Natural ally against global warming not as strong as thought

A natural ally against global warming may provide far less aid than previously hoped.

Researchers estimate that the planet’s soils will soak up about 40 percent less carbon by the end of the century than environmental simulations have predicted. That means the atmosphere in 2100 would hold an extra 4 ½ years’ worth of carbon dioxide emissions from fossil fuel burning at current rates, the researchers report in the Sept. 23 Science.

“Future climate change may be underestimated,” says study coauthor Yujie He, a carbon cycle researcher at the University of California, Irvine. “Whatever mitigation we should do, we should do it faster.”

Soils are the largest land-based reservoir of organic carbon on the planet, storing around 1,500 billion metric tons of organic carbon — about twice the amount of carbon in the atmosphere. When a plant dies and decomposes, some of its carbon stays locked up in soil, but microbes munch on some of it and release it back into the atmosphere, typically as CO₂. The question for scientists is how much carbon is released and how much stays locked up for the long term.

Some of the leading climate simulations used by scientists and policy makers expect that rising atmospheric CO₂ concentrations will fertilize plants and boost photosynthesis, increasing plant growth and ultimately pumping more organic carbon into soils and slowing global warming. How much warming will slow down depends on how long the carbon remains underground. He and colleagues worked out that if the simulations are accurate, today’s soil carbon should be on average about 430 years old.

Scientists can determine the actual age of carbon in soil — how long ago it was part of a living plant — thanks to atmospheric nuclear tests that blasted radioactive carbon around the world during the mid-20th century. The fraction of radiocarbon inside a sample of dirt divulges the average age of the soil’s carbon.

Assembling the radiocarbon ages of soil down to a meter deep from 157 locations around the world, the team found that the simulations are off base. The average age of soil carbon is around 3,100 years old — more than seven times the age predicted by the simulations.

That means the rate of new carbon entering long-term storage in soils must be much slower than previously thought, He says, though a lot of uncertainty remains. The remainder of the carbon probably stays in the soil for a short time — potentially only a few months or years —before reentering the atmosphere.

Predictions of future warming need to more carefully account for the variety of soils around the world, says Joanna Clark, a biogeochemist at the University of Reading in England. Complex interactions with minerals and earthworms can affect how long carbon remains underground, she says, and unusual soils such as peatlands need to be more accurately represented in simulations.