Scientists at Oregon State University have observed a severe decrease of evapotranspiration in dry areas which could lead to an acceleration in the drying up of the land. Photo: Sewell/Oculi/VU/laif
The soils in large areas of the southern hemisphere, including major parts of Australia, Africa and South America, have been drying up in the past decade, a group of international researchers conclude in the first major study to ever examine "evapotranspiration" on a global basis. Most climate models have suggested that evapotranspiration, which is the movement of water from the land to the atmosphere, would increase with global warming. The new research, published online 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 – which had been seven millimetres 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.
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. But one possibility is that on a global level, a limit to the acceleration of the hydrological cycle on land has already been reached.
The consequences could be serious say the scientists. 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.
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.
Evapotranspiration 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.
Longer term observations will be needed to determine if these changes are part of decadal-scale variability or a longer-term shift in global climate, the researchers said.
(wi)
Source: www.scienceinafricaafrica/Oregon State University
