Study Downgrades Groundwater Contribution to Sea Level Rise

Groundwater pumping for agriculture and other uses has risen sharply. But a new study says it isn’t contributing as much as previously thought to sea level rise.

(First published on May 3, 2016, on State of the Planet.)

Some research suggests that, along with melting ice sheets and glaciers, the water pumped from underground for irrigation and other uses, on the rise worldwide, could contribute substantially to rising sea levels over the next 50 years. A new study published in Nature Climate Change says the magnitude is much lower than previously estimated.

The question of how much groundwater depletion will contribute to sea level rise is “highly uncertain and heavily debated,” note the authors of the study, published online May 2. Oceans already are rising at a rate of just over 3 millimeters a year due to the expansion of seawater warmed by climate change and the runoff from melting ice sheets and glaciers. But exactly how much other groundwater actually winds up in the oceans and contributes to sea level rise is not well known.

Yoshihide Wada, from the NASA Goddard Institute for Space Studies and the Center for Climate Systems Research, and his colleagues used a model simulation based on information about hydrological and climate systems and found the contribution to sea level rise from groundwater depletion increased from 0.02 millimeters a year to 0.27 millimeters a year between 1900 and 2000. While that’s more than a ten-fold increase, it means that existing studies “have substantially overestimated” how much groundwater depletion is contributing to sea level rise “by a cumulative amount of at least 10 millimeters during the 20th century and early 21st century.” That includes some of Wada’s own work. His earlier studies relied on a hydrology-based model, he said, while this time, the model combined information about hydrology, oceans and atmosphere that accounted better for climate processes and feedbacks.

Schematic diagram of global water budget over the land and the ocean. The values (in km3 yr−1) show the long-term averages for a control (CTR) or natural run without pumping (a) and a GWD run with pumping (b) over the period 1900–1999. From Wada et al., 2016.

“Land water contribution including groundwater has been overestimated by a factor of three by the latest [Intergovernmental Panel on Climate Change] AR5 report,” said Wada, lead author of the study. “When using our new estimate, the land water contribution becomes negative (−0.10 mm/year) over the period 1971-2010, while the IPCC estimate was positive (0.12 mm/year). As a result, the residual of the global seal level budget becomes larger, which means that we can’t explain an even wider gap in the observed and modeled global sea level budget.

“Projecting accurate sea level rise is important,” Wada said, because rising seas can threaten tens of millions of people living on the world’s coasts and on small islands with increased flooding, storm surges and immersion. The impact on coastal infrastructure is likely to be hugely destructive and expensive. Higher sea levels will also threaten vital ecosystems such as mangrove forests and coral reefs, along with sources of freshwater.

Groundwater depletion results from people pulling water from reserves such as underground aquifers faster than the water is replenished, for both agriculture and to provide water for growing urban areas. Other factors come into play as well: drainage of lakes and wetlands, deforestation and changes in soil moisture, permafrost and snow. Construction of new dams that increase the amount of water held back offsets some of the loss. And not all groundwater pumped from aquifers runs off into or ends up in the oceans. Some is evaporated and returns to earth—or falls into the ocean—as precipitation. Calculating the net results involves some very complex calculations.

The researchers found that about 80 percent of groundwater depletion winds up in the oceans, compared to earlier assumptions of nearly 100 percent. Most gets there by runoff; a third through evaporation and precipitation.

Other factors have been far more significant in raising sea levels: melting ice sheets on Greenland and West Antarctica, and shrinking glaciers around the world. Current estimates suggest we could see the oceans rise by at least 3-4 feet by the end of the century if nothing is done to curb the use of fossil fuels, which discharge CO2 into the atmosphere and add to warming global temperatures.

Wada said the new study accounts more accurately for hydrological and atmospheric processes than previous studies. The bottom line: The increasing depletion of groundwater for expanded agricultural activity, as well as other uses, is adding to sea level rise, but much less than from other sources.

But, Wada noted, the increase in use of groundwater is itself a big problem. He compared it to money kept in a bank account: “If you withdraw money at a faster rate than you deposit new money, you will eventually start having account-supply problems.

“The volume of groundwater in storage is decreasing in many areas of the world in response to excessive pumping. We are using groundwater unsustainably that there might not be enough groundwater to use for food production for growing population. Groundwater depletion can also cause severe environmental problems like reduction of water in streams and lakes, deterioration of water quality, increased pumping costs, and land subsidence,” he said.

Wada’s co-authors are Min-Hui Lo of National Taiwan University; Pat J.-F. Yeh of the National University of Singapore; John T. Reager of the NASA Jet Propulsion Laboratory in Pasadena, Calif.; James S. Famiglietti of the Jet Propulsion Laboratory and the University of California, Irvine; Ren-Jie Wu of National Taiwan University; and Yu-Heng Tseng of the National Center for Atmospheric Research in  Boulder, Colo.

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