The Arctic climate is changing. Permafrost is warming, hydrological processes are changing and biological and social systems are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates convincingly; the Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions are being experienced, a range of biophysical states and processes influenced by the threshold and phase change of freezing point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to isolate due to compounding factors. In almost every discipline represented, we show Climatic Change (2005) 72: 251-298 how the biocomplexity of the Arctic system has highlighted and challenged a paucity of integrated scientific knowledge, the lack of sustained observational and experimental time series, and the technical and logistic constraints of researching the Arctic environment. This study supports ongoing efforts to strengthen the interdisciplinarity of arctic system science and improve the coupling of large scale experimental manipulation with sustained time series observations by incorporating and integrating novel technologies, remote sensing and modeling.

Permafrost soils in boreal and Arctic ecosystems store almost twice as much carbon as is currently present in the atmosphere. Permafrost thaw and the microbial decomposition of previously frozen organic carbon is considered one of the most likely positive climate feedbacks from terrestrial ecosystems to the atmosphere in a warmer world. The rate of carbon release from permafrost soils is highly uncertain, but it is crucial for predicting the strength and timing of this carbon-cycle feedback effect, and thus how important permafrost thaw will be for climate change this century and beyond. Sustained transfers of carbon to the atmosphere that could cause a significant positive feedback to climate change must come from old carbon, which forms the bulk of the permafrost carbon pool that accumulated over thousands of years. Here we measure net ecosystem carbon exchange and the radiocarbon age of ecosystem respiration in a tundra landscape undergoing permafrost thaw to determine the influence of old carbon loss on ecosystem carbon balance. We find that areas that thawed over the past 15 years had 40 per cent more annual losses of old carbon than minimally thawed areas, but had overall net ecosystem carbon uptake as increased plant growth offset these losses. In contrast, areas that thawed decades earlier lost even more old carbon, a 78 per cent increase over minimally thawed areas; this old carbon loss contributed to overall net ecosystem carbon release despite increased plant growth. Our data document significant losses of soil carbon with permafrost thaw that, over decadal timescales, overwhelms increased plant carbon uptake at rates that could make permafrost a large biospheric carbon source in a warmer world.

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Permafrost degradation associated with a warming climate is second only to wildfires as a major disturbance to boreal forests. Permafrost temperatures have risen to 4 °C since the “Little Ice Age”, resulting in widespread thawing of permafrost. The mode of permafrost degradation is highly variable, and its topographic and ecological consequences depend on the interaction of slope position, soil texture, hydrology, and ice content. We partitioned this variability into 16 primary modes: (1) thermokarst lakes from lateral thermomechanical erosion; (2) thermokarst basins after lake drainage; (3) thaw sinks from subsurface drainage of lakes; (4) glacial thermokarst of ice-cored moraines; (5) linear collapse-scar fens associated with shallow groundwater movement; (6) round isolated collapse-scar bogs from slow lateral degradation; (7) small round isolated thermokarst pits from surface thawing; (8) polygonal thermokarst mounds from advanced ice-wedge degradation; (9) mixed thermokarst pits and polygons from initial ice-wedge degradation; (10) irregular thermokarst mounds from thawing of ice-poor silty soils; (11) sinkholes and pipes resulting from groundwater flow; (12) thermokarst gullies and water tracks from surface-water flow; (13) thaw slumps related to slope failure and thawing; (14) thermo-erosional niches from water undercutting of ice-rich shores; (15) collapsed pingos from thawing of massive ice in pingos; and (16) nonpatterned ground from thawing of ice-poor soils. These modes greatly influence how thermokarst changes or disrupts the ground surface, ecosystems, human activities, infrastructure, and the fluxes of energy, moisture, and gases across the land–air interface.

[1] Tentative answers are provided to questions concerning the recent warming of permafrost in Alaska, particularly those regarding timing, duration, magnitude, spatial distribution, seasonality, active layer effects, thawing, thermokarst terrain, and causes. Permafrost warmed at most sites north of the Brooks Range from the Chukchi Sea to the Alaska-Canada border, south along a transect from Prudhoe Bay to Gulkana and at sites up to 300 km from the transect. The warming was coincident with the statewide warming of air temperatures that began in 1976/1977 and appears to have occurred statewide with some exceptions. Magnitude of the warming was 3 to 4°C for the Arctic Coastal Plain, 1 to 2°C for the Brooks Range including its northern and southern foothills, and 0.3 to 1°C south of the Yukon River. This suggests a total warming of >6°C at Prudhoe Bay during the last century. The warming was seasonal (primarily in winter) with little change in summer conditions. Consequently, active layer thicknesses did not increase and were not correlated with warming permafrost conditions. Natural thawing at the permafrost surface ($0.1 m/yr) occurred at both a tundra and forest site. Basal thawing at one site was $0.04 m/yr until 2000 when it accelerated to $0.09 m/yr. New thermokarst terrain has been observed in interior and northern Alaska. Probable causes of the warming include increased air temperatures, snow cover effects, and combinations of these. New investigations are needed to further determine the characteristics, especially the causes, of this recent permafrost warming.

[1] Air temperatures at high latitudes are expected to rise significantly as anthropogenic carbon builds up in the atmosphere. There is concern that warming of the ground in permafrost regions will result in additional release of carbon to the atmosphere. Recent emphasis has thus been on predicting the magnitude and spatial distribution of future warming at high latitudes. Modeling results show that changes in below ground temperatures can be influenced as much by temporal variations of snow cover as by changes in the near-surface air temperature. The recent (1983 -1998) changes in permafrost temperatures on the North Slope of Alaska are consistent with decadal scale variability in snow cover. The implication of these results is that a better understanding of how winter precipitation patterns at high latitudes will change over the coming decades is needed to comprehend evolving permafrost temperatures.

Observations and measurements were made of physical and ecological changes that have occurred since 1985 at a tundra site near Healy, Alaska. Air temperatures decreased (1985 through 1999) while permafrost warmed and thawed creating thermokarst terrain, probably as a result of increased snow depths. Permafrost, active layer and ground‐ice conditions at the Healy site are the result of the interaction of climatic, ecologic and other factors. The slow accumulation of ground ice in an intermediate permafrost layer formed by upward freezing from the permafrost surface leads to long‐term differential frost heave and microrelief. When ground ice in the permafrost melts, the ground surface settles differentially resulting in thermokarst terrain (pits, gullies). Windblown snow fills the thermokarst depressions causing further warming and thawing of the underlying permafrost — a positive feedback effect that enhances permafrost degradation. Thermokarst‐induced changes in relief alter the near‐surface hydrology and ecological processes. Changes in vegetation included differential tussock growth and mortality and a shift in moss species abundance and relative productivity, depending on microtopographic position created by the thermokarst terrain. Water redistribution towards thermokarst depressions caused adjacent higher areas to become drier and resulted in increased moss mortality and shrub abundance. Copyright © 2009 John Wiley & Sons, Ltd.