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Engineered weathering process could mitigate global warming

November 08, 2007

Researchers at Harvard University and Pennsylvania State University have invented a technology, inspired by nature, to reduce the accumulation of atmospheric carbon dioxide (CO2) caused by human emissions.

By electrochemically removing hydrochloric acid from the ocean and then neutralizing the acid by reaction with silicate (volcanic) rocks, the researchers say they can accelerate natural chemical weathering, permanently transferring CO2 from the atmosphere to the ocean. Unlike other ocean sequestration processes, the new technology does not further acidify the ocean and may be beneficial to coral reefs.

The innovative approach to tackling climate change is reported in the Nov. 7 issue of the journal Environmental Science and Technology by Kurt Zenz House, a Ph.D. candidate in Harvard's Department of Earth and Planetary Sciences; Christopher H. House, associate professor of geosciences at Pennsylvania State University; Daniel P. Schrag, professor of earth and planetary sciences in Harvard's Faculty of Arts and Sciences, professor of environmental science and engineering in Harvard's School of Engineering and Applied Sciences, and director of the Harvard University Center for the Environment; and Michael J. Aziz, Gordon McKay Professor of Materials Science in Harvard's School of Engineering and Applied Sciences.

"The technology involves selectively removing acid from the ocean in a way that might enable us to turn back the clock on global warming -- removing CO2 directly from the atmosphere while simultaneously limiting the rate at which man-made CO2 emissions are acidifying the ocean," Kurt Zenz House says. "Essentially, our technology dramatically accelerates a cleaning process that Nature herself uses for greenhouse gas accumulation."

In natural silicate weathering, atmospheric carbon dioxide dissolves into fresh water, forming a weak carbonic acid. This acid is neutralized as rain water percolates through continental rocks, producing an alkaline solution of carbonate salts. The dissolution products eventually flow into the ocean, where the added alkalinity enables the ocean to hold the dissolved carbon instead of releasing it into the atmosphere. As weathering dissolves more continental rock, more carbon is permanently transferred from the atmosphere to the ocean and ultimately to the sediments.

"In the engineered weathering process we have found a way to swap the weak carbonic acid with a much stronger one (hydrochloric acid) and thus accelerate the pace to industrial rates," Kurt Zenz House says. "To minimize the potential for adverse side effects on the environment we combine it with other chemical processes, the net result of which is identical to the natural weathering process. As a result, the ocean's alkalinity would increase, enabling the uptake and storage of more atmospheric CO2 in the form of bicarbonate, the most plentiful and innocuous form of carbon already dissolved in the earth's waters. That means we may be able to safely and permanently remove excess CO2 in a matter of decades rather than millennia."

Unlike other climate engineering schemes that propose reflecting sunlight back into space to cool the planet, the weathering approach counteracts the continued ocean acidification that threatens coral reefs and their rich biological communities. Moreover, the process works equally well on all sources of CO2, including the two-thirds of human emissions that do not emanate from power plants, and could be run in remote locations and powered by stranded energy, such as geothermal and flared natural gas.

The team cautions, however, that while they believe their scheme for reducing global warming is achievable, implementation would be ambitious, costly, and would carry some environmental risks that require further study. Replicating natural weathering would involve building dozens of facilities, akin to large chlorine gas industrial plants, on coasts of volcanic rock.

"The least risky trajectory is to significantly cut our carbon dioxide emissions -- but we may not be able to cut them rapidly enough to avoid unacceptable levels of climate change," says Aziz. "If it looks like we're not going to make it, the 'House Process' has the potential to let us rescind a portion of those emissions while mitigating some of the chemical impacts the excess CO2 will have on the oceans. It won't be ready in time, though, if we wait until we're sure we'll need it before pursuing R&D on the technical and environmental issues involved."

Harvard University