I urval från Stockholms universitets publikationsdatabas

• Consistent centennial-scale change in European sub-Arctic peatland vegetation toward Sphagnum dominance—Implications for carbon sink capacity

  2023. Sanna R. Piilo (et al.). Global Change Biology 29 (6), 1530-1544

  Artikel

  Climate warming is leading to permafrost thaw in northern peatlands, and current predictions suggest that thawing will drive greater surface wetness and an increase in methane emissions. Hydrology largely drives peatland vegetation composition, which is a key element in peatland functioning and thus in carbon dynamics. These processes are expected to change. Peatland carbon accumulation is determined by the balance between plant production and peat decomposition. But both processes are expected to accelerate in northern peatlands due to warming, leading to uncertainty in future peatland carbon budgets. Here, we compile a dataset of vegetation changes and apparent carbon accumulation data reconstructed from 33 peat cores collected from 16 sub-arctic peatlands in Fennoscandia and European Russia. The data cover the past two millennia that has undergone prominent changes in climate and a notable increase in annual temperatures toward present times. We show a pattern where European sub-Arctic peatland microhabitats have undergone a habitat change where currently drier habitats dominated by Sphagnum mosses replaced wetter sedge-dominated vegetation and these new habitats have remained relatively stable over the recent decades. Our results suggest an alternative future pathway where sub-arctic peatlands may at least partly sustain dry vegetation and enhance the carbon sink capacity of northern peatlands.

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• Permafrost development in northern Fennoscandian peatlands since the mid-Holocene

  2023. Marit Hichens-Bergström, A. Britta K. Sannel. Arctic, Antarctic and Alpine research 55 (1)

  Artikel

  Increased permafrost temperatures have been reported in the circum-Arctic, and widespread degradation of permafrost peatlands has occurred in recent decades. The timing of permafrost aggradation in these ecosystems could have implications for the soil carbon lability upon thawing, and an increased understanding of the permafrost history is therefore needed to better project future carbon feedbacks. In this study, we have conducted high-resolution plant macrofossil and geochemical analyses and accelerator mass spectrometry radiocarbon dating of active layer cores from four permafrost peatlands in northern Sweden and Norway. In the mid-Holocene, all four sites were wet fens, and at least three of them remained permafrost-free until a shift in vegetation toward bog species was recorded around 800 to 400 cal. BP, suggesting permafrost aggradation during the Little Ice Age. At one site, Karlebotn, the plant macrofossil record also indicated a period of dry bog conditions between 3300 and 2900 cal. BP, followed by a rapid shift toward species growing in waterlogged fens or open pools, suggesting that permafrost possibly was present around 3000 cal. BP but thawed and was replaced by thermokarst.

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• Regional variability in peatland burning at mid-to high-latitudes during the Holocene

  2023. Thomas G. Sim (et al.). Quaternary Science Reviews 305

  Artikel

  Northern peatlands store globally-important amounts of carbon in the form of partly decomposed plant detritus. Drying associated with climate and land-use change may lead to increased fire frequency and severity in peatlands and the rapid loss of carbon to the atmosphere. However, our understanding of the patterns and drivers of peatland burning on an appropriate decadal to millennial timescale relies heavily on individual site-based reconstructions. For the first time, we synthesise peatland macrocharcoal records from across North America, Europe, and Patagonia to reveal regional variation in peatland burning during the Holocene. We used an existing database of proximal sedimentary charcoal to represent regional burning trends in the wider landscape for each region. Long-term trends in peatland burning appear to be largely climate driven, with human activities likely having an increasing influence in the late Holocene. Warmer conditions during the Holocene Thermal Maximum (∼9–6 cal. ka BP) were associated with greater peatland burning in North America's Atlantic coast, southern Scandinavia and the Baltics, and Patagonia. Since the Little Ice Age, peatland burning has declined across North America and in some areas of Europe. This decline is mirrored by a decrease in wider landscape burning in some, but not all sub-regions, linked to fire-suppression policies, and landscape fragmentation caused by agricultural expansion. Peatlands demonstrate lower susceptibility to burning than the wider landscape in several instances, probably because of autogenic processes that maintain high levels of near-surface wetness even during drought. Nonetheless, widespread drying and degradation of peatlands, particularly in Europe, has likely increased their vulnerability to burning in recent centuries. Consequently, peatland restoration efforts are important to mitigate the risk of peatland fire under a changing climate. Finally, we make recommendations for future research to improve our understanding of the controls on peatland fires.

