At the close of the Fourth International Polar Year, we take stock of the ecological consequences of recent climate change in the Arctic, focusing on effects at population, community, and ecosystem scales. Despite the buffering effect of landscape heterogeneity, Arctic ecosystems and the trophic relationships that structure them have been severely perturbed. These rapid changes may be a bellwether of changes to come at lower latitudes and have the potential to affect ecosystem services related to natural resources, food production, climate regulation, and cultural integrity. We highlight areas of ecological research that deserve priority as the Arctic continues to warm.

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Thawing permafrost represents a poorly understood feedback mechanism of climate change in the Arctic, but with a potential impact due to stored carbon being mobilised (1-5). We have quantified the long-term loss of C from thawing permafrost in NE Greenland from 1996 to 2008 by combining repeated sediment sampling to assess changes in C stock and >12 years of CO 2 production in incubated permafrost samples. Field observations show that the active layer thickness has increased by >1 cm per year but thawing has not resulted in a detectable decline in C-stocks. Laboratory mineralisation rates at 5º C resulted in a C loss between 9% and 75%, depending on drainage highlighting the potential of fast mobilisation of permafrost C under aerobic conditions, but also that C at near-saturated conditions may remain largely immobilised over decades. This is confirmed by a three-pool C dynamics model that projects a potential C loss between 13% and 77% for 50 years of incubation at 5º C.

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[1] We have measured the land-atmosphere CO 2 exchange using the eddy covariance technique in a high Arctic tundra heath in northeast Greenland (Zackenberg). On the basis of 11 years of measurements (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), it was found that snow cover dynamics was important for the CO 2 exchange. The start of CO 2 uptake period correlated significantly with timing of snowmelt. Furthermore, for years with deep and long-lasting snowpacks, the following springs showed increased CO 2 emission rates. In the first part of the study period, there was an increase of approximately 8 g C m À2 yr À1 in both accumulated gross primary production (GPP) and CO 2 sink strength during summer. However, in the last few years, there were no significant changes in GPP, whereas ecosystem respiration (R eco ) increased (8.5 g C m À2 yr À1) and ecosystem CO 2 sink strength weakened (À4.1 g C m). It was found that temperature and temperature-related variables (maximum thaw depth and growing degree days) controlled the interannual variation in CO 2 exchange. However, while R eco showed a steady increase with temperature (5.8 g C m À2 C À1 ), the initial increase in GPP with temperature leveled off at the high end of observed temperature range. This suggests that future increases in temperature will weaken the ecosystem CO 2 sink strength or even turn it into a CO 2 source, depending on possible changes in vegetation structure and functioning as a response to a changing climate. If this trend is applicable also to other Arctic ecosystems, it will have implications for our current understanding of Arctic ecosystems dynamics.