The soils in large areas of the Southern Hemisphere, including large parts of Australia, Africa and South America, have been drying up in the past decade, a new study finds.
The study is the first major one of its kind to look at the movement of water from the land to the atmosphere, called "evapotranspiration," on a global scale. This phenomenon returns about 60 percent 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.
Most climate models have suggested that evapotranspiration would increase with global warming, because of increased evaporation of water from the ocean and more precipitation overall (water that can evaporate). The new research, published online this week in the journal Nature, found that's exactly what was happening from 1982 to the late 1990s.
But in 1998, this significant increase in evapotranspiration — about 0.3 inches (7 millimeters) of water per year — slowed dramatically or stopped.
In large portions of the world, soils are now becoming drier than they used to be, releasing less water and offsetting some moisture increases elsewhere.
Because the data only goes back for a few decades, the researchers say they can't be certain whether the change is part of the natural variability of climate or part of a longer-lasting global change. One possibility, though, is that on a global level, a limit to the acceleration of the hydrological cycle (the transfer of water between land, air and sea) on land has already been reached.
If that's the case, the consequences could be serious. They could include reduced terrestrial vegetation growth, less carbon absorption, a loss of the natural cooling mechanism provided by evapotranspiration, more 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 evapotranspiration over such a large area of the Southern Hemisphere," said study co-author Beverly Law, a professor of global change forest science at Oregon State University. "It is critical to continue such long-term observations, because until we monitor this for a longer period of time, we can't be sure why this is occurring."
Some of the areas with the most severe drying include southeast Africa, much of Australia, central India, large parts of South America, and some of Indonesia. Most of these regions are historically dry, but some are actually tropical rain forests.
The rather abrupt change from increased global evapotranspiration to a near halt in this process coincided with a major El Nino event in 1998, the researchers note in their report, but they are not suggesting that the El Nino is the cause for the phenomenon, since the dry-up has been going on for more than a decade now.
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