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• A strong mitigation scenario maintains climate neutrality of northern peatlands

  2022. Chunjing Qiu (et al.). One Earth 5 (1), 86-97

  Artikel

  Northern peatlands store 300-600 Pg C, of which approximately half are underlain by permafrost. Climate warming and, in some regions, soil drying from enhanced evaporation are progressively threatening this large carbon stock. Here, we assess future CO2 and CH4 fluxes from northern peatlands using five land surface models that explicitly include representation of peatland processes. Under Representative Concentration Pathways (RCP) 2.6, northern peatlands are projected to remain a net sink of CO2 and climate neutral for the next three centuries. A shift to a net CO2 source and a substantial increase in CH4 emissions are projected under RCP8.5, which could exacerbate global warming by 0.21 degrees C (range, 0.09-0.49 degrees C) by the year 2300. The true warming impact of peatlands might be higher owing to processes not simulated by the models and direct anthropogenic disturbance. Our study highlights the importance of understanding how future warming might trigger high carbon losses from northern peatlands.

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• Synchronous or Not? The Timing of the Younger Dryas and Greenland Stadial-1 Reviewed Using Tephrochronology

  2022. Simon A. Larsson (et al.). Quaternary 5 (2)

  Artikel

  The exact spatial and temporal behaviour of rapid climate shifts during the Last Glacial–Interglacial Transition are still not entirely understood. In order to investigate these events, it is necessary to have detailed palaeoenvironmental reconstructions at geographically spread study sites combined with reliable correlations between them. Tephrochronology, i.e., using volcanic ash deposits in geological archives as a dating and correlation tool, offers opportunities to examine the timing of events across wider regional scales. This study aims to review the posited asynchrony of the Younger Dryas stadial in comparison with Greenland Stadial-1 by correlating new proxy data from southernmost Sweden to previous palaeoclimate reconstructions in Europe based on the presence of the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra. μ-XRF core-scanning data were projected using a recently published age–depth model based on these tephras and several radiocarbon dates, and compared to previous findings, including by adapting previous chronologies to the recently proposed earlier date of the Laacher See Tephra (13,006 ± 9 cal. a BP). Although the results to some extent support the idea of a more synchronous Younger Dryas event than previously assumed, this issue requires further high-resolution proxy studies to overcome limitations of temporal precision.

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• Expert assessment of future vulnerability of the global peatland carbon sink

  2021. J. Loisel (et al.). Nature Climate Change 11 (1), 70-77

  Artikel

  Peatlands are impacted by climate and land-use changes, with feedback to warming by acting as either sources or sinks of carbon. Expert elicitation combined with literature review reveals key drivers of change that alter peatland carbon dynamics, with implications for improving models. The carbon balance of peatlands is predicted to shift from a sink to a source this century. However, peatland ecosystems are still omitted from the main Earth system models that are used for future climate change projections, and they are not considered in integrated assessment models that are used in impact and mitigation studies. By using evidence synthesized from the literature and an expert elicitation, we define and quantify the leading drivers of change that have impacted peatland carbon stocks during the Holocene and predict their effect during this century and in the far future. We also identify uncertainties and knowledge gaps in the scientific community and provide insight towards better integration of peatlands into modelling frameworks. Given the importance of the contribution by peatlands to the global carbon cycle, this study shows that peatland science is a critical research area and that we still have a long way to go to fully understand the peatland-carbon-climate nexus.

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• Permafrost Thaw Increases Methylmercury Formation in Subarctic Fennoscandia

  2021. Brittany Tarbier (et al.). Environmental Science and Technology 55 (10), 6710-6717

  Artikel

  Methylmercury (MeHg) forms in anoxic environments and can bioaccumulate and biomagnify in aquatic food webs to concentrations of concern for human and wildlife health. Mercury (Hg) pollution in the Arctic environment may worsen as these areas warm and Hg, currently locked in permafrost soils, is remobilized. One of the main concerns is the development of Hg methylation hotspots in the terrestrial environment due to thermokarst formation. The extent to which net methylation of Hg is enhanced upon thaw is, however, largely unknown. Here, we have studied the formation of Hg methylation hotspots using existing thaw gradients at five Fennoscandian permafrost peatland sites. Total Hg (HgT) and MeHg concentrations were analyzed in 178 soil samples from 14 peat cores. We observed 10 times higher concentrations of MeHg and 13 times higher %MeHg in the collapse fen (representing thawed conditions) as compared to the peat plateau (representing frozen conditions). This suggests significantly greater net methylation of Hg when thermokarst wetlands are formed. In addition, we report HgT to soil organic carbon ratios representative of Fennoscandian permafrost peatlands (median and interquartile range of 0.09 +/- 0.07 mu g HgT g(-1) C) that are of value for future estimates of circumpolar HgT stocks.

