Water cycle goes bust as the world gets warmer

Earth's water cycle has been pushed to its limit. The amount of water evaporating off the land and into the atmosphere hit a maximum 12 years ago and is now in decline, new calculations show.

We have only a limited amount of water. The same drops keep moving in a circle from the Earth to the sky and then back to the Earth again as rain or snow. Water disappears from the Earth's surface, turns into clouds, falls again as rain or snow. It then runs back to lakes and rivers, the oceans and even sometimes makes its way underground.

Martin Jung of the Max Planck Institute for Biogeochemistry in Jena, Germany, and colleagues calculated trends in evapo-transpiration – the amount of water vapor that entered the atmosphere – between 1982 and 2008. This moisture is either evaporated off the land by the sun's heat or released by plants.

Evapotranspiration (ET) is the combined process of plant transpiration and soil evaporation. Plant transpiration is the movement of moisture from the plant to the air through tiny pores in the leaves known as stomates. The water enters the plants through the roots in a liquid form and leaves the plants through the stomates in a gaseous form. Soil evaporation is the direct evaporation of water from the surface of the soil into the atmosphere.

They found that evapo-transpiration rose steadily until 1998, as would be expected in a warming global climate. But the trend reversed in 1998, and the amount of moisture being cycled into the atmosphere began to drop.

Team member Steven Running of the University of Montana in Missoula says that in some regions, rising temperatures have sucked all the available water out of the ground. Though that moisture returns to the ground as rain, most of it falls elsewhere, leaving dry regions like Australia parched.

Unmet demand

The team say that because the atmosphere is now warmer, it is able to hold more water, effectively trapping it there. That leaves less water on the ground available to maintain the cycle. "There's demand but no supply," Running explains.

This picture is backed up by satellite measurements showing falling levels of soil moisture globally. The trend is strongest in the southern hemisphere, particularly in South America and Australia. "Globally, we're seeing larger and longer droughts," 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 evapo-transpiration. But these changes leveled off at the end of the decade, around the same time evapo-transpiration stopped rising.

Source:
New Scientist, "Water cycle goes bust as the world gets warmer", accessed October 12, 2010
Journal reference: Nature, DOI: 10.1038/nature09396