Worldwide Rising Seas and the Greenwater Equation

November 2, 2010

Worldwide overpumping of groundwater, particularly in northern India, Iran, Mexico, northeastern China and the American West, more than doubled from 1960 to 2000 and is responsible for about 25 percent of the rise in sea level, according to estimates in a new study by a team of Dutch researchers published in Geophysical Review Letters.

The general idea that groundwater used for irrigation is running off into ocean-bound rivers or evaporating into the clouds, only to end up raining into the ocean, has been around for two decades or so; it was a focus of a 2005 paper in The Journal of Hydrogeology. But Peter H. Gleick, a leading expert on water issues, said the new paper offers a fresh way of quantifying the phenomenon.

Mr. Gleick, president of the Oakland-based Pacific Institute, said that experts on groundwater issues “have known for a long time that that water ultimately ends up in the oceans and contributes to sea level rise. What we haven’t known is the magnitude and severity of the problem.”

This study, by a team of researchers based at the University of Utrecht and the International Groundwater Resources Assessment Center in Utrecht in the Netherlands, suggests that, in Dr. Gleick’s words, “both the magnitude and the severity of the phenomenon are severe”: it estimates that groundwater depletion worldwide went from 99.7 million acre-feet (29.5 cubic miles) in 1960 to 229.4 million acre-feet (55 cubic miles) in 2000.

That volume is almost as much as the combined annual flows of the Ohio and Susquehanna Rivers, as measured by the United States Geological Survey. Put another way, it is 15 times the amount of water used annually by all the users of the Colorado River in the United States, from the cities of Los Angeles, Phoenix and Las Vegas to the farms of the California and Arizona deserts, which produce most of this country’s winter fruit and vegetables.

Barton H. Thompson Jr., a Stanford law professor who is co-director of the university’s Woods Institute for the Environment, said the Dutch study could help broaden the lens through which groundwater problems are examined.

“There has been growing recognition that it is not simply a local issue but at least a regional issue,” he said. “If you are living in an area where maybe you’re not depleting your groundwater but other people nearby are depleting theirs, eventually they are going to have to find other water. They may have to find it nearby, and that may be your water.”

What the new study suggests, he added, “is that groundwater depletion is a global problem. Now we have to worry that it’s also contributing to sea level rise. It changes the scale of the problem in a way that perhaps we haven’t thought about before.”

Both Dr. Gleick and Dr. Thompson emphasized the extent to which large agricultural regions in arid or semi-arid areas, from California’s San Joaquin Valley to the Ogallala Aquifer under the Great Plains the Yuncheng Basin in northern China, have become dependent on groundwater to grow the crops that sustain both livestock and people.

This dependence may make it hard to change current practices that lead to depletion, Dr. Gleick suggests.

“I do think there’s growing awareness of the seriousness of the groundwater over-pumping problem, but I think it’s going to take more than this wake-up call to change policy, because we’re hugely dependent on this unsustainable source of water,” he said, adding, “Forty percent of our groundwater withdrawals are coming from unsustainable sources of water.”

“By definition, unsustainable means it can’t continue forever. This water provides a lot of our food. And we’re basically drawing down the bank account.”

Groundwater’s contribution to sea-level rise will probably diminish, he added, because as groundwater basins are depleted, there won’t be as much water left to send through rain clouds to the oceans.