Historical Trends and Future Projections of Extreme Climate Conditions for the National Climate Assessment
Confident projections of future conditions under selected scenarios of greenhouse gas emissions can be made for some extremes types but not others. In a few cases, confident projections can be made for extreme conditions but not for mean conditions. Precipitation is a case in point. Projections of mean precipitation do not provide a robust signal of either increases or decreases in most of the U.S. This lack of a robustly simulated direction of change extends to the end of the 21st Century. However, there are aspects of the precipitation climatology that exhibit a robust direction of change. Model simulations of extreme rainfall indicate future increases of 20% or more in the magnitude of the largest rainfall events under a high emissions scenario. Such increases are simulated even where mean annual precipitation is projected to decrease. Also, the length of dry spells is projected to increase in most areas, even in some areas where mean annual precipitation is projected to decrease. Thus, overall the precipitation climate is projected to become more extreme.