Warming soils in the tropics could cause microbes to release carbon dioxide from storage. One scientist called the finding “another example of why we need to worry more.”
Humble dirt could pack an unexpected climate punch, according to a new study published Wednesday 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 percent 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 an attempt to forecast the future, ecologists began in the early 1990s building apparatuses to artificially heat soils. Such experiments in temperate and boreal forests have shown that carbon-rich soils almost always belch carbon dioxide when warmed. In 2016, a group of researchers estimated that, by 2050, soils could release so much of the planet-warming gas that it would be like adding the carbon emissions of a new country the size of the United States.
But that study left out the perpetually warm, mega-biodiverse tropics, where a third of all soil carbon resides. Figuring out the fate of this carbon would require grappling with the many pitfalls of doing research in the tropics: humidity, storms and a multitude of hungry animals that can take a toll on research equipment — chewing through electrical wires or protective coverings, for example— and on researchers themselves.
For understanding soils’ contributions to climate change, the tropics “is a really important region” that “really hasn’t been studied,” said Margaret Torn, an ecologist at Lawrence Berkeley Lab in California, who was not involved in the study.
In 2014, Dr. Nottingham, then a postdoctoral fellow at the University of Edinburgh, traveled to Barro Colorado Island, a human-created island in the Panama Canal area that’s home to the Smithsonian Tropical Research Institute. He buried electrical wires in five circular plots to a depth of nearly four feet. For protection from the elements and ravenous insects, he shielded the wires inside metal structures shaped like freakishly large spiders. Measurements were logged inside weatherproof boxes.
“Our experiment was basically me as a postdoc making things out of a DIY shop,” Dr. Nottingham said.
The team encountered a number of hiccups, including poor electrical connections that blew up and cost the researchers nearly a year and much of their budget to repair. 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. Other equipment measured the carbon dioxide coming out of both experimental plots and nearby plots that weren’t artificially warmed as well as microbial activity in the plots.ImageNytimes