Drought is spreading fast

Dry areas increase by 250 pe cent in 30 years

Published: Tuesday 15 February 2005

Dry spread: depleting soil moi the area hit by severe drought worldwide has increased from nearly 12 per cent in the 1970s to 30 per cent in the early 2000s, according to scientists at the National Center for Atmospheric Research (ncar), Colorado, usa . Almost half of that change is due to global warming rather than decrease in rainfall or snowfall, they warn.

"As surface air temperatures increase over land, water holding capacity of the air increases and thus its demand of moisture. This has been a primary cause for the widespread drying during the last two-three decades," the team led by Aiguo Dai said in a paper that appeared in the December 2004 issue of Journal of Hydrometeorology (Vol 5, No 6) .

The team found in the last 30 years widespread drying occurred mainly over much of Europe, Asia, western and southern Africa, eastern Australia and Canada. A large jump in areas categorised as very dry occurred in the early 1980s due to an El Nio-induced precipitation decrease and subsequent increase in surface temperatures.

Though most of the Northern Hemisphere has been under a dry spell in recent decades, the us has bucked that trend, becoming wetter overall during the last 50 years. The moistening is especially notable between the Rocky Mountains and the Mississippi River. Other countries showing a moistening trend include Argentina and parts of western Australia. These trends are related more to increased precipitation than to temperature, says Dai, whose team studied not only very dry but also very wet areas from 1950 to 2002.

"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 large number of people each year, it is important to monitor them and perhaps predict their variability."

They found areas experiencing either very dry or very wet conditions decreased slightly by about 7 per cent between 1950 and 1972, whereas their proportion increased from roughly 20 per cent in 1972 to 38 per cent in 2002.

Dai and colleagues used the Palmer index -- a measure of the near-surface moisture conditions -- to see how soil moisture has evolved over the last few decades. The index has been the most widely used yardstick to measure drought in the us, but not in other countries. So the ncar team used long-term records (from 1870 to 2002) of temperature and precipitation from various sources to derive the index for all the countries. A longer time period was necessary to establish the accuracy of the analysis which was used only after its results were found consistent with those from a historical simulation of global land surface conditions, produced by a computer model developed by scientists at ncar, the National Aeronautics and Space Administration of the us, Georgia University of Technology, the University of Texas at Austin, and the University of Arizona.

The scientists then studied the effects of surface warming on dry and wet areas by simulating cases with and without temperature changes. By factoring out rainfall and snowfall, the ncar scientists estimated how much of the global trend in soil moisture was due solely to rising temperatures that enhance evaporation.

"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 Nio, a more frequent visitor since the 1970s, tends to inhibit precipitation.

As the global climate warms, how will the location and duration of droughts be affected? Scientists do not yet have an answer to that. Globally, the average amount of rainfall is expected to increase, as warmer temperatures lead to more evaporation from the sea. However, the same warmth will help dry out land more quickly, so the droughts that do occur may increase in intensity.

Computer-based simulations by another team of ncar scientists have been used to track areas of persistent dryness in the past and to project where these areas might develop in the future. These models do not yet agree on areas most at risk. However, they tend to predict drying in the hearts of mid-latitude continents, such as the central us, eastern Europe and western Asia.

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