By 2050, groundwater pumping will cause a global sea level
rise of about 0.8 millimeters per year.
by
Dr. Nitish Priyadarshi
Slowly but surely, the sea level continues to rise. Recent
research suggests this increase is also driven by the exploitation of
underground water by humans that eventually flowed into the sea.
Climate change, with its associated melting ice caps and shrinking glaciers,
is the usual suspect when it comes to explaining rising sea levels. But a
recent study now shows that human water use has a major impact on sea-level
change that has been overlooked.
Science community was shocked by the claim that 42% of the
sea-level rise of the past decades is due to groundwater pumping for irrigation
purposes. What could this mean for the future – and is it true?
Global warming is melting glaciers and causing sea levels to
rise. The volume of water is also expanding because of heat. This ‘thermal
expansion’ contributes significantly to the surge in the sea levels. But there
is yet another important reason for the rising sea levels, as a team of
hydrologists led by Yadu Pokhrel from Rutgers University (USA) has discovered
for the first time.
Experts had already identified a flaw in existing models. If
one takes ice melting and the expansion of water because of higher temperatures
into account the oceans should have risen by 1.1 mm per year in the second half
of the 20th century. However in reality, they rose by 1.8 mm.
Groundwater makes up about twenty percent of the world's fresh
water supply, which is about 0.61% of the entire world's water, including
oceans and permanent ice. Global groundwater storage is roughly equal to the
total amount of freshwater stored in the snow and ice pack, including the north
and south poles. This makes it an important resource which can act as a natural
storage that can buffer against shortages of surface water, as in during times
of drought.
Most of the Earth’s liquid freshwater is found, not in lakes
and rivers, but is stored underground in aquifers. Indeed, these aquifers
provide a valuable base flow supplying water to rivers during periods of no
rainfall. The contribution from groundwater is vital; perhaps as many as two
billion people depend directly upon aquifers for drinking water, and 40 per
cent of the world’s food is produced by irrigated agriculture that relies
largely on groundwater.
During the last 30 to 40 years there has been an enormous
rise in food production in many countries through the increased use of
irrigation. Much of this irrigation water has been drawn from groundwater as
people realise the advantages to increased productivity of timely irrigation
and security of application.
Due to human usage, groundwater reaches the ocean through
the sewage system and rivers as well as the hydrological cycle in the
atmosphere- and contributes about 42 per cent of the rise in the sea levels.
Because of population growth and increased irrigation,
ground and drinking water consumption has doubled over the last few
decades.
A recent study from Yoshihide Wada and other researchers
from Utrecht University attempted to assess the status of global groundwater
depletion—that is, the amount of water that is being drawn out from underground
reservoirs that is not being replaced by precipitation—and came up with some
startling conclusions. Chief among them that depletion of groundwater may be
contributing to as much as 25 percent of observed sea-level rise in recent
years.
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.
Taking into account the seepage of groundwater back into the aquifers, as
well as evaporation and runoff, the researchers estimated that groundwater
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 amount of groundwater pumped out by Delhiites and others across northern India is highest in the world and is contributing as much as 5% to the total rise in sea levels.
A new study using satellite data has found that the region - a swathe of over 2,000km from west Pakistan to Bangladesh along north India - extracts a mind boggling 54 trillion litres from the ground every year, a figure that's likely to cause serious concern over the future of water availability.
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