Researchers writing in Geophysical Research Letters say groundwater, once pumped to the surface for use, doesn't just seep back into the ground but eventually ends up in the world's oceans.
"Other than ice on land, the excessive groundwater extractions are fast becoming the most important terrestrial water contribution to sea level rise," lead study author Yoshihide Wada of Utrecht University in the Netherlands said.
Sea level rise caused by groundwater pumping from 1970 to 1990 was canceled out as people built dams, where water was trapped instead of emptying into the sea, Wada said. His research shows that changed in the 1990s as populations started pumping more groundwater and building fewer dams.
Earlier researchers didn't factor groundwater into sea level rise because estimates for those sources were too uncertain, he said.
"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."
Wada estimates by 2050, the cumulative effect of non-ice, land-based water sources will have added 1.2 inches to sea level rise since 1900.