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This paper provides a snapshot of the permafrost thermal state in the Nordic area obtained during the International Polar Year (IPY) [2007][2008][2009]. Several intensive research campaigns were undertaken within a variety of projects in the Nordic countries to obtain this snapshot. We demonstrate for Scandinavia that both lowland permafrost in palsas and peat plateaus, and large areas of permafrost in the mountains are at temperatures close to 08C, which makes them sensitive to climatic changes. In Svalbard and northeast Greenland, and also in the highest parts of the mountains in the rest of the Nordic area, the permafrost is somewhat colder, but still only a few degrees below the freezing point. The observations presented from the network of boreholes, more than half of which were established during the IPY, provide an important baseline to assess how future predicted climatic changes may affect the permafrost thermal state in the Nordic area. Time series of active-layer thickness and permafrost temperature conditions in the Nordic area, which are generally only 10 years in length, show generally increasing active-layer depths and rising permafrost temperatures.

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.

[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.

The northern latitudes are experiencing disproportionate warming relative to the mid-latitudes, and there is growing concern about feedbacks between this warming and methane production and release from high-latitude soils. Studies of methane emissions carried out in the Arctic, particularly those with measurements made outside the growing season, are underrepresented in the literature. Here we present results of 5 yr (2006–2010) of automatic chamber measurements at a high-Arctic location in Zackenberg, NE Greenland, covering both the growing seasons and two months of the following freeze-in periods. The measurements show clear seasonal dynamics in methane emission. The start of the growing season and the increase in CH₄ fluxes were strongly related to the date of snowmelt. Within each particular growing season, CH₄ fluxes were highly correlated with the soil temperature (R² > 0.75), which is probably explained by high seasonality of both variables, and weakly correlated with the water table. The greatest variability in fluxes between the study years was observed during the first part of the growing season. Somewhat surprisingly, this variability could not be explained by commonly known factors controlling methane emission, i.e. temperature and water table position. Late in the growing season CH₄ emissions were found to be very similar between the study years (except the extremely dry 2010) despite large differences in climatic factors (temperature and water table). Late-season bursts of CH₄ coinciding with soil freezing in the autumn were observed during at least three years. The cumulative emission during the freeze-in CH₄ bursts was comparable in size with the growing season emission for the year 2007, and about one third of the growing season emissions for the years 2009 and 2010. In all three cases the CH₄ burst was accompanied by a corresponding episodic increase in CO₂ emission, which can compose a significant contribution to the annual CO₂ flux budget. The most probable mechanism of the late-season CH₄ and CO₂ bursts is physical release of gases accumulated in the soil during the growing season. In this study we discuss possible links between growing season and autumn fluxes. Multiannual dynamics of the subsurface CH₄ storage pool are hypothesized to be such a link and an important driver of intearannual variations in the fluxes, capable of overruling the conventionally known short-term control factors (temperature and water table). Our findings suggest the importance of multiyear studies with a continued focus on shoulder seasons in Arctic ecosystems

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The land-atmosphere exchange of methane (CH 4 ) and carbon dioxide (CO 2 ) in a high-Arctic wet tundra ecosystem (Rylekaerene) in Zackenberg, north-eastern Greenland, was studied over the full growing season and until early winter in 2008 and from before snow melt until early winter in 2009. The eddy covariance technique was used to estimate CO 2 fluxes and a combination of the gradient and eddy covariance methods was used to estimate CH 4 fluxes. Small CH 4 bursts were observed during spring thawing 2009, but these existed during short periods and would not have any significant effect on the annual budget. Growing season CH 4 fluxes were well correlated with soil temperature, gross primary production, and active layer thickness. The CH 4 fluxes remained low during the entire autumn, and until early winter. No increase in CH 4 fluxes were seen as the soil started to freeze. However, in autumn 2008 there were two CH 4 burst events that were highly correlated with atmospheric turbulence. They were likely associated with the release of stored CH 4 from soil and vegetation cavities. Over the measurement period, 7.6 and 6.5 g C m À2 was emitted as CH 4 in 2008 and in 2009, respectively. Rylekaerene acted as a C source during the warmer and wetter measurement period 2008, whereas it was a C sink for the colder and drier period of 2009. Wet tundra ecosystems, such as Rylekaerene may thus play a more significant role for the climate in the future, as temperature and precipitation are predicted to increase in the high-Arctic.