The input and output of CO2 into the soil were largely balanced before humanity began to increase emissions. Deadwood and decaying leaves released gasses were balanced by microorganisms that feed on them.

But climate change is changing this balance now, scientists are warning.

“Carbon retained in tropical soils is more vulnerable than historically understood to warming,” Lead Author Andrew Nottingham, an Edinburgh University academic, told AFP. “A modest rise in breathing in tropical forest soils may have a huge effect on atmospheric CO2 concentrations with global warming implications.”

Previous studies have shown that higher temperatures that release locked carbon into colder or frozen soils, such as the Arctic tundra. A 2016 study predicted that the soils could emit as much CO2 as the United States by 2050, but the report also did not look at tropical soils.

Carbon was thought to be less vulnerable to climate change losses in tropical soils than soil carbon in higher latitudes, however experimental evidence was lacking. In the current research, Nottingham and his team have reported evidence that tropical-forest soils could be more fragile than originally expected to rise, particularly if they tend to increase in temperatures.

The authors put heat rods around the perimeter of barren land on Barro Colorado Island, Panama, in a tropical climate, and for two years raised the soil profile temperature by 4oC. They measured the CO2 efflux regularly with a set of chambers on the ground. Their results showed an unexpected 55% increase in CO2 emissions from soil. In an effort to understand growth, they excluded the roots of the soil below the chambers, and realized that most extra CO2 was due to a greater than expected increase in soil microbes’ respiration.

Extrapolating the results , the researchers estimated that the emission in the atmosphere of 65 billion tons of carbon, equivalent to approximately 240 billion tons of CO2, if all the world’s tropical soils were warmed up by 4o C for two years prior to 2100.

“The current annual emissions from human sources are more than six times higher. This could be an underestimation because we could see substantial continuing declines over two years in our trial, “Nottingham said, noting that the analysis takes no account of deeper carbon stocks under two meters.

The world’s average temperature is just over one degree Celsius higher than pre-industrial. This was sufficient to increase the frequency of droughts, heat waves and superstorms, and other climatic phenomena. The temperature on the land alone rose by 2oC, doubling the world average.