NOAA 97-3

CONTACT:  Patricia Viets            FOR IMMEDIATE RELEASE


As the global climate continues to warm, extreme flooding like that recently experienced in the western United States is expected to become more frequent, reports a senior scientist with the Commerce Department's National Oceanic and Atmospheric Administration.

Although it is impossible to link any particular weather or climate event to global warming, and present-day climate models are not sophisticated enough to accurately pinpoint regions of the globe where changes will be the largest, extreme flooding is expected to become more frequent across the United States due to an increase in precipitation extremes, said Thomas Karl, senior scientist at NOAA's National Climatic Data Center in Asheville, N.C.

Observations since the beginning of the 20th century for the United States indicate that intense precipitation events have already increased by about 20 percent, and cold season precipitation has increased by nearly 10 percent, Karl said. An increase in the intensity of precipitation has led to an increased flood potential.

Why does this happen? Increased concentrations of greenhouse gases in the atmosphere lead to an increase in mean global temperatures. As the global climate warms, the hydrological cycle is affected because a portion of the heating will go into evaporating larger quantities of water from the earth's surface. As global temperatures increase, the atmosphere can also support greater amounts of water vapor. In general, an increase in the proportion of extreme and heavy precipitation events would occur where there is enough atmospheric instability to trigger precipitation events. This means more flooding with an increase in extreme precipitation events, but also more droughts. Droughts arise where and when the atmosphere is not favorable to precipitation, and the evaporated moisture is transported to other regions. The additional evaporation from the surface leads to a drying of the soil, and more severe and widespread droughts.

Comparisons of climatologies, from climate models run with present-day and doubled carbon dioxide concentrations, reveal some dramatic changes in the hydrologic cycle as the global climate warms, Karl said. When carbon dioxide concentrations are doubled, the expected frequency and areal extent of extreme droughts and intense precipitation in the United States increase (more than two inches per day) and Canada (more than one inch per day), some models showing a three to four-fold increase. There is also a distinct increase in wintertime or cold season precipitation.

Recent events, like the flooding in the Northeast last autumn and the flooding this winter, offer examples of the kind of situations that are expected to be associated with an increased risk of occurrence, Karl said.



Additional details of the heavy precipitation and flooding across the northwestern United States, along with an analysis of the direct causes of the floods, are discussed in a Special Climate Summary issued by NOAA's Climate Prediction Center on Jan. 14, 1997. The summary entitled "Flooding in the Pacific Northwest" is available on the Internet at under Special Summaries. For interviews on the Special Climate Summary, contact Stephanie Kenitzer. To obtain satellite data of the Soil Wetness Index, call Patricia Viets , or fax your request to 301-457-5006. Data on the flooding can be found on the World Wide Web at: Further information about the National Climatic Data Center can be found at: