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Winter 2000

Population and wealth, more than climate, drive soaring flood costs

by Anatta

The fivefold increase in U.S. annual flood damages from the 1940s to the 1990s is much more the result of social changes than of increased precipitation, according to NCAR scientists Roger Pielke Jr. and Mary Downton (both in the Environmental and Societal Impacts Group). The authors published their results in the Journal of Climate last October.

Pielke and Downton examined ten different measures of precipitation. They found a strong relationship between flood damage and the number of two-day heavy rainfall events and wet days. They also found a somewhat weaker relationship between flood damage and two-inch (five-centimeter) rainfall events in most regions. However, they say, these relationships could not explain the dramatic growth in flood losses—from $1 billion in the 1940s (adjusted for inflation) to $5 billion in the 1990s.

In a series of recent articles, including this one in the Journal of Climate, Pielke, Downton, and colleagues have looked at the role of increasing precipitation, population, and national wealth. They found that population growth alone accounts for 43% of the rise in flood damages from 1932 to 1997, with a much smaller effect from increased precipitation. "Most of the other 57% increase is due to burgeoning national wealth," says Pielke. Downton's work suggests that more detailed disaster reporting also contributes to the trend.

Climate scientists have observed a rise in precipitation in some areas of the United States and elsewhere over the past century. The Intergovernmental Panel on Climate Change (IPCC) has written that a warmer climate could lead to more heavy-rain events. The Pielke-Downton paper confirmed that flooding increases with precipitation, depending greatly on the time and location of the rain or snowfall. However, "even without an increase in precipitation," they write, "total flood damage will continue to rise with the nation's growing population and wealth unless actions are taken to reduce vulnerability."

Pielke, a political scientist, has often stated that his work is consistent with the IPCC's consensus view that the earth's climate is changing at least partly because of human activity. In a recent presentation at NCAR, he pointed out, "We can manage spiraling flood costs without waiting for precise answers from climate change research." Disaster mitigation policies regarding floodplain management are already available and can curtail the rising costs, he explained. "We know enough to act now."

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Wed Feb 7 15:34:33 MST 2001