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By Michael Marshall

Richard Roscoe/Alamy

Earth contains 1.85 billion billion tonnes of carbon, according to a 10-year research project. If it were all combined into a single sphere, it would be larger than many asteroids.

The new estimate comes from the Deep Carbon Observatory (DCO), a huge international research programme established in 2009. Its main goal has been to estimate the scale of the carbon cycle, “from where carbon gets released in volcanic eruptions and fractures, to where it goes into the atmosphere, gets filtered down into the biosphere and gets buried as sediment and rock”, says Celina Suarez at the University of Arkansas.

This has involved everything from measuring the release of carbon dioxide gas from volcanoes to studying diamonds (a solid form of carbon) from deep in the mantle. The observatory has also looked at carbon isotopes in rocks laid down at different times to understand how the carbon cycle has changed over time.

While carbon’s movements around the planet are well understood, estimating the total amounts in each bit of the world has been a monumental job. “All the work the DCO has been doing in the past 10 years has been trying to document actual numbers of where this carbon is stored,” says Suarez.

“The majority of carbon is very deep in the mantle and in the core,” says Suarez. In contrast, the carbon in the air, land and ocean amounts to just 43.5 trillion tonnes – less than 1 per cent of the total.

Throughout the last 500 million years, the period when complex animal life has existed on Earth, the carbon cycle has been in balance for more than 99 per cent of the time. “What comes out goes back in,” says Suarez.

Extinction events

However, four periods are known when the cycle has become unbalanced for about a million years, for example because of major volcanic eruptions releasing more carbon to the air. “Those occurrences are correlated to mass extinction events,” says Suarez. For example, the end-Permian extinction 252 million years ago killed 80 to 90 per cent of species, and was almost certainly caused by volcanic eruptions.

We are now destabilising the carbon cycle by burning fossil fuels and deforesting large areas of the globe. “The balance is getting a bit out of whack,” says Suarez. Furthermore, we seem to be adding carbon to the air at a faster rate than in any of the previous events, including the end-Permian, she says.

“It’s pretty extensive, when you think about how many millions of years it took for the carbon that is being burned today to form,” says Suarez. Coal seams formed over millions of years, but “most of that has been burned in the last 200 years”.

Journal reference: Elements , DOI: 10.2138/gselements.15.5.301