Huge parts of world drying up due to land 'evapo-transpiration': study
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COMMENTS 
The soil in large parts of the southern hemisphere has been drying up over the last decade, according to researchers who have released the first major global study on “evapo-transpiration”
A group of researchers examining the phenomenon of ‘evapo-transpiration’ on a global basis have concluded that the soil in large areas of the southern hemisphere, including major portions of Australia, Africa and South America, has been drying up over the last decade.
Most climate models have suggested that evapo-transpiration, which is the movement of water from the land to the atmosphere, will increase with global warming. The new research found that that’s exactly what was happening from 1982 to the late-1990s. The report was published online in the journal Nature.
In 1998, however, this significant increase in evapo-transpiration -- which has been seven millimetres per year -- slowed dramatically, or stopped. In large portions of the world, soils are becoming drier than they used to be, releasing less water and offsetting moisture increases elsewhere.
Due to the limited number of decades for which data are available, scientists say they can’t be sure whether this is a natural variability or part of a longer-lasting global change. One possibility is that, on a global level, a limit to the acceleration of the hydrological cycle on land has already been reached.
If that’s the case, the consequences could be serious.
It could include reduced terrestrial vegetation growth, less carbon absorption, loss of the natural cooling mechanism provided by evapo-transpiration, greater heating of the land surface, more intense heat waves and a “feedback loop” that could intensify global warming. We didn’t expect to see this shift in evapo-transpiration over such a large area of the southern hemisphere,” said Beverly Law, a professor of global change forest science at Oregon State University.
Some of the areas with the most severe drying include southeast Africa, much of Australia, central India, large parts of South America, and some parts of Indonesia. Although most of these regions are historically dry, some are actually tropical rainforests.
The rather abrupt change from increased global evapo-transpiration to a near halt in the process coincided with a major El Nino event in 1998, the researchers note in their report, although they are not suggesting that is the causative mechanism for a phenomenon that has been going on for more than a decade.
Greater evapo-transpiration was expected with global warming because of increased evaporation of water from the oceans and more precipitation overall. And the data indeed shows that some areas are wetter than they used to be.
However, other huge areas are now drying out, the study shows. This could lead to increased drought stress on vegetation and less overall productivity, Law said. As a result, less carbon absorbed, less cooling through evapo-transpiration, and more frequent or extreme heat waves.
Some of the sites used in this study are operated by Law’s research group in the central Oregon Cascade Range, in the Metolius river watershed, and they are consistent with some of these concerns. In the last decade there have been multiple years of drought, vegetative stress, and significant forest fires in the area.
Evapo-transpiration returns about 60% of annual precipitation back to the atmosphere, in the process using more than half of the solar energy absorbed by land surfaces. This is a key component of the global climate system, linking the cycling of water with energy and carbon cycles.
Source: ANI, October 13, 2010
http://environmentalmanagementnews.net, October 2010