Melting Arctic Could Unleash Vast CO2 Stores

Arctic land and seas may soak up as much as a quarter of the carbon absorbed globally each year, says a comprehensive review of the Arctic carbon cycle.

Global warming threatens the potential of this vast carbon repository to continue sequestering carbon. The Arctic could even begin releasing its carbon stores as it warms, accelerating global temperature rise.

Carbon dioxide (CO2) is released primarily by the burning of fossil fuels. On land, it is consumed by plants through photosynthesis, and in the oceans, by dissolving directly into the water or through uptake by algae and other marine plants.

These carbon sinks are tempered on land by plant decomposition and wildfires, which release CO2. In the oceans, warmer waters release dissolved carbon dioxide, but they also may increase the rate of photosynthesis by marine organisms.

The net effect of these processes is that about half of the CO2 released worldwide is removed from the atmosphere.

The Arctic is responsible for absorbing anywhere from 0 to 25 percent of this total, says the new study, published in Ecological Monographs.

"There is a large uncertainty about the degree to which Arctic lands and oceans are a carbon sink," said lead author David McGuire of the U.S. Geological Survey and the University of Alaska, Fairbanks. "Our best estimate for CO2 is it is on the order of probably 10 to 15 percent."

Because the soils and waters of the Arctic are so cold, decomposition happens more slowly than photosynthesis, which is why the Arctic is a net sink for carbon.

But as the Arctic warms, permafrost melts, which allows faster decomposition, releasing CO2. And soils will dry out, increasing the risk of fire.

"Right now, the biggest vulnerability seems to be the potential effect of fire in terms of decreasing the efficiency of the carbon sink and possibly turning it into a source," McGuire said.

Another climate risk in the Arctic comes from methane, a greenhouse gas 25 times more potent than carbon dioxide. As permafrost melts, it is likely to create waterlogged conditions, which lead to the formation of methane by anaerobic bacteria.

Warming may also cause the release of methane hydrate deposits trapped beneath the ocean floor under high pressure.

"One of the questions, particularly on the continental shelves, is whether some thawing could occur that could cause these hydrates to be released. In general, we think it's quite a low probability, but the stocks of methane are enormous."

"The changes that are projected in the thawing of permafrost are all likely to amplify the rates of climate change," said Terry Chapin, also of the University of Alaska, Fairbanks, but who did not take part in the study. "Things could turn out to be much more serious than the present projections would suggest."

"It's very dangerous to put off any decisions about trying to reduce the rates of climate change," he added. "Once these things begin to happen, as a society we lose a bit of our ability to take preventative actions."