Think big: Australia's Nullarbor Plain (marked red), which stretches across 2000 km at its widest point, is the largest expanse of limestone on the planet.

Credit: Wikimedia

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NEW YORK: A new calculation suggests that adding vast quantities of limestone to the world's oceans could be an effective solution to climate change. Experts remain to be convinced, however.

The revised estimate is based on an existing 'planetary engineering' theory first mooted in 1995 by Haroon Kheshgi, a scientist with oil conglomerate Exxon Mobil. Kheshgi proposed that adding lime to seawater might help reduce atmospheric carbon dioxide (CO₂) levels, by reacting with the dissolved greenhouse gas and locking it away as calcium bicarbonate.

The process would also turn back the clock on ocean acidification, which is caused by excess CO₂ in the water, and poses a threat to the growth or corals and other shelled marine life.

The oceans currently take in approximately one third of the world's excess CO₂, making them the world's greatest carbon sink. They have acted as a buffer to man-made greenhouse gas emissions, absorbing around two billion tonnes a year, but scientists warn this buffering capacity is slowing down.

A new twist

Kheshgi's original proposal never gained ground, because of the expense and energy required – lime (calcium oxide) is produced by heating limestone (calcium carbonate) to 900ºC – and because the process produces CO₂.

Now, an article in the July issue of the U.K. trade magazine Chemistry and Industry resurrects the idea, but with a new twist. The concept is to use renewable energy sources, or those that cannot be exploited economically, to mine the limestone and power the process that produces lime.

According to Tim Kruger, a management consultant at the London firm Corven, who made the new estimate, the addition of lime to seawater could theoretically absorb twice as much CO₂ as is created in the mining process, making it carbon negative.

"The value of energy is location dependent, and our approach is to use energy that is currently unused," said Kruger.

He suggests using cheap energy sources, such as natural gas already burnt-off from in areas so remote as to prevent economical transport, or solar energy from deserts. The energy source would also have to coincide with large limestone deposits available to mine.

Kruger, who is a scientist by training, cites Australia's Nullarbor Plain as one obvious location that meets the requirements. The desert is home to approximately 10,000 km³ of limestone available for mining and 20 MJ/m² of solar energy hits the Earth's surface there daily.