June 20, 2006
The Science of Global Warming: Permafrost -- Another Tipping Point?
Zimov, Schuur, and Chapin noted yesterday in Science the enormous potential contribution of permafrost to the global carbon budget. The amount of carbon stored in permafrost is more than that stored in global vegetation. Science article Thus, as global warming thaws permafrost, it releases this enormous reservoir of carbon to the atmosphere, thus accelerating global warming. This is the type of positive feedback mechanism that causes "tipping points" and "cliffs." See yesterday's post on tipping points. If the 1000 Gt of carbon contained in permafrost thaw quickly over the course of 100 years as Zimov indicates, the annual carbon release from permafrost will be 150% of the annual carbon release from fossil fuels.
The carbon content of Earth's atmosphere has increased from ~360 gigatons (Gt)--mainly as CO2--during the last glacial maximum to ~560 Gt during preindustrial times and ~730 Gt today. These changes reflect redistributions among the main global carbon reservoirs. The largest such reservoir is the ocean (40,000 Gt, of which 2500 Gt is organic carbon), followed by soils (1500 Gt) and vegetation (650 Gt). There is also a large geological reservoir, from which ~6.5 Gt of carbon are released annually to the atmosphere by burning fossil fuels.>
Permafrost (permanently frozen ground) is an additional large carbon reservoir that is rarely incorporated into analyses of changes in global carbon reservoirs. Here we illustrate the importance of permafrost carbon in the global carbon budget...We estimate the carbon reservoir in frozen yedoma to be ~500 Gt (2). Another ~400 Gt of carbon are contained in nonyedoma permafrost (excluding peatlands) (3), and 50 to 70 Gt reside in the peatbogs of western Siberia (4). These preliminary estimates indicate that permafrost is a large carbon reservoir [about 1000Gt], intermediate in size between those of vegetation and soils.... Given this estimate of permafrost carbon storage on land, the redistribution of carbon during glacial periods is a fertile area for reassessment. Permafrost is a globally significant carbon reservoir that responds to climate change in a unique and very simple way: With warming, its spatial extent declines, causing rapid carbon loss; with cooling, the permafrost reservoir refills slowly, a dynamic that mirrors the past atmospheric record of CO2. In a warmer climate, permafrost carbon is thus likely to become part of more actively cycling carbon reservoirs. Factors inducing high-latitude climate warming should be mitigated to minimize the risk of a potentially large carbon release that would further increase climate warming.
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