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• Predicted Vulnerability of Carbon in Permafrost Peatlands With Future Climate Change and Permafrost Thaw in Western Canada

  2021. Claire C. Treat (et al.). Journal of Geophysical Research - Biogeosciences 126 (5)

  Artikel

  Climate warming in high-latitude regions is thawing carbon-rich permafrost soils, which can release carbon to the atmosphere and enhance climate warming. Using a coupled model of long-term peatland dynamics (Holocene Peat Model, HPM-Arctic), we quantify the potential loss of carbon with future climate warming for six sites with differing climates and permafrost histories in Northwestern Canada. We compared the net carbon balance at 2100 CE resulting from new productivity and the decomposition of active layer and newly thawed permafrost peats under RCP8.5 as a high-end constraint. Modeled net carbon losses ranged from -3.0 kg C m(-2) (net loss) to +0.1 kg C m(-2) (net gain) between 2015 and 2100. Losses of newly thawed permafrost peat comprised 0.2%-25% (median: 1.6%) of old C loss, which were related to the residence time of peat in the active layer before being incorporated into the permafrost, peat temperature, and presence of permafrost. The largest C loss was from the permafrost-free site, not from permafrost sites. C losses were greatest from depths of 0.2-1.0 m. New C added to the profile through net primary productivity between 2015 and 2100 offset similar to 40% to >100% of old C losses across the sites. Differences between modeled active layer deepening and flooding following permafrost thaw resulted in very small differences in net C loss by 2100, illustrating the important role of present-day conditions and permafrost aggradation history in controlling net C loss.

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• Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems

  2021. Heather Kropp (et al.). Environmental Research Letters 16 (1)

  Artikel

  Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (>40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw.

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• Warming climate forcing impact from a sub-arctic peatland as a result of late Holocene permafrost aggradation and initiation of bare peat surfaces

  2021. Minna Väliranta (et al.). Quaternary Science Reviews 264

  Artikel

  Effects of permafrost aggradation on greenhouse gas (GHG) dynamics and climate forcing have not been previously quantified. Here, we reconstruct changes in GHG balances over the late Holocene for a sub-arctic peatland by applying palaeoecological data combined with measured GHG flux data, focusing on the impact of permafrost aggradation in particular. Our data suggest that permafrost initiation around 3000 years ago resulted in GHG emissions, thereby slightly weakening the general long-term peatland cooling impact. As a novel discovery, based on our chronological data of bare peat surfaces, we found that current sporadic bare peat surfaces in subarctic regions are probably remnants of more extensive bare peat areas formed by permafrost initiation. Paradoxically, our data suggest that permafrost initiation triggered by the late Holocene cooling climate generated a positive radiative forcing and a short-term climate warming feedback, mitigating the general insolation-driven late Holocene summer cooling trend. Our work with historical data demonstrates the importance of permafrost peatland dynamics for atmospheric GHG concentrations, both in the past and future. It suggests that, while thawing permafrost is likely to initially trigger a change towards wetter conditions and consequent increase in CH4 forcing, eventually the accelerated C uptake capacity under warmer climate may overcome the thaw effect when a new hydrological balance becomes established.

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• Carbon release through abrupt permafrost thaw

  2020. Merritt R. Turetsky (et al.). Nature Geoscience 13 (2), 138-+

  Artikel

  The permafrost zone is expected to be a substantial carbon source to the atmosphere, yet large-scale models currently only simulate gradual changes in seasonally thawed soil. Abrupt thaw will probably occur in <20% of the permafrost zone but could affect half of permafrost carbon through collapsing ground, rapid erosion and landslides. Here, we synthesize the best available information and develop inventory models to simulate abrupt thaw impacts on permafrost carbon balance. Emissions across 2.5 million km(2) of abrupt thaw could provide a similar climate feedback as gradual thaw emissions from the entire 18 million km(2) permafrost region under the warming projection of Representative Concentration Pathway 8.5. While models forecast that gradual thaw may lead to net ecosystem carbon uptake under projections of Representative Concentration Pathway 4.5, abrupt thaw emissions are likely to offset this potential carbon sink. Active hillslope erosional features will occupy 3% of abrupt thaw terrain by 2300 but emit one-third of abrupt thaw carbon losses. Thaw lakes and wetlands are methane hot spots but their carbon release is partially offset by slowly regrowing vegetation. After considering abrupt thaw stabilization, lake drainage and soil carbon uptake by vegetation regrowth, we conclude that models considering only gradual permafrost thaw are substantially underestimating carbon emissions from thawing permafrost.

