which in turn depends on temperature change, vegetation, and the effects of changes in CO2 concentration on evapotranspiration. As a result of the many interrelationships, projections remain somewhat uncertain of how changes in precipitation are likely to affect soil moisture and runoff, although the rising summertime temperature is likely to create additional stress by significantly increasing evaporation.

[next section]

Jump to top of page

US CCSP  logo & link to home USGCRP logo & link to home
US Climate Change Science Program / US Global Change Research Program, Suite 250, 1717 Pennsylvania Ave, NW, Washington, DC 20006. Tel: +1 202 223 6262. Fax: +1 202 223 3065. Email: Web: Webmaster: