Permafrost carbon: Stock and decomposability of a globally significant carbon pool

The magnitude of future CO2-induced climate warming is difficult to predict because of uncertainties in the role of ecosystems and oceans as CO2 sources and sinks. Siberia has extensive areas ( 1 x 10(6) km(2)) of deep ( up to 90 m) deposits of organic-rich frozen loess (wind-blown silt) that accumulated during the Pleistocene but have not been considered in most global carbon ( C) inventories. Similar deposits occur less extensively in Alaska. Recent warming at high latitudes causes this permafrost ( permanently frozen ground) to thaw, raising questions about the fate of C in thawing permafrost. Here we show that Siberian loess permafrost contains a large organic C pool (similar to 450 GT - more than half the quantity in the current atmosphere) that decomposes quickly when thawed, and could act as a positive feedback to climate warming.