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• Modelling past and future peatland carbon dynamics across the pan-Arctic

  2020. Nitin Chaudhary (et al.). Global Change Biology 26 (7), 4119-4133

  Artikel

  The majority of northern peatlands were initiated during the Holocene. Owing to their mass imbalance, they have sequestered huge amounts of carbon in terrestrial ecosystems. Although recent syntheses have filled some knowledge gaps, the extent and remoteness of many peatlands pose challenges to developing reliable regional carbon accumulation estimates from observations. In this work, we employed an individual- and patch-based dynamic global vegetation model (LPJ-GUESS) with peatland and permafrost functionality to quantify long-term carbon accumulation rates in northern peatlands and to assess the effects of historical and projected future climate change on peatland carbon balance. We combined published datasets of peat basal age to form an up-to-date peat inception surface for the pan-Arctic region which we then used to constrain the model. We divided our analysis into two parts, with a focus both on the carbon accumulation changes detected within the observed peatland boundary and at pan-Arctic scale under two contrasting warming scenarios (representative concentration pathway-RCP8.5 and RCP2.6). We found that peatlands continue to act as carbon sinks under both warming scenarios, but their sink capacity will be substantially reduced under the high-warming (RCP8.5) scenario after 2050. Areas where peat production was initially hampered by permafrost and low productivity were found to accumulate more carbon because of the initial warming and moisture-rich environment due to permafrost thaw, higher precipitation and elevated CO2 levels. On the other hand, we project that areas which will experience reduced precipitation rates and those without permafrost will lose more carbon in the near future, particularly peatlands located in the European region and between 45 and 55 degrees N latitude. Overall, we found that rapid global warming could reduce the carbon sink capacity of the northern peatlands in the coming decades.

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• Overlooked organic vapor emissions from thawing Arctic permafrost

  2020. Haiyan Li (et al.). Environmental Research Letters 15 (10)

  Artikel

  Volatile organic compounds (VOCs) play an essential role in climate change and air pollution by modulating tropospheric oxidation capacity and providing precursors for ozone and aerosol formation. Arctic permafrost buries large quantities of frozen soil carbon, which could be released as VOCs with permafrost thawing or collapsing as a consequence of global warming. However, due to the lack of reported studies in this field and the limited capability of the conventional measurement techniques, it is poorly understood how much VOCs could be emitted from thawing permafrost and the chemical speciation of the released VOCs. Here we apply a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) in laboratory incubations for the first time to examine the release of VOCs from thawing permafrost peatland soils sampled from Finnish Lapland. The warming-induced rapid VOC emissions from the thawing soils were mainly attributed to the direct release of old, trapped gases from the permafrost. The average VOC fluxes from thawing permafrost were four times as high as those from the active layer (the top layer of soil in permafrost terrain). The emissions of less volatile compounds, i.e. sesquiterpenes and diterpenes, increased substantially with rising temperatures. Results in this study demonstrate the potential for substantive VOC releases from thawing permafrost. We anticipate that future global warming could stimulate VOC emissions from the Arctic permafrost, which may significantly influence the Arctic atmospheric chemistry and climate change.

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• Permafrost is warming at a global scale

  2019. Boris K. Biskaborn (et al.). Nature Communications 10

  Artikel

  Permafrost warming has the potential to amplify global climate change, because when frozen sediments thaw it unlocks soil organic carbon. Yet to date, no globally consistent assessment of permafrost temperature change has been compiled. Here we use a global data set of permafrost temperature time series from the Global Terrestrial Network for Permafrost to evaluate temperature change across permafrost regions for the period since the International Polar Year (2007-2009). During the reference decade between 2007 and 2016, ground temperature near the depth of zero annual amplitude in the continuous permafrost zone increased by 0.39 +/- 0.15 degrees C. Over the same period, discontinuous permafrost warmed by 0.20 +/- 0.10 degrees C. Permafrost in mountains warmed by 0.19 +/- 0.05 degrees C and in Antarctica by 0.37 +/- 0.10 degrees C. Globally, permafrost temperature increased by 0.29 +/- 0.12 degrees C. The observed trend follows the Arctic amplification of air temperature increase in the Northern Hemisphere. In the discontinuous zone, however, ground warming occurred due to increased snow thickness while air temperature remained statistically unchanged.

