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The soil and oceans hold vastly more carbon than the atmosphere (Image by Bvelevski [CC BY-SA 4.0] via Wikimedia Commons)

26-year study find rising temperatures could cause soils to release carbon on a scale with the potential to accelerate climate change even further. Tim Radford reports.

As the world’s soils warm, they may surrender potentially dangerous amounts of carbon to the atmosphere. Rising temperatures could mean rising levels of carbon dioxide respired by the microbes underfoot.

The world’s longest-running soil-warming experiments deliver no easy assurances forests will continue to absorb atmospheric carbon that pours from vehicle exhausts, power stations and factory chimneys as humans burn fossil fuels, raise greenhouse gas levels and send the planetary thermometer ever higher.

The conclusion is based on a set of experiments described in the journal, Science.

Carbon budget

Since 1991, researchers have been measuring the soil carbon traffic in Harvard Forest, Massachusetts, in the United States. In this forest are sets of plots six metres square. Some are left wild. Some have electric cables dug into the soil to deliver 5°C warmth of the kind that might be expected later this century. Some have soil disturbed but not warmed. Researchers tried every combination and compared the soil carbon loss over time.

They measured phases of substantial carbon loss from the warmed soils, alternating with phases of no detectable loss. That is: they measured soil carbon loss to the atmosphere, and stasis, but never observed evidence that warmed soils might store carbon more efficiently. Altogether, the warmed soils lost 17% more of the carbon stored in the top 60 centimetres than unheated soils.

“We know that microbial soil respiration is a major, and natural, source of greenhouse gases to the atmosphere. Using the long-term warming experiment as a window into future climate change, we see that warming has a profound but discontinuous effect on greenhouse gas emissions,” says one of the authors, Kristen DeAngelis, assistant professor of microbiology at the University of Massachusetts at Amherst.

The carbon budget – the flow of carbon into and from the atmosphere – is at the heart of all climate change forecasting. Higher ratios of greenhouse gases in the atmosphere will raise the planetary temperatures, potentially to catastrophic levels. If humans don’t abandon fossil fuels and switch to solar and wind power at a fast enough rate, then they must hope for help from nature — that forests will soak up more carbon, that wetlands will continue to store plant tissue as peat and so on.

But there is already evidence that warming soils may be less effective at storing carbon dioxide, so the race is on to understand the subterranean economy a little better and identify the agencies that control carbon traffic in the topsoils.

The latest study delivers some more reliable estimates of warming impacts, at least in the soils in one hardwood forest in one part of the temperate world. There is already evidence that tropical forests, above ground, may be releasing more carbon than they store, if only because of continued clearance for cattle ranching and plantations. Now it seems clear that the carbon stored in the forest soils may too find its way into the atmosphere.

World’s soils

“To put this in context, each year, mostly from fossil-fuel burning, we are releasing about 10 billion metric tons of carbon into the atmosphere. That’s what’s causing the increase in atmospheric carbon dioxide concentration and global warming,” says Jerry Melillo, director emeritus at the Ecosystems Centre of the Marine Biological Laboratory at Woods Hole, Massachusetts, who led the study.

“The world’s soils contain about 3,500 billion metric tons of carbon. If a significant amount of that soil carbon is added to the atmosphere, due to microbial activity in warmer soils, that will accelerate the global warming process. And once this self-reinforcing feedback begins, there is no easy way to turn it off. There is no switch to flip.”

Humans could act to reduce emissions, by closing coal-fired power stations, Professor Melillo says.

“But if the microbes in all landscapes respond to warming in the same way as we’ve observed in mid-latitude forest soils, this self-reinforcing feedback phenomenon will go on for a while, and we are not going to be able to turn those microbes off. Of special concern is the big pool of easily decomposed carbon that is frozen in Arctic soils. As those soils thaw out, this feedback phenomenon would be an important component of the climate system, with climate change feeding itself in a warming world.”

This story was originally published by the Climate News Network and has been republished under a Creative Commons licence.

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