Huge Parts of World Are Drying Up: Land 'Evapotranspiration' Taking Unexpected Turn
ScienceDaily (Oct. 11, 2010) — The soils in large areas of the Southern Hemisphere, including major portions of Australia, Africa and South America, have been drying up in the past decade, a group of researchers conclude in the first major study to ever examine "evapotranspiration" on a global basis.
The soils in large areas of the Southern Hemisphere, including major portions of Australia, Africa and South America, have been drying up in the past decade, a group of researchers conclude in the first major study to ever examine "evapotranspiration" on a global basis. (Credit: iStockphoto/Domenico Pellegriti)
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 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 -- which had been seven millimeters 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.
If that's the case, the consequences could be serious.
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Letter
Nature advance online publication 10 October 2010 | doi:10.1038/nature09396; Received 7 October 2009; Accepted 6 August 2010; Published online 10 October 2010
Recent decline in the global land evapotranspiration trend due to limited moisture supply
Martin Jung et al.
Correspondence to: Martin Jung1 Email: mjung@bgc-jena.mpg.de
More than half of the solar energy absorbed by land surfaces is currently used to evaporate water1. Climate change is expected to intensify the hydrological cycle2 and to alter evapotranspiration, with implications for ecosystem services and feedback to regional and global climate. Evapotranspiration changes may already be under way, but direct observational constraints are lacking at the global scale. Until such evidence is available, changes in the water cycle on land—a key diagnostic criterion of the effects of climate change and variability—remain uncertain. Here we provide a data-driven estimate of global land evapotranspiration from 1982 to 2008, compiled using a global monitoring network3, meteorological and remote-sensing observations, and a machine-learning algorithm4. In addition, we have assessed evapotranspiration variations over the same time period using an ensemble of process-based land-surface models. Our results suggest that global annual evapotranspiration increased on average by 7.1 ± 1.0 millimetres per year per decade from 1982 to 1997. After that, coincident with the last major El Niño event in 1998, the global evapotranspiration increase seems to have ceased until 2008. This change was driven primarily by moisture limitation in the Southern Hemisphere, particularly Africa and Australia. In these regions, microwave satellite observations indicate that soil moisture decreased from 1998 to 2008. Hence, increasing soil-moisture limitations on evapotranspiration largely explain the recent decline of the global land-evapotranspiration trend. Whether the changing behaviour of evapotranspiration is representative of natural climate variability or reflects a more permanent reorganization of the land water cycle is a key question for earth system science.
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