Monday, December 17, 2012

Groundwater exploitation is also raising sea levels.

By 2050, groundwater pumping will cause a global sea level rise of about 0.8 millimeters per year.

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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