Over the truly long term, Earth’s climate has a geological thermostat built in that helps moderate change. If things get warmer, chemical weathering of exposed rock speeds up—a reaction that gradually removes carbon dioxide from the atmosphere. But on a timescale much more relevant to our lives, there is actually something sort of similar going on. Humanity’s aging concrete infrastructure is taking up CO 2 , too. It’s not a huge amount, but it’s not nothing.

The manufacturing of cement produces CO 2 emissions. The raw material that goes into cement is principally limestone—calcium carbonate. At high temperature, molecules of CO 2 escape, leaving just calcium oxide behind, which is what we call “lime.” So in addition to the burning of fossil fuels to heat the material, you’re converting some bedrock (the calcium carbonate) into atmospheric CO 2 .

But this process gets reversed as cement sits around and slowly deteriorates—the lime reacts with water and atmospheric CO 2 to make calcium carbonate again. While researchers doing the accounting for global greenhouse gas emissions have worked carefully to track the CO 2 produced by cement manufacturing (it kicks in about five percent of total fossil fuel and industry emissions) the reverse process has never really been tallied at a global scale.

A new study led by Chinese Academy of Sciences researcher Fengming Xi sought to fill in that gap. Adding a new set of measurements from cement in Chinese structures to existing studies elsewhere, the researchers put together a chemical model with estimates of things like exposed surface area, weather conditions, and structure lifespan. The result is a global estimate of how much atmospheric CO 2 has been sucked up by decaying cement during its life, demolition, and re-use or disposal.

The numbers are surprising. As of 2013, the researchers estimate that about a quarter of a billion tons of CO 2 was being absorbed by cement each year. Prior to the 1980s, this was mostly happening in the US and Europe, but China is the story more recently. The huge construction boom there in recent decades means that Chinese cement has come to dominate this process.

In total, the researchers estimate that about 4.5 billion tons of CO 2 carbon has been taken up by cement since 1930. That’s about 43 percent of the amount of CO 2 that has been released by the heating reaction in cement manufacturing—meaning cement hasn’t been quite as big a source of CO 2 emissions as we thought, all things considered.

Interestingly, while concrete makes up about 70 percent of the cement “stuff” out there, mortar does the majority of the CO 2 consumption. Mortar is often applied in thin layers on building walls, giving it a very high exposed surface area and making the CO 2 -absorbing reaction more efficient. While concrete tends to absorb only 17 percent of the CO 2 emitted during manufacturing over its life cycle, mortar consumes just as much CO 2 as it released at birth.

Future accounting of the Earth’s carbon cycle (like the one we covered last week) will be updated to include this information, offsetting some of the emissions from manufacturing. But this also has other interesting implications. The researchers note that policy efforts should prioritize cleaning up the energy sector over cement production, which isn’t quite as bad as we were thinking. If emissions from cement plants were captured, the researchers note, the net effect of cement could actually be to reduce atmospheric CO 2 . We could even help that process along by purposely handling concrete rubble from torn-down structures in a way that allows it to absorb more CO 2 .

If you’re feeling like this is an unusually good piece of news about past and current greenhouse gas emissions, NASA’s Gavin Schmidt provided a bit of a wet blanket on Twitter, noting, “For further context, this is approximately equivalent to getting an extra 6 months before the carbon budget for stabilization is reached.”

Nature Geoscience, 2016. DOI: 10.1038/NGEO2840 (About DOIs).