Global warming may release carbon in tropics

Humble dirt could pack an unexpected climate punch, according to a new study published in the journal Nature.

An experiment that heated soil underneath a tropical rainforest to mimic temperatures expected in the coming decades found that hotter soils released 55% more planet-warming carbon dioxide than did nearby unwarmed areas.

If the results apply throughout the tropics, much of the carbon stored underground could be released as the planet heats up.

"The loss rate is huge," said Andrew Nottingham, an ecologist at the University of Edinburgh, who led the study. "It's a bad news story."

The thin skin of soil that covers much of our planet's land stores vast amounts of carbon — more, in total, than in all plants and the atmosphere combined. That carbon feeds hordes of bacteria and fungi, which build some of it into more microbes while respiring the rest into the atmosphere as carbon dioxide. Many of these microbes grow more active at warmer temperatures, increasing their digestion and respiration rates.

The finding "is another example of why we need to worry more" about how fast the globe is warming, said Eric Davidson, an environmental scientist at the University of Maryland College of Environmental Science in Frostburg who was not involved in the research.

In 2014, Nottingham, then a postdoctoral fellow at the University of Edinburgh, traveled to Barro Colorado Island, a human-created island in the Panama Canal Zone that's home to the Smithsonian Tropical Research Institute. He buried electrical wires in five circular plots to a depth of nearly 4 feet.

Starting in November 2016, the wires' electrical resistance began warming the soil by almost 6 degrees Fahrenheit, within the range of how much the tropics are projected to warm by century's end according to current climate models.

The results are sobering: Over two years, warmed soils spewed out 55% more carbon than control plots.

If the entire tropics were to behave similarly, the researchers estimate that 65 billion metric tons of carbon would enter the atmosphere by 2100 — more than six times the annual emissions from all human-related sources.

Scaling the results to account for the entire tropics is complicated, however. The soils on Barro Colorado Island are richer in nutrients than many others, such as those of much of the vast Amazon rainforest, Davidson noted. That could make it easier for the Panamanian microbes to ramp up their activity.