More plant growth will absorb some of the carbon dioxide emitted by a warmer Arctic.Punchstock

Thawing Arctic soils could release a billion tonnes of carbon every year by the end of this century, new evidence from test plots in Alaska suggests.

The study by researchers in the United States is one of the first to use radiocarbon dating to calculate the rate of carbon loss from melting tundra soils in situ.

"Previous studies have calculated carbon loss as tundra thaws in the laboratory," says Edward Schuur from the University of Florida in Gainesville, who led the research. "This study is different because we measured the ecosystem response in the real world."

Scientists have long debated how the global climate might be affected by thawing of the Arctic's permanently frozen soils, known as permafrost. When permafrost melts, microbes decompose organic matter in the soil, producing greenhouse gases. But when plants have access to warmer, deeper soils, they grow faster and take in carbon dioxide. Scientists have postulated that CO₂ release by microbes would outweigh any greening of the Arctic by plant life, but the precise balance between the two was not known, says Schuur.

The study by Schuur and his colleagues, published today in Nature¹, shows that after 15 years of thaw, plants initially grow faster and take in more carbon than is released by the melting tundra, so the ecosystem is an overall carbon sink. But after a few decades, the balance shifts and the ecosystem becomes a source of carbon.

"The plants are growing faster, but after a few decades the rate of carbon loss from the soils is so high the plants can't keep up," says Schuur.

It's estimated that permafrost soils store about twice as much carbon than is currently present in the atmosphere², and the stores of carbon are unlikely to run out any time soon. "It's a slow-motion time bomb," says Schuur.

Ecosystem effects

Schuur's team determined how long each chosen test site had been thawing using long-term temperature data and historical aerial photographs. The long-term data enabled the group to determine the ecosystem response to thawing several years into the process, despite conducting the experiment over only three years.

The team calculated the total carbon lost or gained at each test site on the tundra owing to permafrost thaw, using an infrared gas analyser to measure CO₂ concentrations.

The team then used radiocarbon dating to calculate the percentage of total lost or gained carbon that came from the permafrost. Because the isotope carbon-14 is subject to radioactive decay over time, lower levels of carbon-14 suggest it is 'old carbon' that has been locked away from the atmosphere in permafrost for hundreds or even thousands of years.

Extrapolations of the experimental findings to the whole Arctic region suggest that CO₂ emissions from future permafrost thawing could be roughly a billion tonnes per year — of the same order of magnitude as emissions from current deforestation of the tropics. Burning of fossil fuels releases about 8.5 billion tonnes of CO₂ a year.

"This represents an important finding for our predictions of what may happen to tundra ecosystems as the permafrost thawing progresses over the coming decades," says Torben Christensen, a biogeochemist at Lund University in Sweden.

But Christensen points out that future permafrost studies should include measurements of methane gas as well, because of its potential effects on global warming.

Martin Heimann from the Max Planck Institute of Biogeochemistry in Jena, Germany, adds that although Schuur's study might be accurate in describing his Alaskan test site, there are problems in extrapolating his results across the Arctic. "The Arctic is very heterogeneous," Heimann says. "These studies have to be made at other places."

Heimann also points out that the data from Schuur and his team show interannual variability, which makes it challenging to deduce long-term trends. "The measurements need to be conducted for decades," he says. "But it's a start.

Schuur, E. A. G. et al. Nature 459, 556–559 (2009). | Article |

Schuur, E. A. G. et al. Bioscience 58, 701–714 (2008). | Article |