How is the global climate changing? How is climate changing in the United States? Does climate warming mean we will see more weather variations and extremes?

SPEAKER: Thomas Karl, NOAA National Climate Data Center, Asheville, NC

INTRODUCTION: Michael MacCracken, Director, Office of the USGCRP

There is increasing evidence that the global climate is changing�global temperatures have risen about 1 F over the past century, mountain glaciers are melting back, sea level is rising. But how is the climate of the United States changing? Are these changes like others being experienced around the world? Is the US climate becoming more or less variable? Are we having more or fewer climatic extremes? This seminar will address these questions in the context of the anthropogenic influences on atmospheric composition and climate.

An increasing number of studies are tying the global warming of the last 100 years to human influences. These studies do so by analyzing the global patterns of change, drawing on new analyses from around the world. One very important study has focused on climate changes in the United States. Based on an analysis of national weather-related trends since 1910, Thomas Karl and his colleagues at the National Climate Data Center have found, with high statistical confidence (typically 19 chances out of 20) that trends of climate in the United States are consistent with projected trends due to a human-induced warming effect resulting from increased concentrations of heat-trapping greenhouse gases.

To help better define how these changes in climate will affect people in their day to day lives, Tom Karl and his colleagues have looked very closely at nearly nine decades of weather and temperature trends. They have devised two climate change indices: (1) a Climate Extremes Index, which takes into account extreme heat, extreme cold, extreme drought, and extreme wetness; and (2) a Greenhouse Climate Response Index, which is a set of measures that permit a comparison between model predictions of climate change due to increases of anthropogenic greenhouse gases, and observed changes.

Climate models make a number of predictions about how the climate will change due to the rising concentrations of greenhouse gases. These include: (1) an increase of mean surface temperature, somewhat more pronounced during the night; (2) an increase in precipitation, especially during the cold season; (3) more severe and longer lasting droughts during the warm season; (4) a greater portion of warm season precipitation derived from heavy convective rainfall (showers or thunderstorms) compared with gentler, longer-lasting rainfalls; and (5) a decrease in the day-to-day variability of temperature.

To test whether the climate is changing in these ways, the Greenhouse Climate Response Index is an average of five indicators related to these predictions: (1) percent of the US with much above normal minimum temperatures; (2) percent of the US with much above normal precipitation from October through April (cold season); (3) percent of the US with much greater than normal area in extreme drought during May through September (warm season); (4) percent of the US with a much greater than normal proportion of precipitation derived from extreme 1-day precipitation events exceeding 2 inches, and (5) percent of the US with day-to-day temperature differences much less than normal.

Results of this work indicate that, since 1970, precipitation in the US has remained above the twentieth century mean, averaging 5% higher than in the previous 70 years. Since the 1970s, temperature has rapidly increased, and has remained as high as some of the very high temperatures recorded during the "Dust Bowl" era of the 1930's. However, the more recent warmth is accompanied by relatively high amounts of precipitation, unlike the 1930s. Since 1976 the Climate Extremes Index (CEI) has averaged about 1.5% higher than the average of the previous 65 years, indicating that the climate of the US has become more extreme in recent decades. The proportion of the US with much above normal daily minimum temperatures has been at high levels since the late 1970s. During these same years sea-surface temperatures in the central and eastern equatorial Pacific have remained anomalously warm. There seems clear evidence that the climate is changing.

Since 1976, the US Greenhouse Climate Response Index (GCRI) has been about 2.5% above the average from previous years in this century. The tendency toward increased values of the GCRI over the past two decades is suggestive of a climate driven by greenhouse warming. Comparing US climate records for the last 30-80 years with records from China, Australia, and the former Soviet Union, Tom Karl and his colleagues have found that seasonal temperature variability has generally decreased over the entire Northern Hemisphere. In the US and former Soviet Union declines in temperature variability are seen primarily in the spring and summer. No significant increases in temperature variability were found in any country.

Thomas Karl is Senior Scientist at the National Climatic Data Center in Asheville NC, where he has been since 1980. Prior to joining NCDC, Tom Karl was at the National Weather Service, the NOAA Environmental Resources Laboratory and the Univ. of North Carolina and he served a brief stint as a TV/Radio Weather Forecaster in 1975. Tom Karl has published extensively, being author or co-author on 77 peer-reviewed journal articles, approximately 200 scientific reports and other publications, and 7 commercial books (3 as co-author, 1 as co-editor, 3 as contributor). He is a fellow of the American Meteorological Society, and a member of the Amer. Assoc. of State Climatologists. He has won the Landsberg Award (1993), the Amer. Meteor. Soc. Editors Award, the Dept. of Commerce Gold Medal (1991), the NOAA Administrator�s Award (1989), and the Dept. of Commerce Bronze Medal (1988). He serves as Associate Editor of the Journal of Climate and of Climatic Change, and is a member of the National Research Council�s Climate Research Committee, Panel on EOSDIS, Panel on Policy Implications of Greenhouse Warming, and serves as a co-author of the assessments of the Inter-governmental Panel on Climate Change.