Thawing Permafrost a Significant Source of Carbon
15 June 2006
|Circumarctic map of permafrost. Click to enlarge. Source: UNEP-GRIDA|
With global temperatures rising, the Arctic’s “permanently” frozen soil—permafrost—isn’t staying frozen. A type of soil contained deep within thawing permafrost—loess—may be releasing significant, and previously unaccounted for, amounts of carbon into the atmosphere, according to authors of a paper published this week in the journal Science.
Some have been warning about the danger of melting permafrost for a number of years. For example, Svein Tveitdal, managing director of GRID-Arendal in Norway, a United Nations Environment Program (UNEP) information center, warned of the potential in 2001:
Permafrost has acted as a carbon sink, locking away carbon and other greenhouse gases like methane, for thousands of year. But there is now evidence that this is no longer the case, and the permafrost in some areas is starting to give back its carbon. This could accelerate the greenhouse effect.
The just-published work by the scientists from Russia, the University of Florida, and the University of Alaska Fairbanks found that loess permafrost—extending more deeply into the permafrost layers and covering more than a million square kilometers in Siberia and Alaska—is a very large carbon reservoir with the potential to be a significant contributor of atmospheric carbon, yet one seldom incorporated into analyses of changes in global carbon reservoirs.
The unique aspect of the Siberian loess permafrost is that it is quite deep—20 to 40 meters—and has a surprisingly high carbon concentration at depth for a mineral soil. This paper explains the processes that led to the accumulation of large amounts of soil carbon and the processes that could lead to its return to the atmosphere.—Terry Chapin, co-author from the Institute of Arctic Biology at UAF
People know about carbon in permafrost—it’s not a trivial amount. Normally, scientists look for carbon in the upper layers of permafrost where organic matter decomposes.—Ted Schuur, co-author from the University of Florida
The largest carbon reservoir on Earth is the ocean, which scientists estimate holds about 40,000 gigatons (Gt); soils contain about 2,500 Gt and vegetation about 650 Gt. According to the authors, about 500 Gt of carbon are contained in the thaw-threatened loess, also called yedoma, of Siberia and Alaska.
By comparison, in 2003, total worldwide emissions of carbon from the combustion of fossil fuels was 6.8 Gt (25.1 Gt CO2), according to the US Energy Information Administration. Total 2003 worldwide emissions of carbon from the combustion of petroleum was 2.5 Gt (9.3 Gt CO2). The release of even a portion of an extra 500 Gt of carbon—equivalent in total to the release of 1.8 trillion tons of CO2—would, in other words, be a significant addition, even spread out over time.
I was surprised, because it is unusual to find major new large carbon stocks. We have spent more than five years discussing among ourselves all the details of the calculations, because initially I did not believe that the pool could be both so large and so decomposable (once thawed).—Terry Chapin
Laboratory and field experiments by the scientists demonstrate that the organic matter in yedoma decomposes quickly when it is thawed and produces rates of carbon release similar to those of productive northern grassland soils.
Permafrost has been seldom incorporated into global carbon budgets in part because the “... size of the carbon pool was so poorly quantified ... and in part because global data bases for soils have been standardized to provide data only for the top meter of soil,” Chapin said.
If these rates continue as field observations suggest, most carbon in recently thawed yedoma will be released within a century—a striking contrast to the preservation of carbon for tens of thousands of years when frozen in permafrost.
“Permafrost and the Global Carbon Budget”; Sergey A. Zimov, Edward A. G. Schuur, F. Stuart Chapin III; Science 16 June 2006: Vol. 312. no. 5780, pp. 1612 - 1613 DOI: 10.1126/science.1128908
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