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.

Interest from industry

Kruger's calculations estimate that 500 km³ of limestone would be needed in order to take the CO₂ levels back to pre-industrial levels, although "the exact amount will depend on the source of the heat used to drive the process," he said.

If the calculations are correct, 1.5 km³ of limestone would be enough to sequester 1 billion tonnes of carbon dioxide. Working at the rate of 1 km³ per year, it would therefore take 750 years alone to take current levels of CO₂ in the atmosphere back to historic levels.

Despite the seemingly implausible scale and potential expense of the project, Kruger said that he has already had some interest from industry. Multinational oil company Shell is now funding several aspects of research on the feasibility of the project. These include exploring the potentially dangerous impacts on marine life and the overall economic feasibility.

"We think it's a promising idea," said Shell's Gilles Bertherin, who is involved with the project. "There are potentially huge environmental benefits from addressing climate change – and adding calcium hydroxide to seawater will also mitigate the effects of ocean acidification, so it should have a positive impact on the marine environment."

Some scientists continue to have grave reservations about the concept though – as many do for other planetary-scale engineering projects.

Miscalculation

Daniel Schrag, a climate scientist at Harvard University in Cambridge, U.S., said that despite Kruger's revisions, the concept remains "fundamentally flawed."

"In theory it is a great idea. The problem is that the lime is so basic that it is impossible to dilute it enough to keep precipitation from happening immediately," he said. "It's possible on a small scale, but as soon as one wants to do enough to make a difference to the climate problem, it becomes impossible to prevent immediate precipitation."

Schrag added that the calculation that lime could absorb twice the CO₂ created in the mining process is a "miscalculation" and that the saturation factor is closer to one. If proved correct, this means that the process would not be carbon negative at all.

Regardless of the criticism, Kruger said the next is to test the theory using both a large-scale aquarium and computer models to estimate the possible effects on the world's oceans.

Barry Brook, director of the Research Institute for Climate Change and Sustainability at the University of Adelaide in South Australia, agreed that, "there are risks, but we need to be trailing such ideas."

He added that "we are already in the danger zone, and may need to geo-engineer our way out of it," in addition to drastically cutting greenhouse gas emissions. Brook himself is a supporter of the idea to bury CO₂ in the deep oceans.