People around the world are pumping so much water out of the ground, and releasing it back into the environment, that it's causing oceans to rise, a new study reveals. The effect is bigger than you might expect. The research estimates that by 2050, seas will rise about one-third of an inch (0.8 mm) per year due to groundwater pumping, and it may rival melting glaciers as a primary cause for rising seas.

Groundwater pumped for irrigation, drinking water and industrial uses does not typically end up back underground. Instead, it flows into streams or rivers or evaporates into the atmosphere, eventually finding its way to the ocean.

Other studies have shown that many aquifers — natural underground lakes that have built up water over millions of years — are being pumped dry. The ground tends to compact when the water is extracted, and once pumped dry, often an aquifer can never store as much water as it once did — sort of like a sponge that’s lost its sponginess.

“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, with Utrecht University in the Netherlands and lead author of the study, detailed in Geophysical Research Letters, a journal of the American Geophysical Union.

The researchers estimate that in the year 2000, humans pumped about 49 cubic miles (204 cubic kilometers) of groundwater. Most was used for irrigation.

In the coming decades, Wada said 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. Already rising seas are causing some islands to disappear. In March, a study found that 4 million Americans are threatened by rising seas.

Between around 1970 and 1990, sea level rise caused by groundwater pumping was cancelled out by the construction of dams, which trap water in reservoirs so the water so less water goes to the sea, Wada explained. A study in 2008 confirmed this effect.

Wada’s research shows that starting in the 1990s, this changed as humans began pumping more groundwater and building fewer dams.

The researchers looked not only at the contribution of groundwater pumping, which they had investigated before, but also at other factors that influence the amount of terrestrial water entering the oceans, including marsh drainage, forest clearing, and new reservoirs. They calculate that by mid-century, the net effect of these additional factors is an additional 0.05 mm per year of annual sea level rise, on top of the contribution from groundwater pumping alone.

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.

In the current study, the researchers estimated the impact of groundwater depletion since 1900 using data from individual countries on groundwater pumping, model simulations of groundwater recharge, and reconstructions of how water demand has changed over the years. They also compared and corrected those estimates with observations from sources such as the GRACE satellite, which uses gravity measurements to determine variations in groundwater storage.