Harvard researchers propose a method for tinkering with ocean chemistry to lower carbon dioxide levels and combat climate change.

Mason Inman

Imagine a remote, rocky island with a huge chemical plant that extracts acid from the ocean, pours it onto the island’s rocks, and lets the runoff drain into the ocean.

It might sound at first like some mad scientist’s scheme. But it’s actually a serious plan by Harvard researchers for pulling carbon dioxide, the primary greenhouse gas, out of the air. The chemical process would lock up CO~2~ in the form of chalk that would be buried naturally at sea, so it could help lower carbon dioxide levels in the atmosphere and fight climate change.

Harvard researchers have an idea to exploit ocean chemistry to lower levels of greenhouse gases. (Source: Flickr_congress/2179930076/)

The amount of carbon dissolved in the oceans depends on the acidity of the water; if the oceans were less acidic, they would be able to take up more CO~2~. Kurt House, a Harvard graduate student in the earth and planetary sciences department, and colleagues have devised a way to remove acid from the ocean to drive and speed up this process.

“It’s a form of geoengineering, changing the chemistry of the oceans, in order to pull CO~2~ out of the air,” says House.

Carbon to chalk

The first step is to draw water from the sea and run it through an electrochemical plant. It would use electricity to drive reactions that pull hydrochloric acid out of the ocean. This in turn would enhance the ocean’s natural ability to soak up carbon dioxide and store it in the form of bicarbonate.

Over hundreds or thousands of years, the bicarbonate would react further to form insoluble calcium carbonate—or chalk—that would become buried under the ocean floor.

To neutralize the acid from the ocean, the plant would run it over rock, producing sand and salt that can be safely disposed of. The procedure would resemble natural processes of rock weathering, but put into overdrive.

Energy sink

However, the process requires a lot of energy to drive the electrochemical reactions. Unless it’s powered by low-carbon electricity—from wind, geothermal, or natural gas—it would be counterproductive. So the Harvard team proposes putting the chemical plants in remote regions where they could harness wind or geothermal energy that would otherwise go untapped.

One major question is how much the process can be scaled up. Even under the most optimistic projections, at most it could reduce about 10 percent of current CO~2~ emissions, says Michael Aziz, a Harvard materials scientist who worked with House on the idea. “So we can’t relax and all go out and buy SUVs,” he adds.

“Their process seems basically feasible,” says Ken Caldeira, a climate scientist with the Carnegie Institution for Science in Stanford, CA. “I think it’s relatively benign for the oceans.” He adds, however, “the main drawback is that it’s very energy intensive, so it’s likely to be very expensive.”

Peter Brewer, an ocean chemist with the Monterey Bay Aquarium Research Institute in California, is more circumspect about the idea. “It is a bit of a stretch—perhaps an A-plus for ingenuity and a C for practicality,” he says. One problem is that the process involves a number of complex steps that haven’t been shown to be manageable on large scales. “I doubt very much that this can be practical and cost effective,” Brewer says.

Pros and cons

Climate scientists have studied other ways of spurring the ocean to absorb more carbon dioxide, such as “fertilizing” it with iron to stimulate the growth of plankton, which take up CO~2~. House’s proposal would certainly cost more and require more energy, Caldeira says. But, he points out, “fertilization of the ocean with iron seems to be limited to, at most, absorbing about 2 percent of current emissions.” House’s idea is, in principle, more scalable and won’t interfere as much with natural ecosystems, he says. And, he adds, since it would work by reducing the acidity of seawater, it could counteract ocean acidification, a side effect of rising atmospheric CO~2~ levels that threatens marine life.

Despite the challenges, Aziz and House are pushing the idea forward. They say that a financial incentive to sequester carbon is first needed. “If and when a carbon tax or cap-and-trade system [for greenhouse gas emissions] creates a high enough price for carbon emissions, the story will be different,” Aziz says.