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• Widespread global peatland establishment and persistence over the last 130,000 y

  2019. Claire C. Treat (et al.). Proceedings of the National Academy of Sciences of the United States of America 116 (11), 4822-4827

  Artikel

  Glacial-interglacial variations in CO2 and methane in polar ice cores have been attributed, in part, to changes in global wetland extent, but the wetland distribution before the Last Glacial Maximum (LGM, 21 ka to 18 ka) remains virtually unknown. We present a study of global peatland extent and carbon (C) stocks through the last glacial cycle (130 ka to present) using a newly compiled database of 1,063 detailed stratigraphic records of peat deposits buried by mineral sediments, as well as a global peatland model. Quantitative agreement between modeling and observations shows extensive peat accumulation before the LGM in northern latitudes (> 40 degrees N), particularly during warmer periods including the last interglacial (130 ka to 116 ka, MIS 5e) and the interstadial (57 ka to 29 ka, MIS 3). During cooling periods of glacial advance and permafrost formation, the burial of northern peatlands by glaciers and mineral sediments decreased active peatland extent, thickness, and modeled C stocks by 70 to 90% from warmer times. Tropical peatland extent and C stocks show little temporal variation throughout the study period. While the increased burial of northern peats was correlated with cooling periods, the burial of tropical peat was predominately driven by changes in sea level and regional hydrology. Peat burial by mineral sediments represents a mechanism for long-term terrestrial C storage in the Earth system. These results show that northern peatlands accumulate significant C stocks during warmer times, indicating their potential for C sequestration during the warming Anthropocene.

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• Holocene development and permafrost history in sub-arctic peatlands in Tavvavuoma, northern Sweden

  2018. Anna Britta Kristina Sannel (et al.). Boreas 47 (2), 454-468

  Artikel

  Under changing climatic conditions permafrost peatlands can play an important role in the global carbon budget through permafrost carbon feedbacks and shifts in carbon assimilation. To better predict future dynamics in these ecosystems an increased understanding of their Holocene carbon and permafrost history is needed. In Tavvavuoma, northern Sweden, we have performed detailed analyses of vegetation succession and geochemical properties at six permafrost peatland sites. Peatland initiation took place around 10000 to 9600cal. a BP, soon after retreat of the Fennoscandian Ice Sheet, and the peatlands have remained permafrost-free fens throughout most of the Holocene. At the four sites that showed a continuous accumulation record during the late Holocene radiocarbon dating of the shift from wet fen to dry bog vegetation, characteristic of the present permafrost peatland surface, suggests that permafrost developed at around 600-100cal. a BP. At the other two sites peat accumulation was halted during the late Holocene, possibly due to abrasion, making it more difficult to imply the timing of permafrost aggradation. However also at these sites there are no indications of permafrost inception prior to the Little Ice Age. The mean long-term Holocene carbon accumulation rate at all six sites was 12.3 +/- 2.4 gC m(-2) a(-1) (+/- SD), and the mean soil organic carbon storage was 114 +/- 27kg m(-2).

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• Holocene development of subarctic permafrost peatlands in Finnmark, northern Norway

  2018. Sofia E. Kjellman (et al.). The Holocene 28 (12), 1855-1869

  Artikel

  Subarctic permafrost peatlands are important soil organic carbon pools, and improved knowledge about peat properties and peatland sensitivity to past climate change is essential when predicting future response to a warmer climate and associated feedback mechanisms. In this study, Holocene peatland development and permafrost dynamics of four subarctic peat plateaus in Finnmark, northern Norway have been investigated through detailed analyses of plant macrofossils and geochemical properties. Peatland inception occurred around 9800 cal. yr BP and 9200 cal. yr BP at the two continental sites Suossjavri and Iskoras. Younger basal peat ages were found at the two coastal locations Lakselv and Karlebotn, at least partly caused by the time lag between deglaciation and emergence of land by isostatic uplift. Here, peatland development started around 6150 cal. yr BP and 5150 cal. yr BP, respectively. All four peatlands developed as wet fens throughout most of the Holocene. Permafrost aggradation, causing frost heave and a shift in the vegetation assemblage from wet fen to dry bog species, probably did not occur until during the last millennium, ca. 950 cal. yr BP in Karlebotn and ca. 800 cal. yr BP in Iskoras, and before ca. 150 cal. yr BP in Lakselv and ca. 100 cal. yr BP in Suossjavri. In Karlebotn, there are indications of a possible earlier permafrost phase around 2200 cal. yr BP due to climatic cooling at the late Subboreal to early Subatlantic transition. The mean long-term Holocene carbon accumulation rate at all four sites was 12.3 +/- 4.1 gC m(-2) yr(-1) (+/- SD) and the mean soil organic carbon storage was 97 +/- 46 kgC m(-2).

