In August 2002, parts of central Europe experienced unprecedented flooding after record rains fell upon saturated soils and brimming reservoirs. Damages on the continent added up to more than 25 billion Euros, and in Dresden, Germany, the Elbe River reached 9.4 meters above flood stage, a level not seen since the Middle Ages.
Despite the 2002 season, a new analysis by German researchers suggests that extreme summer floods in the region aren’t becoming more frequent. In fact, the scientists say, widespread inundations have been on the wane for the past century or so.
For the study, team leader Manfred Mudelsee of the University of Leipzig in Germany and his coworkers considered regional floods along the central stretches of the Elbe and Oder Rivers dating back at least 700 years. The scientists culled data from historical archives and modern instruments, and they analyzed summer and winter floods separately because they have different causes.
Floods that occur from May through October typically arise during or just after long periods of precipitation, says Mudelsee. The frequency of such inundations hasn’t changed significantly on the Elbe since 1820 or on the Oder since 1920. Other researchers have reported 10 major summer floods on the Elbe in the past 500 years, and 4 of those high-water events occurred between 1500 and 1550.
So-called winter floods in the region have declined, says Mudelsee. These cold-season floods are often caused by ice dams that form when frozen rivers break up in the spring. Before 1850, 91 of 103 severe winter floods on the Elbe, and 28 of 34 of those on the Oder, were influenced by river ice. Between 1930 and 1970, however, just 2 of 13 winter floods on the Elbe and 3 of 20 on the Oder were affected by ice.
Mudelsee and his colleagues also estimated the ameliorating influence of reservoirs and other flood-control measures and found that they had little effect during large inundations. The scientists report their findings in the Sept. 11 Nature.
“I’m not surprised that there’s not been a rise in floods,” says Phil Jones, a climatologist at the University of East Anglia in Norwich, England. Current amounts of precipitation aren’t very different from those measured during other wet periods of the past 2 centuries. Modern floods may appear more serious than past ones largely because more people live on floodplains now than in earlier periods, he notes.
Results of many computerized climate models suggest that continuation of the current increase in average global temperature will boost evaporation from the oceans. An accompanying rise in precipitation could increase the frequency and severity of floods, some scientists say.
“I’m convinced the climate is changing, but I’m not convinced that the frequency of floods has changed,” says Kenneth W. Potter, an environmental engineer at the University of Wisconsin–Madison.
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