Two new studies flag an underreported factor in global ocean change

By Janet Raloff

Web edition: May 21, 2012

Pumping groundwater, some 70 percent of it to irrigate crops, has recently become a potent force in global sea-level rise, two new studies conclude.

It sounds obvious: Once brought to the surface, water will eventually run into the seas, says hydrologist Yoshihide Wada of Utrecht University in the Netherlands. But until now, most major assessments of factors affecting sea-level rise — such as those reported by the Intergovernmental Panel on Climate Change — ignored the role of groundwater extraction, he says. For instance, the IPCC has assumed that groundwater extraction would be largely balanced by river water impounded by dams.

Such an assumption was probably accurate until about 1980, says Yadu Pokhrel of Rutgers University in Piscataway, N.J. Since then, the balance has shifted, he says, and groundwater extraction is now developing into a growing force behind sea-level rise.

In recent years, sea level has been rising around 3.1 millimeters each year. Besides groundwater depletion, other major contributors include the melting of glaciers and polar ice fields, and the expansion of ocean water as it heats up. By 2003, groundwater removal was responsible for about 34 percent of that sea-level rise, Pokhrel and his colleagues report online May 20 in Nature Geoscience. They added all human water-use components to a computer program that projects climate impacts, assuming that any water demands not met by surface water came from groundwater sources.

Wada’s team also found groundwater pumping to be a bigger contributor than thought. By 2000, these researchers report online May 8 in Geophysical Research Letters, global groundwater depletion was causing sea levels to rise some 0.57 millimeters each year — around 18 percent of current annual sea-level increases and an amount of water roughly equivalent to twice as much as dams were holding back. This analysis, unlike Pokhrel’s, used globally recorded data on groundwater extraction.

This imbalance stands to only grow, Wada notes. His team adapted computer programs to, for the first time, project how humans’ water use would affect sea level through 2050. By then, the scientists calculate, reservoir impoundment would prevent about 0.1 millimeters rise in sea level, while groundwater depletion would increase sea level by more than eight times that amount.

The new studies confirm groundwater extraction as an important and growing factor in sea-level rise, says Leonard Konikow of the U.S. Geological Survey in Reston, Va. But he challenges the magnitude reported by the new studies. His own analysis, published last September in Geophysical Research Letters, pegged groundwater extraction’s impacts at only 13 percent of the recent increase.

Konikow notes that groundwater depletion must be estimated indirectly, such as by adding regional measurements of groundwater and pumping to larger-scale satellite and other data. He charges that the potentially more-simplistic techniques employed in the new papers overestimate groundwater pumping — and hence its sea level impacts.

That’s true, Pokhrel acknowledges. His and Wada’s approaches assume, for the sake of calculations, that groundwater reservoirs are infinite and affordable. Obviously, he says, that’s not true.

Wada agrees: “There are no data on global groundwater levels. So we assume in the future that there will be [sufficient] groundwater — which may not be true, in some cases, if groundwater levels become too low” or if farmers can no longer budget to tap increasingly deep sources.

 “Many people think that groundwater pumping effects will be small or localized,” Pokhrel says. But the new data, he says, show that cumulatively, these withdrawals are changing sea level globally.