Earth is swallowing up more carbon from its atmosphere than scientists previously thought, new research suggests. This discovery may change some of the equations and balances around our projections of climate change, although it doesn't mean we can breathe a sigh of relief.
The updated findings indicate that around a third of carbon rolled into Earth's interior stays locked away long term. Previously, it was thought that almost all of it reappeared through volcanic eruptions.
As deep carbon stores are where most of our planet's carbon is located, knowing more about how these stores operate and evolve will help us in figuring out the knock-on effects of atmospheric CO2 and habitability here on the surface.
"We currently have a relatively good understanding of the surface reservoirs of carbon and the fluxes between them, but know much less about Earth's interior carbon stores, which cycle carbon over millions of years," says materials scientist Stefan Farsang, from the University of Cambridge in the UK.
There's only one way that carbon gets pulled deep into Earth, and that's via plate subduction: the slow-motion collision and warping of tectonic plates, which take the carbon-storing remains of organisms and seashells into the ground as they go.
What the researchers did here was simulate the chemical reactions happening in the tectonic plate rock, using the European Synchrotron Radiation Facility particle accelerator. They were able to create the intense pressure and super-high temperatures of subduction zones, thus modelling what might be happening in Earth's interior.
Specifically, the team found that carbonate rocks become less calcium-rich and more magnesium-rich when channeled deeper into the mantle – that makes them less soluble, and less likely to be drawn into the fluids supplying volcanoes.
Instead, the majority of the carbonate apparently sinks deeper, and can eventually turn into diamond – taking the carbon gathered from the atmosphere, via ocean sediments, along with it.
"Our results show that these minerals are very stable and can certainly lock up CO2 from the atmosphere into solid mineral forms that could result in negative emissions," says mineral physicist Simon Redfern, from the Nanyang Technological University (NTU) in Singapore.
"These results will also help us understand better ways to lock carbon into the solid Earth, out of the atmosphere. If we can accelerate this process faster than nature handles it, it could prove a route to help solve the climate crisis."
Carbon is constantly captured from the atmosphere in all kinds of ways – through soils and through oceans for example – and scientists are looking at how this might be accelerated artificially in the future.
On its own, this sort of process is far from enough to save our rapidly warming planet from a climate crisis (globally reducing emissions is still the single most important thing we must do); but a better understanding of the carbon cycle happening between the atmosphere, oceans and Earth's interior should prove useful in plotting a future course.
Of course, figuring out what is happening deep below the surface of Earth across long time scales is incredibly hard science, and no subduction zone is the same in terms of its geological and chemical make-up. The scientists are ready to carry out further studies to gather more data.
"There is still a lot of research to be done in this field," says Farsang. "In the future, we aim to refine our estimates by studying carbonate solubility in a wider temperature, pressure range and in several fluid compositions."
The research has been published in Nature Communications.