Water conservation may have some hitherto unknown benefits. It can chip in with its bit in the global fight against increasing carbon emissions.

A new study, accepted for publication in the American Geophysical Union’s journal Earth’s Future, for the first time showed that groundwater depletion releases a significant amount of carbon dioxide into the atmosphere — a fact so far overlooked by scientists who tabulate carbon sources.

It may be minuscule if compared with industrial emissions, but it still ranks among the top 20 sources of carbon emissions documented by the US Environmental Protection Agency (EPA) and the Intergovernmental Panel on Climate Change (IPCC), according to two hydrogeologists with Michigan State University (MSU) who authored the study.

The MSU scientists estimated groundwater depletion in the US could be responsible for releasing 1.7 million tonnes of carbon dioxide. This would mean the carbon dioxide emitted through groundwater depletion is comparable to the carbon generated from aluminium, glass and zinc production in the US, said the scientists.

The CO2 emissions relating to groundwater depletion could be much higher in India considering that most of parts of the country witness rampant exploitation of sub-surface water. But the numbers are not readily available.

Understanding all sources

“We were somewhat surprised that this hasn’t been accounted for in the literature and in the [EPA and IPCC] evaluations,” said David Hyndman, a co-author of the study, in a statement.

The authors are of the opinion that carbon release should be counted in even if it’s small. “Understanding all sources of carbon dioxide emissions is important for making accurate climate change projections and finding solutions,” they argued.

“It’s not going to change the way we think about global climate change. It’s just another factor involved that we need to consider,” said other co-author Warren Wood.

“This is an idea that a number of us have knocked around a little bit, but I think the approach here is really novel,” said Bill Simpkins, a hydrogeologist at Iowa State University in Ames, Iowa who was not involved in the study. “[Groundwater depletion] is certainly not a documented source that people feel obligated to put in their climate estimates.”

Rain falling from the sky contains the same amount of carbon dioxide as is present in the atmosphere. But soil carbon dioxide levels are up to 100 times greater than carbon dioxide levels in the atmosphere, because soil microbes degrade organic carbon into carbon dioxide. When rainwater hits the ground and percolates through Earth’s rocks and sediments, the water dissolves extra carbon produced by these microbes.

If left to its own devices, this carbon-rich water remains below ground for hundreds to thousands of years before surfacing in oceans or freshwater bodies. But humans are now extracting groundwater at an unprecedented pace to sustain a growing population. The US alone sucks up nearly 303 billion litres of water from the earth every day to supply drinking water and irrigate crops.

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