EARTH鈥橲 water cycle has been pushed to its limit. The amount of water transported from the land into the atmosphere hit a maximum 12 years ago and is now in decline, new calculations show.
Martin Jung of the Max Planck Institute for Biogeochemistry in Jena, Germany, and colleagues calculated trends in evapotranspiration 鈥� the amount of water vapour that entered the atmosphere 鈥� between 1982 and 2008. This moisture is either evaporated by the sun鈥檚 heat or released by plants.
Evapotranspiration rose steadily until 1998, as would be expected in a warming global climate. But from then on, the amount of moisture being cycled into the atmosphere began to drop (Nature, ).
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Team member Steven Running of the University of Montana in Missoula says that in regions such as parts of Australia, the increased evaporation as temperatures rise has left the ground parched. Though the moisture returns to the ground as rain, most of it falls elsewhere, leaving the arid areas unable to contribute to the cycle.
鈥淚n some regions, the increased evaporation as temperatures rise has left the ground parched鈥�
The calculations are supported by satellite measurements that show falling levels of soil moisture in many parts of the world. The trend is strongest in the southern hemisphere. 鈥淕lobally, we鈥檙e seeing larger and longer droughts,鈥� says Running.
Martin Wild of the Swiss Federal Institute of Technology in Zurich says rising temperatures are a plausible explanation, but adds that trends in air pollution, which blocks solar energy, could also be to blame. Pollution fell throughout the 1990s, thanks to measures to improve air quality in the developed world. This increased the amount of sunlight reaching the ground and boosted evaporation. But these changes levelled off at the end of the decade, around the same time evaporation stopped rising.