Global average sea level rose at an
average rate of around 1.7 ± 0.3 mm per year over 1950 to 2009 and
at a satellite-measured average rate of about 3.3 ± 0.4 mm per year
from 1993 to 2009.
Two main factors
are known to contribute to observed sea level rise. The first is
thermal expansion: as ocean water warms, it expands. The second is
from the contribution of land-based ice due to increased melting.
The major store of water on land is found in glaciers and ice
sheets. Now there may be a third reason: groundwater extraction.
Groundwater extraction is more common in more arid regions of the
world, where there is less available surface water. It is used for
crop irrigation, drinking water or industrial purposes. Aquifer
levels will decline if over a prolonged period more groundwater is
extracted at more locations than can be replenished by means of
rainwater recharge. As a consequence, rivers and wetlands will run
dry and aquifer levels will fall to such a depth that pumping
becomes impossible. Aquifer depletion can eventually spell
ecological disaster or even lead to famine.
With the knowledge that most of the extracted groundwater eventually
winds up in the sea, the researchers at Utrecht University and
Deltares calculated the contribution groundwater extraction makes to
rising sea levels. Researcher Marc Bierkens says, “We calculated it
at eight-tenths of a millimetre per year. This is surprisingly large
when compared to the current annual rise in sea levels, which the
IPCC estimates at 3.1 mm.”
"Although the role of groundwater depletion in rising sea levels had
already been acknowledged, it was not addressed in the most recent
IPCC report due to a lack of reliable data to illustrate the
severity of the situation. Our study confirms that groundwater
depletion is, in fact, a significant factor."
The researchers also looked at a combination of information to
identify the areas in the world where groundwater extraction leads
to groundwater depletion. An estimate of the amount of groundwater
extracted annually in most of the world’s countries could be
obtained from a database of the International Groundwater Resources
Assessment Center (IGRAC), which is affiliated with Deltares.
Combining this information with the estimated demand for water,
based on population density and data on irrigated areas, the
researchers were able to produce a map of global groundwater
extraction. A water balance model was then used to map out global
groundwater aquifer recharge, i.e. precipitation that seeps through
the soil to recharge groundwater aquifers. By subtracting the
figures of the groundwater extraction map from the figures of the
groundwater aquifer recharge map, the researchers were able to
compile a map of global groundwater depletion.
According to Bierkens, "The study reveals that depletion is the most
acute in areas of India, Pakistan, the US and China, which are also
the regions without sustainable levels of food production and water
consumption and which are expected to experience major problems in
the long run."
Environment News Network