Capturing carbon dioxide and storing it underground could help address climate change, but some experts worry that the gas will leak back out.

Research described in the journal Science points to a more secure way of storing it—as rock. The scientists showed that when carbon dioxide is pumped along with water into certain types of underground formations, it reacts with the surrounding rock and forms minerals that could sequester the carbon dioxide for hundreds or thousands of years.

Last week, a major U.N. climate report called attention to the importance of carbon capture and storage technology (CCS) for dealing with climate change, and suggested that the cost of limiting warming to two degrees Celsius would greatly increase if CCS isn’t used (see “The Cost of Limiting Climate Change Could Double Without Carbon Capture Technology”). But the report also noted that concerns about leaks could slow or block large-scale use of the technology.

In the new work, researchers from University College London and the University of Iceland added carbon dioxide to a stream of water being pumped underground at a large geothermal power plant in Iceland, as part of normal plant operations. The carbon dioxide quickly dissolves in the water, and in that state it no longer has a tendency to rise to the surface. Once underground, the carbon dioxide-laden water reacts with basalt, a type of volcanic rock. The researchers showed that, within a year, 80 percent of it had reacted with magnesium, calcium, and iron to form carbonate minerals such as limestone.

Researchers have proposed storing carbon dioxide by reacting it with basalt and other types of rock before. What’s surprising about this study is just how fast the reactions occurred, says Sigurdur Gislason, a professor at the University of Iceland. The researchers report that 80 percent of the carbon dioxide they’d injected had formed carbonates in just one year.

One challenge with the new approach is that it requires very large amounts of water—10 to 20 times the mass of the carbon dioxide being stored, says Eric Oelkers, a professor of aqueous geochemistry at University College London. The researchers estimate that this will make it twice as expensive as conventional approaches to storing carbon dioxide—at least in the short run.

Mark Zoback, a professor of earth sciences at Stanford University, says there may be other challenges. While basalt is common, especially on the ocean floor, basalt that is porous enough to accommodate the large volumes of water and carbon dioxide might be hard to come by. If the approach were to be used at a large scale, it “would probably necessitate transport of CO2 in pipelines for thousands of miles.”

Yet Zoback, whose research suggests that earthquakes could cause carbon dioxide gas to leak out of underground storage sites, says, “the advantages of storing carbon in a mineral form are absolutely clear. It would be great if this could scale up” (see “Researchers Say Earthquakes Would Let Stored CO2 Escape”).