Permafrost Organic Carbon Content Double Prior Estimates

14 September 2008

Conceptual diagram of the effect of permafrost thawing on climate. Decomposition in oxic soils releases primarily CO2, whereas anoxic decomposition produces both CH4 and CO2, but at a lower total emission rate. Fire releases mostly CO2, but also some CH4. Click to enlarge. Credit: BioScience

An international, three-year study involving collaboration between scientists from Australia, Russia, the US, the UK, Canada and Europe has estimated that the amount of frozen organic carbon locked away in the world’s permafrost regions—a major potential source of atmospheric carbon dioxide (CO₂) and methane (CH₄)—is 1,672 petagrams (1,672 billion metric tons). This is more than double prior estimates of the world’s high-latitude carbon inventory, and more than twice the size of the current atmospheric carbon pool.

In an open access paper published in the September 2008 edition of Bioscience, the team concludes that whereas some of the CO₂ produced as a result of decomposition of previously frozen vegetation would be absorbed by increased, global warming-induced plant growth, it is likely the net effect would be a significant net increase in atmospheric CO₂.

Permafrost C, once thawed, can enter ecosystems that have either predominantly oxic (oxygen present) or predominantly anoxic (oxygen limited) soil conditions. [See diagram above.] There is a gradient of water saturation on the landscape that ranges from fully oxic to fully anoxic, and ecosystems can become drier as permafrost thaws (shrinking lake area, drying wetland/peatlands), or wetter (thermokarst lakes). The soil oxygen status is a key determinant of the rate and form of C loss to the atmosphere.

Decomposition in oxic soils releases primarily CO₂, whereas anoxic decomposition produces both CH₄ and CO₂, but at a lower total emission rate. Fire releases mostly CO₂, but also some CH₄, and can burn upland and wetland ecosystems, although burning of organic soils at depth is restricted in wetter environments unless there is severe drought.

These emissions of C through decomposition are offset by gross and net primary productivity (photosynthesis and net plant growth). Under some local conditions, it is possible that C will enter the permafrost pool...although this total amount is small relative to C that is expected to thaw from permafrost as a result of climate change.

—Schuur et al. (2008)

Under a warming climate, the researchers note, release of carbon from permafrost to the atmosphere occurs primarily through accelerated microbial decomposition of organic matter. However, the rate and form of this C release is dependent on a number of landscape-level processes that are only beginning to be understood quantitatively. The paper explores four: active layer thickening; talik formation; river and coastal erosion; and thermokarst development.

...accurately predicting the magnitude and effect of thawing permafrost on the world’s climate is difficult for several reasons. While global carbon models may include simple permafrost dynamics they do not adequately represent the broader consequences, such as the decomposition of organic matter in thawing permafrost and the transformation of landscapes.

—Dr Pep Canadell, The Centre for Australian Weather and Climate Research, co-author

Dr Canadell says that despite such limitations, scientists now know that even the release of a small fraction of this vast frozen reservoir of carbon would significantly accelerate climate change.

At current rates of warming in the higher latitudes, the evidence indicates that this is likely to happen.

—Pep Canadell

Resources

• Edward A. G. Schuur, James Bockheim, Josep G. Canadell, Eugenie Euskirchen, Christopher B. Field, Sergey V. Goryachkin, Stefan Hagemann, Peter Kuhry, Peter M. Lafleur, Hanna Lee, Galina Mazhitova, Frederick E. Nelson, Annette Rinke, Vladimir E. Romanovsky, Nikolay Shiklomanov, Charles Tarnocai, Sergey Venevsky, Jason G. Vogel, and Sergei A. Zimov (2008) Vulnerability of Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle. BioScience Vol. 58. No. 8