As people pump ground water 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, ground water pumping will cause a global sea level rise of about 0.8 millimeters per year.
"Other than ice on land, the
excessive ground water 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 notes, ground water 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.
Between around 1970 and 1990, sea level
rise caused by ground water pumping was cancelled out as people built dams,
trapping water in reservoirs so the water wouldn't empty into the sea, Wada explains.
His research 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 ground water 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. Wada
and his colleagues 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 ground water pumping alone.
The research team's article is published
inGeophysical Research Letters, a journal of the American Geophysical
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 says.
But it didn't quantify the future contribution from other terrestrial water
sources, such as ground water, reservoirs, wetlands and more, he reveals,
because the report's authors thought the estimates for those sources were too
"They assumed that the positive and
negative contribution from the ground water and the reservoirs would cancel
out," Wada says. "We found that wasn't the case. The contribution
from the ground water is going to increase further, and outweigh the negative
contribution from reservoirs."
In the current study, the researchers
estimated the impact of ground water depletion since 1900 using data from
individual countries on ground water pumping, model simulations of ground water
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 ground water storage.
With these ground water depletion rates,
Wada and his colleagues estimate that in 2000, people pumped about 204 cubic
kilometers (49 cubic miles) of ground water, most of which was used for
irrigation. Most of this, in turn, evaporates from plants, enters the
atmosphere and rains back down. Taking into account the seepage of ground water
back into the aquifers, as well as evaporation and runoff, the researchers
estimated that ground water pumping resulted in sea level rise of about 0.57 mm
in 2000 – much greater than the 1900 annual sea level rise of 0.035 mm.
The researchers also projected ground water
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 ground water 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 ground water pumping, marsh drainage,
dams and more – will have added 31 mm to sea level rise since 1900.
The new study assumes that, where there
is ground water, people will find a way to extract it, Wada says, but some of
his colleagues are investigating the limits of ground water extraction. One way
to decrease ground water's contribution to sea level rise, he notes, is to
improve water efficiency in agriculture – to grow more with less ground water.