Water stress & climate change

In Ground Water and Climate Change, international scientists explain how several human-driven factors, if not rectified, will combine with climate change to significantly reduce useable groundwater availability for agriculture globally. The paper was published in the journal Nature Climate Change.

The authors note that inadequate groundwater supply records and mathematical models for predicting climate change and associated sea-level-rise make it impossible to forecast groundwater's long-range fate globally. "Over-pumping of groundwater for irrigation is mining dry the world's ancient Pleistocene-age, ice-sheet-fed aquifers and, ironically, at the same time increasing sea-level rise, which we haven't factored into current estimations of the rise," says a team member. "Groundwater pumping reduces the amount of stored water deep underground and redirects it to the more active hydrologic system at the land-surface. There, it evaporates into the atmosphere, and ultimately falls as precipitation into the ocean."

Current research estimates oceans will rise by about a metre globally by the end of the century due to climate change. But that estimation doesn't factor in another half-a-centimetre-a-year rise, says this study, expected due to groundwater recycling back into the ocean globally.

Increasing climate-change-induced storm surges will also flood coastal areas, threatening the quality of groundwater supplies and compromising their usability. This is the second study from the team to assist the Intergovernmental Panel on Climate Change (IPCC) in assessing the impact of climate change on the world's groundwater supply.

Running out of fresh water

In the last few decades, the rate at which humans worldwide are pumping dry the vast underground stores of water that billions depend on has more than doubled. Not only are we running out of fresh water but also adding to sea-level rise!

In the new study, which compares estimates of groundwater added by rain and other sources to the amounts being removed for agriculture and other uses, the team taps a database of global groundwater information including maps of groundwater regions and water demand. The rate at which global groundwater stocks are shrinking has more than doubled between 1960 and 2000, increasing the amount lost from 126 to 283 cubic kilometers (30 to 68 cubic miles) of water per year.

The new assessment shows the highest rates of depletion in some of the world's major agricultural centers, including northwest India, northeastern China, northeast Pakistan, California's central valley, and the midwestern United States.

Today, people are drawing so much water from below that they are adding enough of it to the oceans (mainly by evaporation, then precipitation) to account for about 25 percent of the annual sea level rise across the planet, the researchers find. The team estimates the contribution of groundwater depletion to sea level rise to be 0.8 millimeters per year, which is about a quarter of the current total rate of sea level rise of 3.1 millimeters per year. That's about as much sea-level rise as caused by the melting of glaciers and icecaps outside of Greenland and Antarctica.

Groundwater represents about 30 percent of the available fresh water on the planet, with surface water accounting for only one percent. The rest of the potable, agriculture friendly supply is locked up in glaciers or the polar ice caps. This means that any reduction in the availability of groundwater supplies could have profound effects for a growing human population. Especially since we have no idea of how much groundwater exists!