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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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.