In the really long run, Earth’s climate has a built-in geological thermostat that helps moderate change. As it warms, the chemical weathering of exposed rock accelerates — a reaction that gradually removes carbon dioxide from the atmosphere. But on a timescale much more relevant to our lives, something similar is actually going on. Humanity’s aging concrete infrastructure absorbs CO2, at. It’s not a huge amount, but it’s not nothing either.
The manufacturing cement produces CO2 emissions. The raw material for cement is mainly limestone – calcium carbonate. At high temperature, molecules of CO2 escape and leave behind only calcium oxide, which we call “lime”. So in addition to burning fossil fuels to heat the material, you convert some rock (the calcium carbonate) into atmospheric CO2.
But this process is reversed if cement is left to sit and slowly deteriorate – the lime reacts with water and atmospheric CO2 to make calcium carbonate again. While researchers who keep accounts of global greenhouse gas emissions have worked diligently to track the CO2 produced by cement manufacturing (it causes about five percent of total fossil fuel and industry emissions), the reverse process has never really been counted on a global scale.
A new study led by Chinese Academy of Sciences researcher Fengming Xi sought to fill that gap. Adding a new set of measurements of cement in Chinese structures to existing studies elsewhere, the researchers put together a chemical model that included estimates of things like exposed surface, weather conditions and structure life. The result is a rough estimate of how much CO is in the atmosphere2 has been soaked up by decaying cement during its life, demolition and reuse or disposal.
The numbers are surprising. As of 2013, the researchers estimate that about a quarter of a billion tons of CO2 was absorbed by cement each year. Before the 1980s, this happened mainly in the US and Europe, but more recently China is the story. Due to the huge construction boom there in recent decades, Chinese cement has come to dominate this process.
In total, the researchers estimate that about 4.5 billion tons of CO2 carbon has been absorbed by cement since 1930. That is about 43 percent of the amount of CO2 that has been released by the heating reaction in cement production, which means that cement has not been such a big source of CO2 emissions as we thought, all things considered.
Interestingly, while concrete makes up about 70 percent of the cement material out there, Mortar does most of the CO2 consumption. Mortar is often applied in thin layers to building walls, giving it a very high exposed surface and reducing CO2-absorbing reaction more efficient. While concrete tends to absorb only 17 percent of the CO2 emitted during production during its life cycle, mortar consumes the same amount of CO2 as it is released at birth.
Future accounts 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 production. 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 as bad as we thought. If emissions from cement plants were captured, the researchers note, so could the net effect of cement Reduce atmospheric CO2. We could even help that process by purposely treating concrete rubble from demolished structures in such a way that it can absorb more CO2.2.
If you feel like this is some unusually good news about greenhouse gas emissions past and present, NASA’s Gavin Schmidt provided a bit of a wet blanket on Twitterand notes, “For further context, this is roughly equivalent to getting an extra 6 months before the stabilization carbon budget is reached.”
Natural Geosciences2016. DOI: 10.1038/NGEO2840 (About DOIs).