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Scientists: global warming is causing spreading drought 

The researchers, with the National Center for Atmospheric Research in Boulder, Colo., said widespread drying occurred over much of Europe and Asia, Canada, western and southern Africa.

Aiguo Dai, a scientist with the center, presented the findings on Jan. 12 the American Meteorological Society annual meeting in San Diego, Calif. The work also appeared in a paper published in the December issue of the Journal of Hydrometeorology.

 “The results reconfirm the complexity of the climate system,” said Cliff Jacobs, program director in division of atmospheric sciences of the National Science Foundation, U.S.A. “We need to continue to develop a wide variety of research tools to understand these changes.”

Dai and his colleagues found that the fraction of global land experiencing very dry conditions rose from about 10-15 percent in the early 1970s to about 30 percent by 2002. Almost half of that change is due to rising temperatures rather than decreases in rainfall or snowfall, according to Dai.

“These results point to increased risk of droughts as human activity contributes to global warming,” says Dai.

Even as drought has expanded across Earth’s land areas, the amount of water vapor in the air has increased over the past few decades, the researchers said. Average global precipitation has also risen slightly. However, as Dai noted, “surface air temperatures over global land areas have increased sharply since the 1970s.”

The large warming increases the tendency for moisture to evaporate from land areas. Together, the overall area experiencing either very dry or very wet conditions could occupy a greater fraction of Earth’s land areas in a warmer world, Dai says.

“Droughts and floods are extreme climate events that are likely to change more rapidly than the average climate,” says Dai. “Because they are among the world’s costliest natural disasters and affect a very large number of people each year, it is important to monitor them and perhaps predict their variability.”

Dai and colleagues used long-term records of temperature and precipitation from a variety of sources to estimate soil moisture for the period 1870–2002. The results were consistent with those from a historical computer simulation of global land surface conditions. By factoring out rainfall and snowfall, Dai and colleagues estimated how much of the changes moisture changes were due solely to rising temperatures.

“The warming-induced drying has occurred over most land areas since the 1970s,” says Dai, “with the largest effects in northern mid- and high latitudes.” In contrast, rainfall deficits alone were the main factor behind expansion of dry soils in Africa’s Sahel and East Asia. These are regions where El Niño, a more frequent visitor since the 1970s, tends to inhibit rainfall.

Though most of the Northern Hemisphere has shown a drying trend in recent decades, the United States has bucked that trend, becoming wetter overall during the past 50 years, says Dai. The trend is especially notable between the Rocky Mountains and Mississippi River. Other parts of the world showing a moistening trend include Argentina and parts of western Australia. These trends are related more to increased precipitation than to temperature, says Dai.

“Global climate models predict increased drying over most land areas during their warm season,” Dai said. This occurs because of a general increase in “greenhouse gases,” such as carbon dioxide, which trap heat in the atmosphere, he added. “Our analyses suggest that this drying may have already begun.” 

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