A 26-year research project suggests that as global temperatures rise and heat the soil, the released carbon will trigger even more warming, leading to a dangerous feedback loop.
In 1991, a team of scientists began measuring carbon levels in the soil of the Harvard Forest, a field laboratory nestled in the hills of Massachusetts. The team laid underground electrical cables to heat small plots of soil and they have monitored these test plots ever since. During this time, they have measured two periods of rapid carbon loss, separated by one period of no carbon loss.
Jerry Melillo, distinguished scientist at the Marine Biological Laboratory at Woods Hole, led the study. He describes the aim of the experiment this way: “The big question is, as the world warms, will the microbes in the soils decay soil organic matter matter more rapidly, thereby putting carbon dioxide into the atmosphere, which would accelerate warming, which would then feed back to the soil system, such that, in the end, warming would feed itself?”
Melillo and his colleagues set up 18 plots of soil to study. Six of the plots received a heating cable with electricity turned on, which raised the temperature of the soil 5 degrees Celsius (about 9 degrees Fahrenheit); six plots had the cables installed, but no electricity turned on. The final six plots were laid out, but nothing was done to them.
Melillo chose the 5 degrees Celsius mark because of his early involvement in the Intergovernmental Panel on Climate Change (IPCC). At the time, he says, the climate models used in IPCC had identified that temperature increase as the high end of what the world might expect by the end of the 21st century in a “business as usual” scenario — that is, continuing to emit large amounts of heat-trapping gases into the atmosphere, including CO2, methane and nitrous oxide.
“I was trying to give the system a push to see whether or not there was any microbial response to soil warming that would be significant in the climate system,” Melillo explains.
Melillo says the study's results show a considerable amount of carbon loss from the soil.
“We have reduced the soil carbon stock by about 17 percent to a depth of about a half a meter,” he says. “This is significant because that carbon goes from organic matter in the soils to CO2 in the atmosphere. By putting more CO2 in the atmosphere, we are warming the planet, and by putting CO2 in the atmosphere from warm soils, we're making the problem of climate mitigation much more difficult.”
While 17 percent may not sound like much, Melillo points out that the world’s soils contain about 3,000 to 3,200 billion metric tons of carbon, according to global estimates. As a reference, the world currently emits about 10 billion metric tons of carbon into the atmosphere as CO2 from fossil fuel burning. “So, you can think about the soil carbon stores as basically being the equivalent of about 320 years of emissions of fossil fuel carbon at the rate of 10 billion metric tons per year,” he explains.
If that much carbon, now locked in the Earth’s soil, gets released by warming temperatures, humanity is going to have a hard time containing climate disruption, Melillo says.
This soil feedback problem is ubiquitous because microbes all across the globe share the ability to decompose soil organic matter and “there are no obvious switches for turning that behavior off,” he adds.
“So, it's not like [when you] shut down a few hundred coal-fired power plants by flipping the switches in their control rooms,” he points out. “Here, you're dealing with microorganisms that respond to their environment, and it's going to be very difficult to control that response.”
The soil feedback phenomenon could be particularly important in high-latitude soils, including the soils of the Arctic, Melillo says. Much of this soil organic matter is not decomposing right now because it is locked in ice. But, Melillo says, “we know that high-latitude ecosystems are warming more rapidly than other places on the planet and that this frozen soil — this permafrost soil — is thawing. We also know that once that permafrost soil thaws, the organic matter in that thawed soil is full of relatively easy-to-decompose compounds.”
Melillo expects “very rapid decomposition and a flux of carbon dioxide into the atmosphere” if these soils, once they thaw, are well drained. If the soils, once they thaw, remain wet, however, then methane, another heat-trapping gas, could become the dominant product.
“That news is not good,” Melillo points out, “because methane is a more powerful greenhouse gas than is CO2.”