About one-third of the total global soil carbon is locked up in cold soils in northern high-latitude tundra. Cold but seasonally above-freezing temperatures allow plant growth to proceed at rates that lead to carbon accumulation in vegetation at a rate that exceeds carbon loss by decomposition. Hence the tundra historically has been a sink for atmospheric carbon dioxide. A question arises about the future direction gain or loss in this vast carbon pool under the influence of global warming, which by both measurements and models, is proceeding higher rates at high latitude. In a careful long term study of the both above-ground and below-ground soil pools, Mack et al (2004) show that enhanced nutrient availability that is expected to accompany warming of high latitudes leads to enhanced plant growth (gaining carbon) but also increased soil microbial activity (losing carbon) and that the net result is a loss of carbon to tundra ecosystems. Mack et al. (2004), and also Loya and Grogan (2004) in a summary article, point out that this points to a positive feedback to global warming: a warmer climate releases carbon dioxide through decay processes, which leads to enhanced trapping of longwave radiation by elevated levels of greenhouse gases, which in turn leads to further warming.
References
Loya, W.M., and P. Grogan, 2004: Carbon conundrum on the tundra. Nature,
431, 406-408.
Mack, M.C., E.A.G. Schuur, M.S. Bret-Harte, G.R. Shaver, and F.S. Chapin III,
2004: Ecosystem carbon storage in Arctic tundra reduced by long-term nutrient
fertilization. Nature, 431, 440-443.