Soil’s potential to soak up planet-warning carbon dioxide has been overestimated by as much as 40%, say scientists

Hopes that large amounts of planet-warming carbon dioxide could be buried in soils appear to be grossly misplaced, with new research finding that the ground will soak up far less carbon over the coming century than previously thought.

Radiocarbon dating of soils, when combined with previous models of carbon uptake, has shown the widely assumed potential for carbon sequestration to combat climate change has been overestimated by as much as 40%.

Scientists from the University of California, Irvine (UCI) found that models used by the UN’s Intergovernmental Panel on Climate Change (IPCC) assume a much faster cycling of carbon through soils than is actually the case. Data taken from 157 soil samples taken from around the world show the average age of soil carbon is more than six times older than previously thought.

This means it will take hundreds or even thousands of years for soils to soak up large amounts of the extra CO2 pumped into the atmosphere by human activity – far too long to be relied upon as a way to help the world avoid dangerous global warming this century.

“A substantial amount of the greenhouse gas that we thought was being taken up and stored in the soil is actually going to stay in the atmosphere,” said study co-author Steven Allison, UCI associate professor of ecology and evolutionary biology and Earth system science.

Soil is the largest land-based reservoir of carbon on Earth, absorbing it from trees and vegetation as they die and decay. The IPCC calculated that should the mass deforestation of recent centuries be completely reversed, around 40 parts per million (ppm) of CO2, from the current 400ppm levels, could be removed from the atmosphere. Other studies have shown large amounts of carbon could be soaked up with changes in agricultural practices.

But the University of California work, published in the journal Science, went beyond the models to explicitly date radiocarbons, one of the two carbon isotopes.

“It will take a very long time for soil to soak up the carbon, there is a timescale mismatch in terms of climate change,” said Yujie He, a UCI postdoctoral scholar and lead author of the study. “The soil will eventually be a large carbon sink, but it won’t be present in the next century.”

Nations have agreed to limit global temperature increase to 2C compared to pre-industrial times, in order to prevent a dangerous shift in the climate that will lead to more heatwaves, extreme weather and ruinous sea level rise as glaciers melt and seawater expands.

An international aspiration to cap the rise to 1.5C, seen as crucial to the viability of low-lying nations, already appears to be slipping out of reach. As-yet undeveloped technology, such as geo-engineering of landscapes, carbon capture from power plants or direct removal of CO2 from the atmosphere, may be required even if emissions are radically cut.

But the prospect of adapting soils so they suck up more carbon is “unlikely”, especially in the short-term, according to He. “I don’t think we can increase that absorption ability, so we may want to make more proactive action to mitigate climate change, such as cuts to fossil fuel emissions, for example,” she said.