A new method for combatting climate change feels like a bit of modern-day alchemy: scientists have figured out how to take carbon dioxide out of the ocean and turn it into harmless rock.<\/p>\n
For every tonne of carbon dioxide we pump into the air, roughly a quarter<\/a> of it gets absorbed by the ocean like a giant, watery sponge. All of this excess carbon dioxide is acidifying the water and threatening organisms, such as those with calcium carbonate shells, that are sensitive to the change.<\/p>\n To avert this fate, carbon emissions need to drop\u2014fast. But many scientists also believe that active carbon capture\u2014deliberately pulling carbon dioxide out of the environment\u2014will be a necessary step to help curb, and potentially even reverse, the rise in emissions responsible for countless environmental impacts. However, capturing enough carbon to make a difference is a massive task, one that has so far proved challenging and expensive<\/a>.<\/p>\n \u201cYou\u2019re talking about removing some 10 to 20 gigatonnes of [carbon dioxide] per year, starting from 2050, probably for the next century,\u201d says Gaurav Sant, a civil and environmental engineering professor and director of the Institute for Carbon Management at the University of California, Los Angeles.<\/p>\n To date, most efforts to capture carbon have focused on direct air capture\u2014trying to pull the gas out of the atmosphere<\/a>. But to make carbon capture more efficient, Sant\u2019s research team is turning to the ocean for help.<\/p>\n Oceans and other large bodies of water can hold more than 150 times<\/a> more carbon dioxide than the air. Sant and his colleagues\u2019 idea is that if you can remove carbon from the ocean, the water will absorb more from the atmosphere to maintain a state of equilibrium. Now, they\u2019re proposing an innovative way of getting carbon out of the ocean\u2014by turning it into rock.<\/p>\n Seawater contains a lot of calcium and magnesium. When the calcium or magnesium ions combine with carbon dioxide, they form calcite or magnesite. The chemical reaction is similar to how many marine organisms build their shells. But by introducing a third ingredient, electricity, Sant and his team can make that reaction happen quickly, efficiently and, perhaps eventually, on a large scale. Putting this all together, the scientists have proposed a new technology<\/a> that will run seawater through an electrically charged mesh, using electrolysis to trigger the chemical reactions needed to form carbonate rocks.<\/p>\n So far, the team has built a 1.5-by-1.5-meter prototype that they can flood with simulated seawater. They are collecting data on the amount of carbon dioxide that can be removed over various periods of time, analyzing the process efficiency and the amount of energy required. Aside from simply demonstrating the concept, they are using the model to determine what operational variables might impact the process.<\/p>\n \u201cThis is the formative step towards building larger systems and proving the process at a larger scale,\u201d says Sant.<\/p>\n The process is a bit like a water treatment plant, but instead of taking in water and sifting out impurities, the proposed plant would use electricity to force carbon, calcium, and magnesium to react and become solids. The \u201cpurified\u201d water would then be returned to the ocean.<\/p>\n \u201cYou are actually returning water that is slightly more alkaline than what you put in,\u201d says Alan Hatton, a chemical engineer at the Massachusetts Institute of Technology who has worked on several unrelated carbon capture technologies. This more alkaline water could help mitigate the effects of ocean acidification in the immediate vicinity, he adds.<\/p>\n As well as pulling carbon out of seawater, the chemical reaction has a useful byproduct: hydrogen gas. By producing and selling the hydrogen, a plant could help offset its costs. Sant says that even if a proposed ocean carbon capture plant is powered by natural gas instead of renewable energy, the whole process could still be carbon negative because of this hydrogen gas byproduct.<\/p>\n While ocean carbon capture is a newer technology, a few other groups are also experimenting with it. Some of their projects, such as one by Halifax, Nova Scotia\u2013based startup Planetary Hydrogen, are showing promise.<\/p>\n Like Sant\u2019s team, Planetary Hydrogen is extracting carbon from seawater, trapping it in a solid, and indirectly making hydrogen gas. Rather than using electrolysis, however, they\u2019re doing it with hydroxide. Hydroxide is an alkaline material that speeds up what is otherwise a natural process\u2014rocks reacting with carbon dioxide and water to form alkaline forms of carbon\u2014which would typically take place over geological timescales, says Greg Rau, the company\u2019s lead researcher. While neither team is past the early stages of development, the two proposals seem to have a few benefits over trying to capture carbon out of the air.<\/p>\n Carbon dioxide is much less concentrated in the atmosphere than in the ocean, so direct air capture efforts typically need to be quite large<\/a> to have a significant impact. Neither Hatton nor Sant believes ocean capture plants will require such real estate. And, according to Sant, his process will require half the energy cost of direct air capture and it won\u2019t need a storage reservoir for the carbon dioxide.<\/p>\n There are some drawbacks to Sant\u2019s proposal, though, that could make it difficult for the technology to progress. The biggest seems to be the amount of solids the process would create once it\u2019s operating at a scale meaningful enough to affect climate change.<\/p>\n Removing 10 gigatonnes of carbon dioxide from the ocean, for instance, would yield 20 gigatonnes of carbonates\u2014at a minimum, says Sant. He does have an idea for what to do with all these solids, though.<\/p>\n