If current trends continue, global warming will cause a massive release of carbon from soils in tropical forests — soils that contain one-third of the carbon stored in soils globally.

The input and outflow of CO2 into the soil used to be mostly in balance before humanity started ramping up emissions. The gases released by deadwood and decaying leaves were balanced by the microorganisms that fed on that matter.

But climate change is now altering such balance, researchers are warning.

“Carbon held in tropical soils is more sensitive to warming than previously recognized,” lead author Andrew Nottingham, a researcher at the University of Edinburgh, told AFP. “Even a small increase in respiration from tropical forest soils could have a large effect on atmospheric CO2 concentrations, with consequences for global climate.”

Previous research showed that rising temperatures could release carbon locked away in cooler or frozen soils such as in the Arctic tundra. A study from 2016, estimated that by 2050, soils could release as much CO2 as the US — but that study didn’t even consider tropical soils.

Carbon in tropical soils was thought to be less vulnerable to loss under climate change than is soil carbon at higher latitudes, but experimental evidence for this was lacking. In the new study, Nottingham and his team published evidence showing that tropical-forest soils might be more vulnerable to warming than was initially thought, especially if temperatures continue to rise.

The authors placed warming rods around the perimeter of undisturbed soil plots in a tropical forest on Barro Colorado Island, Panama, increasing the temperature of the soil profile by 4ºC for two years. They periodically measured the efflux of CO2 with a set of chambers placed over the soil. Their results showed observed an unexpectedly large increase of 55% in soil CO2 emissions. Trying to understand the growth, they excluded the roots from the soil under the chambers and realized that most of the extra CO2 was because of a greater-than-expected increase in the respiration of soil microbes.

Extrapolating from the findings, the researchers estimated that if all the world’s tropical soils warmed by 4ºC for a two-year period sometime before 2100, it would release 65 billion tons of carbon, equivalent to about 240 billion tons of CO2, into the atmosphere.

“That is more than six times the current annual emissions from human-related sources. This could be an underestimation, because we might see large continued loss beyond the two years in our experiment,” Nottingham said, also adding that deeper stores of carbon below two meters weren’t taken into account in the study.

The surface temperature of the planet has grown just over one degree Celsius above preindustrial levels. This has been enough to boost the severity of droughts, heatwaves and superstorms, among other climate events. On land alone, the temperature rose 2ºC, doubling the global average.

The study was published in the journal Nature.