Cement is a climate villain. Making it is thought to produce 5% of all global greenhouse gas emissions from fossil fuels and factories. But this building block of modern civilization may eventually suck some of that carbon dioxide (CO 2 ) back up—enough to cancel nearly a quarter of the gases released making cement, according to a new study.

To make cement, limestone (calcium carbonate) is turned into lime (calcium oxide) by baking it at temperatures topping 1000°C. That conversion releases copious amounts of CO 2 —half cement’s total greenhouse gases. The other half comes from fossil fuels used to heat cement kilns.

But there’s a silver lining: The mortar, concrete, and rubble from demolished buildings can gradually absorb CO 2 through a process called carbonation. As CO 2 from the air enters tiny pores in the cement, it encounters a variety of chemicals and water trapped there. The ensuing reactions convert the CO 2 into other chemicals, including water. Still, just how much CO 2 the world’s cement soaked up had never been estimated.

So a team of Chinese scientists, including physicist Zhu Liu, now at the California Institute of Technology in Pasadena, set out to do just that. Those researchers eventually teamed up with Steve Davis, an earth systems scientist at the University of California, Irvine, and other U.S. and European researchers. Together, they compiled data from studies of how cement is used around the world, including the thickness of concrete walls, the quality of concrete used in different structures, the life spans of concrete buildings, and what happens to the concrete after the buildings are torn down. The scientists also visited construction sites around China—the world’s largest producer of cement—to get more accurate estimates of a variety of factors that influence how much CO 2 the cement absorbs. That included everything from the size range of concrete rubble and how long it was left in the open air, to how much cement was used in thick concrete versus thin layers of mortar spread on walls, where it’s exposed more readily to CO 2 .

Then they took things to the laboratory. Here, they calculated the carbonation rate in mortar and concrete in different settings—buried, in the open air, and enclosed in a room. The information formed the underpinnings of a computer model that the scientists ran 100,000 times to see how the final estimates changed as different variables were tweaked.

The results cast a different light on the cumulative impact cement has on the climate. The researchers estimate that between 1930 and 2013, cement has soaked up 4.5 gigatons of carbon or more than 16 gigatons of CO 2 , 43% of the total carbon emitted when limestone was converted to lime in cement kilns, they report online today in Nature Geoscience . That’s more than 20% of the carbon soaked up by forests in recent decades, they write.

The findings don’t represent a dramatic change in the overall picture of greenhouse gas emissions, says Rob Jackson, an earth systems scientist at Stanford University in Palo Alto, California, and chairman of the Global Carbon Project, a consortium of researchers that tracks the planet’s carbon. But, he says, it adds another piece of information to the part of carbon models that is particularly prone to uncertainty—how much carbon is soaked up on land. In future inventories, cement will need to be added to the list of things that absorb carbon from the atmosphere. “It’s important,” Jackson says. “It’s an opportunity to improve what we know.”