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• Latitudinal limits to the predicted increase of the peatland carbon sink with warming

  2018. Angela Gallego-Sala (et al.). Nature Climate Change 8 (10), 907-+

  Artikel

  The carbon sink potential of peatlands depends on the balance of carbon uptake by plants and microbial decomposition. The rates of both these processes will increase with warming but it remains unclear which will dominate the global peatland response. Here we examine the global relationship between peatland carbon accumulation rates during the last millennium and planetary-scale climate space. A positive relationship is found between carbon accumulation and cumulative photosynthetically active radiation during the growing season for mid- to high-latitude peatlands in both hemispheres. However, this relationship reverses at lower latitudes, suggesting that carbon accumulation is lower under the warmest climate regimes. Projections under Representative Concentration Pathway (RCP)2.6 and RCP8.5 scenarios indicate that the present-day global sink will increase slightly until around AD 2100 but decline thereafter. Peatlands will remain a carbon sink in the future, but their response to warming switches from a negative to a positive climate feedback (decreased carbon sink with warming) at the end of the twenty-first century.

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• PeRL: a circum-Arctic Permafrost Region Pond and Lake database

  2017. Sina Muster (et al.). Earth System Science Data 9 (1), 317-348

  Artikel

  Ponds and lakes are abundant in Arctic permafrost lowlands. They play an important role in Arctic wetland ecosystems by regulating carbon, water, and energy fluxes and providing freshwater habitats. However, ponds, i. e., waterbodies with surface areas smaller than 1.0 x 10(4) m(2), have not been inventoried on global and regional scales. The Permafrost Region Pond and Lake (PeRL) database presents the results of a circum-Arctic effort to map ponds and lakes from modern (2002-2013) high-resolution aerial and satellite imagery with a resolution of 5m or better. The database also includes historical imagery from 1948 to 1965 with a resolution of 6m or better. PeRL includes 69 maps covering a wide range of environmental conditions from tundra to boreal regions and from continuous to discontinuous permafrost zones. Waterbody maps are linked to regional permafrost landscape maps which provide information on permafrost extent, ground ice volume, geology, and lithology. This paper describes waterbody classification and accuracy, and presents statistics of waterbody distribution for each site. Maps of permafrost landscapes in Alaska, Canada, and Russia are used to extrapolate waterbody statistics from the site level to regional landscape units. PeRL presents pond and lake estimates for a total area of 1.4 x 10(6) km(2) across the Arctic, about 17% of the Arctic lowland (<300ma. s.l.) land surface area. PeRL waterbodies with sizes of 1.0 x 10(6) m(2) down to 1.0 x 10(2) m(2) contributed up to 21% to the total water fraction. Waterbody density ranged from 1.0 x 10 to 9.4 x 10(1) km(-2). Ponds are the dominant waterbody type by number in all landscapes representing 45-99% of the total waterbody number. The implementation of PeRL size distributions in land surface models will greatly improve the investigation and projection of surface inundation and carbon fluxes in permafrost lowlands. Waterbody maps, study area boundaries, and maps of regional permafrost landscapes including detailed metadata are available at https://doi.pangaea.de/10.1594/PANGAEA.868349.

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• Permafrost Map for Norway, Sweden and Finland

  2017. Kjersti Gisnås (et al.). Permafrost and Periglacial Processes 28 (2), 359-378

  Artikel

  A research-based understanding of permafrost distribution at a sufficient spatial resolution is important to meet the demands of science, education and society. We present a new permafrost map for Norway, Sweden and Finland that provides a more detailed and updated description of permafrost distribution in this area than previously available. We implemented the CryoGRID1 model at 1km(2) resolution, forced by a new operationally gridded data-set of daily air temperature and snow cover for Finland, Norway and Sweden. Hundred model realisations were run for each grid cell, based on statistical snow distributions, allowing for the representation of sub-grid variability of ground temperature. The new map indicates a total permafrost area (excluding palsas) of 23 400km(2) in equilibrium with the average 1981-2010 climate, corresponding to 2.2 per cent of the total land area. About 56 per cent of the area is in Norway, 35 per cent in Sweden and 9 per cent in Finland. The model results are thoroughly evaluated, both quantitatively and qualitatively, as a collaboration project including permafrost experts in the three countries. Observed ground temperatures from 25 boreholes are within +/- 2 degrees C of the average modelled grid cell ground temperature, and all are within the range of the modelled ground temperature for the corresponding grid cell. Qualitative model evaluation by field investigators within the three countries shows that the map reproduces the observed lower altitudinal limits of mountain permafrost and the distribution of lowland permafrost.

