Global Warming Linked to Increasing Drought

By J.R. Pegg

WASHINGTON, DC, January 11, 2005, (ENS) - The amount of land suffering from severe drought has more than doubled in the past 30 years, say U.S. researchers who point to rising global temperatures as a key factor in the increase.

Some 30 percent of the Earth's surface experienced drought in 2002, compared to 10 to 15 percent in the early 1970s, according to climate scientists at the National Center for Atmospheric Research (NCAR).

Almost half of the increase is due to rising temperatures rather than decreases in rainfall or snowfall, said Aiguo Dai, an atmospheric scientist with NCAR and lead author of the new study.

Dai said global climate models predict increase drying over most land areas during their warm season, as emissions of heat-trapping carbon dioxide and other greenhouse gases increase and cause temperatures to rise.

Warming increases the tendency for moisture to evaporate from land areas, said Dai, and "our analyses suggest that this drying may have already begun."

Dai will present the new findings Wednesday at the American Meteorological Society's annual meeting in San Diego.

The work also appears in the December issue of the "Journal of Hydrometeorology" in a paper also authored by NCAR's Kevin Trenberth and Taotao Qian. grouse

According to the United Nations, drought and desertification threaten the livelihood of some one billion people in more than 110 countries around the world. (Photo by Evan Schneider courtesy UN)
The researchers found that even as drought has expanded over the past few decades, the amount of water vapor in the air has increased.

The average global precipitation has also risen slightly, but "surface air temperatures over global land area have increased sharply since the 1970s," Dai said.

The last 10 years are among the hottest on record.

In the past three decades drought has hit much of Europe and Asia, Canada, western and southern Africa, and eastern Australia.

By factoring out rainfall and snowfall, Dai and colleagues estimated how much of this drying was due solely to rising temperatures through the extra evaporation they produce.

"The warming-induced drying has occurred over most land areas since the 1970s, with the largest effects in northern mid and high latitudes," Dai said.

Rainfall deficits alone were the main factor behind expansion of dry soils in Africa's Sahel and East Asia, Dai said.

Though most of the Northern Hemisphere has shown a drying in recent decades, the United States - except for parts of the intercontinental West - has bucked that trend, becoming wetter overall during the last 50 years, Dai said.

The moistening in the United States is in particular notable between the Rocky Mountains and Mississippi River.

Other parts of the world showing a moistening trend include Argentina and parts of western Australia. Sun

Heat from the Sun is being trapped close to the planet by a blanket of greenhouse gases. (Photo courtesy NOAA)
Dai said trends in these areas are related more to increased precipitation than to temperature.

The researchers used long-term records of temperature and precipitation from a variety of sources to derive the drought index for the period 1870-2002.

They found their results were consistent with those from a historical simulation of global land surface conditions, produced by a comprehensive computer model developed by scientists at NCAR, NASA, and several other U.S. research institutions.

"Droughts and floods are extreme climate events that are likely to change more rapidly than the average climate," Dai said. "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."