Zimov-permafrost-200 - PERSPECTIVES requires detailed information on the statespecific collision rate and ortho-to-para conversions of H2 H3 and

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1612 Climate warming will thaw permafrost, releasing trapped carbon from this high- latitude reservoir and further exacerbating global warming. Permafrost and the Global Carbon Budget Sergey A. Zimov, Edward A. G. Schuur, F. Stuart Chapin III CLIMATE CHANGE T he carbon content of Earth’s atmosphere has increased from ~360 gigatons (Gt)—mainly as CO 2 —during the last glacial maximum to ~560 Gt during preindus- trial 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 car- bon 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 impor- tance of permafrost carbon in the global carbon budget by describing the past and potential future dynamics of frozen loess (windblown dust, termed yedoma in Siberia) that was deposited during the glacial age, covering more than 1 million km 2 of the north plains of Siberia and Central Alaska to an average depth of ~25 m. The frozen yedoma represents relict soils of the mammoth steppe-tundra ecosystem that occupied this territory during glacial times ( 1 ). As windblown or river-borne materials accumu- lated on the soil surface, the bottom of the previ- ously thawed soil layer became incorporated into permafrost. These sediments contain little of the humus that characterizes modern ecosys- tems of the region, but they comprise large amounts of grass roots (see the figure) and ani- mal bones, resulting in a carbon content that is much higher than is typical of most thawed min- eral soils. Frozen yedoma deposits across Siberia and Alaska typically have average car- bon contents from 2% to 5%—roughly 10 to 30 times the amount of carbon generally found in deep, nonpermafrost mineral soils. Using an overall average carbon concentra- tion for yedoma of ~2.6%, as well as the typi- cal bulk density, average thickness, and ice- wedge content of the yedoma, we estimate the carbon reservoir in frozen yedoma to be ~500 Gt ( 2 ). Another ~400 Gt of carbon are con- tained in nonyedoma permafrost (excluding peatlands) ( 3 ), and 50 to 70 Gt reside in the peatbogs of western Siberia ( 4 ). These prelim- inary estimates indicate that permafrost is a large carbon reservoir, intermediate in size between those of vegetation and soils. Our laboratory incubations and field experiments show that the organic matter in yedoma decomposes quickly when thawed, resulting in respiration rates of initially 10 to 40 g of carbon per m 3 per day, and then 0.5 to 5 g of carbon per m 3 per day over several years. These rates are similar to those of pro- ductive northern grassland soils. If these rates
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This note was uploaded on 11/07/2011 for the course EAS 8803 taught by Professor Staff during the Spring '08 term at Georgia Institute of Technology.

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Zimov-permafrost-200 - PERSPECTIVES requires detailed information on the statespecific collision rate and ortho-to-para conversions of H2 H3 and

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