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• Circumpolar distribution and carbon storage of thermokarst landscapes

  2016. D. Olefeldt (et al.). Nature Communications 7

  Artikel

  Thermokarst is the process whereby the thawing of ice- rich permafrost ground causes land subsidence, resulting in development of distinctive landforms. Accelerated thermokarst due to climate change will damage infrastructure, but also impact hydrology, ecology and biogeochemistry. Here, we present a circumpolar assessment of the distribution of thermokarst landscapes, defined as landscapes comprised of current thermokarst landforms and areas susceptible to future thermokarst development. At 3.6 x 10(6) km(2), thermokarst landscapes are estimated to cover similar to 20% of the northern permafrost region, with approximately equal contributions from three landscape types where characteristic wetland, lake and hillslope thermokarst landforms occur. We estimate that approximately half of the below-ground organic carbon within the study region is stored in thermokarst landscapes. Our results highlight the importance of explicitly considering thermokarst when assessing impacts of climate change, including future landscape greenhouse gas emissions, and provide a means for assessing such impacts at the circumpolar scale.

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• Effects of permafrost aggradation on peat properties as determined from a pan-Arctic synthesis of plant macrofossils

  2016. C. C. Treat (et al.). Journal of Geophysical Research - Biogeosciences 121 (1), 78-94

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  Permafrost dynamics play an important role in high-latitude peatland carbon balance and are key to understanding the future response of soil carbon stocks. Permafrost aggradation can control the magnitude of the carbon feedback in peatlands through effects on peat properties. We compiled peatland plant macrofossil records for the northern permafrost zone (515 cores from 280 sites) and classified samples by vegetation type and environmental class (fen, bog, tundra and boreal permafrost, and thawed permafrost). We examined differences in peat properties (bulk density, carbon (C), nitrogen (N) and organic matter content, and C/N ratio) and C accumulation rates among vegetation types and environmental classes. Consequences of permafrost aggradation differed between boreal and tundra biomes, including differences in vegetation composition, C/N ratios, and N content. The vegetation composition of tundra permafrost peatlands was similar to permafrost-free fens, while boreal permafrost peatlands more closely resembled permafrost-free bogs. Nitrogen content in boreal permafrost and thawed permafrost peatlands was significantly lower than in permafrost-free bogs despite similar vegetation types (0.9% versus 1.5% N). Median long-term C accumulation rates were higher in fens (23g C m(-2)yr(-1)) than in permafrost-free bogs (18g C m(-2)yr(-1)) and were lowest in boreal permafrost peatlands (14g C m(-2)yr(-1)). The plant macrofossil record demonstrated transitions from fens to bogs to permafrost peatlands, bogs to fens, permafrost aggradation within fens, and permafrost thaw and reaggradation. Using data synthesis, we have identified predominant peatland successional pathways, changes in vegetation type, peat properties, and C accumulation rates associated with permafrost aggradation.

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• Permafrost Warming in a Subarctic Peatland - Which Meteorological Controls are Most Important?

  2016. A. Britta K. Sannel (et al.). Permafrost and Periglacial Processes 27 (2), 177-188

  Artikel

  Because climate change can affect the carbon balance and hydrology in permafrost peatlands, a better understanding of their sensitivity to changes in temperature and precipitation is needed. In Tavvavuoma, northernmost Sweden, meteorological parameters and ground thermal properties have been monitored in a peat plateau from 2006 to 2013. During this time period, the air temperature record shows no warming trend, and the late-season thaw depth has been relatively stable at around 55-60cm. Meanwhile, the mean annual ground temperature at 1m depth has increased by 0.06 degrees C/yr and at 2-5m depth the permafrost is currently warmer than -0.3 degrees C. Statistical analyses suggest that interannual changes in thaw depth and ground temperatures are affected by different meteorological factors. Summer air temperatures and annual thawing degree-days control thaw depth (p0.05), whereas winter precipitation/snow depth affects ground temperatures (p0.1). The permafrost in this peat plateau is likely relict and not in equilibrium with current climatic conditions. Since the early 20(th) century, there has been a regional increase in air temperature and snow depth. If the ongoing permafrost warming in Tavvavuoma is a result of these long-term trends, short-term variability in meteorological parameters can still have an impact on the rate of permafrost degradation, but unless pronounced climate cooling occurs, thawing of the peat plateau is inevitable.

