Groundwater depletion is fast becoming the most important contribution of terrestrial water to sea-level rise. It currently outweighs the negative contribution of storage behind dams and will be of the same magnitude as the present contribution of mountain glaciers. This is the main conclusion form a study by a team of researchers, led by Utrecht University scientists, that was recently published in Geophysical Research Letters.
The scientists reconstructed the global effect of groundwater over-exploitation on sea-level rise during the 20th century and, for the first time, provided projections of the effect into the 21st century and placed their results in context with other major sources of terrestrial storage change.
Groundwater pumping and sea level rise
As people pump groundwater for irrigation, drinking water, and industrial uses, the water doesn’t just seep back into the ground — it also evaporates into the atmosphere, or runs off into rivers and canals, eventually emptying into the world’s oceans. This water adds up, and a new study calculates that by 2050, groundwater pumping will cause a global sea level rise of about 0.8 millimeters per year. “Other than ice on land, the excessive groundwater extractions are fast becoming the most important terrestrial water contribution to sea level rise,” says Yoshihide Wada, with Utrecht University in the Netherlands and lead author of the study. In the coming decades, he 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.
Groundwater and other terrestrial water sources
Rens van Beek, one of the co-authors, adds: “Between around 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.” The study shows that starting in the 1990s, that changed as populations started 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.
Future sea level rise
Marc Bierkens, who lead the research team, notes: “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”. “But it didn’t quantify the future contribution from other terrestrial water sources, such as groundwater, reservoirs, wetlands and more, he said, because the report’s authors thought the estimates for those sources were too uncertain. “Also, they assumed that the positive and negative contribution from the groundwater and the reservoirs would cancel out,” Wada adds. “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 says. The researchers also projected groundwater depletion, reservoir storage, and other impacts for the rest of the century, using climate models and projected population growth and land use changes. The increase in groundwater depletion between 1900 and 2000 is due mostly to increased water demands, the researchers find. But the increase projected between 2000 and 2050 is mostly due to climate-related factors like decreased surface water availability and irrigated agricultural fields that dry out faster in a warmer climate. If things continue as projected, Wada estimates that by 2050, the net, cumulative effect of these non-ice, land-based water sources and reservoirs — including groundwater pumping, marsh drainage, dams, and more — will have added 31 mm to sea level rise since 1900.
Publication
Y. Wada, L. P. H. van Beek, F. C. S. Weiland, B. F. Chao, Y.-H. Wu, and M. F. P. Bierkens (2012), Past and future contribution of global groundwater depletion to sea-level rise. Geophysical Research Letter, 39, L09402, doi:10.1029/2012GL051230.
More information
Roy Keeris, Press office Utrecht University, +31 (0)30 253 2411, r.b.keeris@uu.nl.