[1] 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 × 106 km2) 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 (∼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.