A scheme to dump quicklime into the oceans to sequester more carbon in their depths is being revived by a British management consultant with backing from Shell.

First proposed back in the '90s by Exxon engineer Haroon Kheshgi (.pdf), the idea takes advantage of a series of simple chemical reactions. Limestone, at high temperatures, breaks down into carbon dioxide and quicklime, in a process that produces greenhouse gas. But dump that quicklime in seawater, and it absorbs roughly twice as much CO2 as was released in the first reaction.

The heat required to decompose the limestone will probably come from fossil fuel, generating more CO2, but even so, the sum of the process could be a reduction of the CO2 in the atmosphere.

"If we discover we've overshot the amount of CO2 the environment can cope with, the carbon-negative process I'm describing can reduce the amount of carbon dioxide in the atmosphere," said Tim Kruger, founder of Cquestrate.com, which has drawn seed funding from Shell and bills itself as developing an open source solution to climate change.

Geoengineering projections have shown that it might be possible to stop the warming of the Earth, but the workable ones have had a big problem: the oceans. While schemes like shooting sulfur dioxide into the stratosphere to deflect some of the sun's energy could cool the Earth, they don't deal directly with the problem of carbon dioxide in the atmosphere.

Regardless of the greenhouse effect, CO2 buildup will lead to ocean acidification, which could wipe out coral reefs and lead to large-scale oceanic ecosystem collapse.

The quicklime scheme is different. It would go right at the heart of the CO2 buildup problem by removing the gas from the air and sequestering it in the world's oceans. It also makes the oceans more alkaline, directly combating ocean acidification.

Of course, the scale of the project would have to be eye-poppingly large. The early calculations, Kruger told Wired.com, indicate that

56 billion cubic feet of limestone would be required to sequester each gigaton of carbon. Humans put out about 5.5 billion tons of carbon annually by burning fossil fuels, so a limestone offset budget could reach 300 billion cubic feet of limestone per year.

The U.S. Geological Survey estimates limestone reserves as adequate for every country in the world. This scheme, however, would require a major ramp-up in lime production from the 300 million tons now produced in the world.

The energy requirements would also be enormous. Kruger estimates that 2.7 gigajoules of energy will be required for the conversion of each ton of limestone. Multiplied out, his numbers suggest the equivalent of 10 billion barrels of oil would be necessary to generate the heat for decomposing those billions of cubic feet of limestone – although oil would not be the fuel of choice.

Where would that energy come from in a world of rising energy prices? Kruger said that the first place would be "stranded energy," like the natural gas that is flared during oil recovery. The World Bank estimates that 3 trillion cubic feet of natural gas are flared each year. That amount of gas translates into about 3.3 billion exajoules of energy, or about one-third of the energy required to decompose enough limestone to offset all of the carbon emissions generated by human fossil-fuel use. Kruger said that solar thermal and nuclear power plants could be other heat-generating options.

Still, geoengineering experts are skeptical that securing enough energy for the process to scale up will be possible.

"The basic problem is that there is not all that much stranded energy around, so it is at best a niche opportunity," Ken Caldeira, the Stanford professor and geoengineering expert, wrote on the Geoengineering Google Group.

Environmental advocates, too, worry about unforeseen deleterious effects on the ocean ecosystems near lime deposition points.

EcoGeek blogger Hank Green even compared the scheme to a lobotomy for the Earth, opining, "Pouring many tons of calcium hydroxide into the oceans in an attempt to decrease the amount of CO2 in the atmosphere is akin to shoving a rod into your brain and hoping you come out the other side a happier person."

Logistically, the idea would have to work work at the global scale, but use local supplies of limestone that happen to be located near natural gas fields or solar-plant filled deserts. Kruger said that was necessary to make the process cost-effective while remaining net carbon-negative.

"The important point is to locate the process where the energy is cheap," Kruger said. "That's the only way that this is going to be economically feasible."

Whether or not the scheme works, it's likely to spark more debate about how human beings should be approaching the dual problems of ocean acidification and climate change. You can join the official debate around Kruger's idea at Cquestrate.

"I want to make sure that this succeeds, or that if it doesn't succeed, it fails quickly," he said. "Getting people to contribute information, we'll see if there is some fundametal flaw in the idea."

Image: A limestone quarry in Malta. Thanks, flickr user DBarefoot!

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