Streamflow up, extremes down, study shows

Increased media attention to weather over the past 20 years leads to a perception that extreme weather events are increasing

January 20, 1999
Web posted at: 11:00 AM EST

By Environmental News Network staff

(ENN) -- Images of flood-threatened residents desperately constructing sandbag dikes in the face of impending disaster have proliferated in the media lately, but despite conventional wisdom, floods are not becoming more severe, according to a study conducted by two U.S. Geological Survey scientists.

Hydrologist Harry Lins and colleague Jim Slack, a mathematician, calculated trends for 395 climate sensitive streamflow gauging stations in the continental United States during the 20th century to determine whether or not global warming is causing flooding and droughts to become more severe.

The data collected from "climate-sensitive" streams -- those streams that have not been altered by dams or other unnatural adjustments to streamflow -- indicate that global warming is not causing more severe flooding or drought conditions, said Lins.

"The one hypothesis that has been widely disseminated is that under global warming there will be an enhanced hydrologic cycle. This is interpreted by some to mean that there will be more floods and droughts. Our findings contradict this hypothesis," he said.

While the data collected and analyzed by Lins and Slack shows that streamflow is slightly up, and has been going steadily up in most parts of the country for the past 40 years, Lins said that their findings could actually support a less-publicized hypothesis of global warming.

"A less publicized hypothesis is that under global warming the temperature and pressure gradients that contribute to extreme events will become weaker, thereby reducing floods and droughts. Our findings could be used to support this view," he said.

Before being thrust into the debate over global warming, however, Lins cautioned that there is not enough evidence to link the moderately increased streamflows to global warming.

"Quite honestly, at this time it is not possible (either statistically or numerically) to attribute the hydrologic changes that we observed to global warming," he said.

What Lins and Slack hope to accomplish with their study is tone down the language used by Tom Karl, a senior scientist at the National Oceanic and Atmospheric Administration National Climatic Data Center in Asheville, N.C., in characterizing the modest precipitation increases as "extreme."

"We believe that our results are consistent with those reported by Tom Karl of modest precipitation increases in the U.S. during the 20th century," said Lins. "However, based on our findings, we think that Tom's characterization of these precipitation increases as 'extreme' is misleading and inappropriate."

"We feel that it would have been more appropriate to characterize the precipitation increases as increases in 'heavy' rather than 'extreme' precipitation. The term extreme connotes catastrophic conditions that simply are not evident in the hydrologic record."

This is not say that Lins and Slack don't believe there has been an increasing trend in both flood damage and drought vulnerability over the past 20 years.

"However, the flood damage increase stems from continuing urban and suburban development on floodplains and the drought vulnerability increase is from development in regions of lower renewable water supplies," said Lins.

This, combined with increased media attention to weather over the past 20 years leads to a "perception" that extreme weather events are increasing even though Lins and Slack say their data shows that the events themselves have not become more numerous and severe.

Lins and Slack's research appears in the Jan 15, 1999, issue of Geophysical Research Letters, a publication of the American Geophysical Union.

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