Land a better sponge than previously thought

A new study has shown that the Earth’s continents have soaked up and retained an extra 3.2 trillion tonnes of water in soils, lakes and underground aquifers over the past decade.

The finding flies in the face of perceived wisdom. “We always assumed that people’s increased reliance on groundwater for irrigation and consumption was resulting in a net transfer of water from the land to the ocean,” said the research’s lead author, JT Reager, of the Jet Propulsion Laboratory, in Pasadena, California.

“What we didn’t realise until now is that over the past decade, changes in the global water cycle more than offset the losses that occurred from groundwater pumping, causing the land to act like a sponge, at least temporarily,” Reager said.

There is a continuous cycle on Earth, known as the water cycle, in which a large amount of water evaporates from the oceans, falls over land as rain or snow, and returns to the oceans through runoff and river flows.

Atmospheric warming has increased the evaporation from the oceans by 5 percent between 1894 and 1985, but it was not expected that the land would hold more of the rain.

The water gains over land were spread globally and have temporarily slowed the rate of sea level rise by about 20 percent.

The amount of water involved is about 3,500 cubic kilometres, not an easy figure to imagine. It is approximately half of Lake Malawi, the fourth biggest lake in the world in terms of volume, or all of Lake Huron, the third largest of the US Great Lakes.

This water retention has increased the mass of the continents, which in turn affects the gravitational field of the Earth. It is this change that can be detected by an alteration to the pull exerted on satellites.

The combined teams at the Jet Propulsion Laboratory, NASA and the University of California, Irvine, used the twin gravity recovery and climate experiment, or GRACE, satellites to obtain these results. The satellites, launched in March 2002, operate as a collaboration between US and German space agencies.

The observable facts are that drought remains a serious and increasing issue in many countries and weather patterns are changing.

It also seems likely that the increased storage of water within land is not in the places where mankind most needs it.