Is the home of polar bears, seals and Inuit communities already doomed? asks Jon Foley in Tipping Points in the Tundra a recent commentary Science. According to him, several recent sources of evidence show that feedback mechanisms seem to be kicking into high gear as the Arctic warms up. Temperature data illustrate, for example, that from the 1960’s to the 1980’s, the Arctic warmed by 0.15 degrees Celsius per decade, but since then the warming has been nearly 0.3 to 0.4 degrees per decade.
Recent evidence comes from Terry Chapin and his co-workers who have analyzed Arctic data on surface temperature, cloud cover, energy exchange, albedo, and changes in snow cover and vegetation. They concluded that the recent changes in the length of the snow-free season have triggered a set of interlinked feedbacks that will amplify future rates of summer warming. One of these feedbacks relate to that the snowmelt has advanced by around 2.5 days per decade which has lead to an increase in the amount of energy that is absorbed and transferred to the atmosphere. The resulting regional increase in temperature is estimated to be comparable (per unit area) to the global atmospheric heating that is projected from a doubling of CO2 levels in the atmosphere.
Chapin et al. also analyses the role of vegetation change for triggering positive feedbacks. Tall shrublands have increased rapidly in the surrounding region of the Arctic. Tree lines have also moved further north. Although the estimated contributions these have on warming were found to be small, the authors expect that they will continue to increase disproportionally in the future.
Chapin et al. concludes:
…summer warming in the Alaskan sector is occurring primarily on land, where a longer snow-free season has contributed more strongly to atmospheric heating than have vegetation changes. This heating more than offsets the cooling caused by increased cloudiness. However, the high temperature sensitivity of several feedback loops, particularly those associated with shrub expansion, suggests that terrestrial amplification of high-latitude warming will likely become more pronounced in the future. Improved understanding of the controls over rates of shrub expansion would reduce the likelihood of unexpected surprises regarding the magnitude of high-latitude amplification of summer warming.
Global Circulation Models (GCM) have an important role in advancing our understanding on climate change. Foley says that the current GCM’s includes representation of changes relation to reduction in snow cover. But he also argue that
unfortunately, few GCM’s represent the possible feedbacks from changing vegetation cover and the associated changes in land-surface properties. As Chapin et al. suggest, increases in shrub and forest cover in the Arctic could dramatically amplify global warming in the Arctic, but nearly all GCM’s used today do not consider such changes in vegetation cover.
A similar point is also Roger A. Pielke Sr. research group’s Climate Science weblog which has an interesting post on climatic tipping points. The group focuses on land-atmosphere interactions ocurring at local, mesoscale, regional, and global scales. The post discusses an an interesting paper Pielke was also a co-author of last year:
Peters, D.P.C., R.A. Pielke Sr., B.T. Bestelmeyer, C.D. Allen, S. Munson-McGee, and K.M. Havstad, 2004: Cross-scale interactions, nonlinearities, and forecasting catastrophic events. Proceedings of the National Academy of Sciences, 101, No. 42, 15130-15135,