By 2050, groundwater pumping will cause a global sea level rise of about 0.8 mm per year, says a new study.
Pumping groundwater for irrigation, drinking and industrial uses, doesn't just seep back into the ground, it also evaporates into the air or runs off into rivers and canals, eventually emptying into the world's oceans - this water adds up, the study says.
"Other than ice on land, the excessive groundwater extractions are fast becoming the most important terrestrial water contribution to sea level rise," said Yoshihide Wada, from Utrecht University in the Netherlands, who led the study, the journal Geophysical Research Letters reports.
Wada and colleagues estimate that in the year 2000, people pumped about 204 cubic km of groundwater, most of which was used for irrigation.
Most of this, in turn, evaporates from plants, enters the atmosphere and rains back down, according to a Utrecht statement.
In the coming decades, researchers noted, groundwater contributions to sea level rise are expected to become as significant as those of melting glaciers and ice caps outside of Greenland and the Antarctic.
Between 1970 and 1990, sea level rise caused by groundwater pumping was cancelled out as people built dams, trapping water in reservoirs so the water wouldn't empty into the sea, Wada said.
His research shows that starting in the 1990s, that changed as populations started pumping more groundwater and building fewer dams.
The last report of the United Nations Intergovernmental Panel on Climate Change in 2007 addressed the effect on sea level rise of melting ice on land, including glaciers and ice caps, Wada said.
But it didn't quantify the future contribution from other terrestrial water sources, such as groundwater, reservoirs, wetlands and more, he said, as the report's authors thought the estimates for those sources were too uncertain.
"They assumed that the positive and negative contribution from the groundwater and the reservoirs would cancel out," Wada said.
"We found that wasn't the case. The contribution from the groundwater is going to increase further, and outweigh the negative contribution from reservoirs."