Research assistant Will Werner takes air samples at the soil warming experiment in the Harvard Research Forest.

Credit: Craig LeMoult/WGBH News

Global Warming Could Kick Off Vicious Cycle Of Carbon Release From Soil

October 6, 2017

A new study in the journal "Science" raises some serious concerns around climate change. The study concludes that if global warming increases the temperature of the earth’s soil, it could set off a vicious cycle leading to more greenhouse gas emissions and more global warming.

Will Werner walked through a forest in central Massachusetts and arrived at an unusual looking spot. On the ground, between the trees are 18 squares measuring about 20 feet in length on each side, marked in different colored tapes.

“So this is the Prospect Hill soil warming experiment,” he said. Buried under a third of these squares are electric cables.

“If you look over here you can see one place where the heating cable is exposed because it had to go around a trunk,” Werner pointed out.

Buried cables warm the earth in some of the plots in the experiment.
Caption
Photo Credit: Craig LeMoult/WGBH News

Werner’s been working as a research assistant here for six years. But the experiment started 20 years before he arrived.

“It was the –  to the best of our knowledge – the first experiment of its type in the world,” he said.

And it was the brainchild of Jerry Melillo of the Marine Biological Laboratory in Woods Hole. Melillo wanted an answer to a pretty scary question.

Jerry Melillo, Distinguished Scientist and Director Emeritus of the Ecosystems Center at the Marine Biological Laboratory in Woods Hole, Mass.
Caption
Photo Credit: Tom Kleindinst, Courtesy of the Marine Biological Labratory

“We were arguing, based on theory, that it was possible if the world warmed that the warming would cause increased decay of soil organic matter," Melillo said. "And the soils of the world hold very large amounts of carbon.”

He said it’s estimated in the top two meters of the world’s soil, there somewhere in the range of 3,000 to 4,000 billion metric tons of carbon – five times as much as there is in the atmosphere as carbon dioxide, or CO2. And he worried that warming global temperatures might prompt microbes in the soil to use up more of that carbon, and then release it into the atmosphere as CO2.

“And given that CO2 is a greenhouse gas, the concern was that we could develop a self-reinforcing feedback that would then be very difficult to control,” he said. So global warming causes carbon to be released from soil, which leads to more warming, and so on and so on. At least, that was the theory. To test it, Melillo came up with the idea of running those heated cables in the Harvard Research Forest.

To find out if heating the soil is making a difference, Will Werner ran quickly from spot to spot taking air samples from small enclosures above the soil squares.

“I have the syringe in the sample port now," he said, crouching over the sample spot. "In about two seconds I’m going to pump it up and down three times. That's done. I plug the hole with the next syringe,” he said, and ran off to the next spot. The samples have to be taken on a tight schedule. 

The heated plots are raising the ground temperature 5 degrees Celsius, which is considered the upper limit of what can be expected this century. In addition to those squares, there are two control groups. One has no heating cables, and the other does have them, but they’re not turned on.

Will Werner at the Harvard Research Forest site.
Caption
Photo Credit: Craig LeMoult/WGBH News

“Initially, during the first decade, we saw a changing phenomenon starting out with a very, very rapid response of CO2 emissions from the warm soils relative to the controls,” Melillo said.

So the warming did, in fact, cause the microbes in the soil to go nuts, and emit a lot of carbon dioxide. And they kept it up for about a decade. But then the researchers saw those emissions drop off and return to normal. So they wrote a paper in the journal "Science" with their results, which showed a decade of increased carbon. “But perhaps that would be it,” Melillo said.

Here’s the thing, though – they kept going with the study. For five or six more years, they kept taking measurements without seeing anything unusual from the heated plots. And then, suddenly, things changed.

“And we began to see carbon loss from the heated plots increasing once again,” Melillo said.

About seven years later, it stopped again. So what they began to see is a cyclical pattern in how the microbes in the soil reacted to the warming. Over the course of the experiment, Melillo said about 17 percent of the carbon stored in the heated plots has been released as CO2. On a global scale, that would be a huge flow into the atmosphere.

“And CO2 being a greenhouse gas would help to amplify the warming that is being primarily driven by the burning of fossil fuels,” he said.

He’s published a new study with the latest update – once again in the journal "Science," this week. Melillo said while we could turn off sources of greenhouse gases like coal-fired power plants, once the world’s soils start heating up, kicking off a vicious cycle, it’s going to be hard, if not impossible, to stop.

“There's no off switch to flip,” he said. 


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