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• Thermal effects of groundwater flow through subarctic fens: A case study based on field observations and numerical modeling

  2016. Ylva Sjöberg (et al.). Water resources research 52 (3), 1591-1606

  Artikel

  Modeling and observation of ground temperature dynamics are the main tools for understanding current permafrost thermal regimes and projecting future thaw. Until recently, most studies on permafrost have focused on vertical ground heat fluxes. Groundwater can transport heat in both lateral and vertical directions but its influence on ground temperatures at local scales in permafrost environments is not well understood. In this study we combine field observations from a subarctic fen in the sporadic permafrost zone with numerical simulations of coupled water and thermal fluxes. At the Tavvavuoma study site in northern Sweden, ground temperature profiles and groundwater levels were observed in boreholes. These observations were used to set up one- and two-dimensional simulations down to 2 m depth across a gradient of permafrost conditions within and surrounding the fen. Two-dimensional scenarios representing the fen under various hydraulic gradients were developed to quantify the influence of groundwater flow on ground temperature. Our observations suggest that lateral groundwater flow significantly affects ground temperatures. This is corroborated by modeling results that show seasonal ground ice melts 1 month earlier when a lateral groundwater flux is present. Further, although the thermal regime may be dominated by vertically conducted heat fluxes during most of the year, isolated high groundwater flow rate events such as the spring freshet are potentially important for ground temperatures. As sporadic permafrost environments often contain substantial portions of unfrozen ground with active groundwater flow paths, knowledge of this heat transport mechanism is important for understanding permafrost dynamics in these environments.

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• A database and synthesis of northern peatland soil properties and Holocene carbon and nitrogen accumulation

  2014. Julie Loisel (et al.). The Holocene 24 (9), 1028-1042

  Artikel

  Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45 degrees N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 +/- 3% (standard deviation) for Sphagnum peat, 51 +/- 2% for non-Sphagnum peat, and at 49 +/- 2% overall. Dry bulk density averaged 0.12 +/- 0.07 g/cm(3), organic matter bulk density averaged 0.11 +/- 0.05 g/cm(3), and total carbon content in peat averaged 47 +/- 6%. In general, large differences were found between Sphagnum and non-Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 +/- 2 (standard error of mean) g C/m(2)/yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25-28 g C/m(2)/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu.

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• Expert assessment of vulnerability of permafrost carbon to climate change

  2013. E. A. G. Schuur (et al.). Climatic Change 119 (2), 359-374

  Artikel

  Approximately 1700 Pg of soil carbon (C) are stored in the northern circumpolar permafrost zone, more than twice as much C than in the atmosphere. The overall amount, rate, and form of C released to the atmosphere in a warmer world will influence the strength of the permafrost C feedback to climate change. We used a survey to quantify variability in the perception of the vulnerability of permafrost C to climate change. Experts were asked to provide quantitative estimates of permafrost change in response to four scenarios of warming. For the highest warming scenario (RCP 8.5), experts hypothesized that C release from permafrost zone soils could be 19-45 Pg C by 2040, 162-288 Pg C by 2100, and 381-616 Pg C by 2300 in CO2 equivalent using 100-year CH4 global warming potential (GWP). These values become 50 % larger using 20-year CH4 GWP, with a third to a half of expected climate forcing coming from CH4 even though CH4 was only 2.3 % of the expected C release. Experts projected that two-thirds of this release could be avoided under the lowest warming scenario (RCP 2.6). These results highlight the potential risk from permafrost thaw and serve to frame a hypothesis about the magnitude of this feedback to climate change. However, the level of emissions proposed here are unlikely to overshadow the impact of fossil fuel burning, which will continue to be the main source of C emissions and climate forcing.

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• Warming-induced destabilization of peat plateau/thermokarst lake complexes

  2011. A. B. K. Sannel, Peter Kuhry. Journal of Geophysical Research 116, G03035

  Artikel

  Peat plateaus are widespread at high northern latitudes and are important soil organic carbon reservoirs. A warming climate can cause either increased ground subsidence (thermokarst) resulting in lake formation or increased drainage as the permafrost thaws. A better understanding of spatiotemporal variations in these landforms in relation to climate change is important for predicting the future thawing permafrost carbon feedback. In this study, dynamics in thermokarst lake extent during the last 35-50 years has been quantified through time series analysis of aerial photographs and high-resolution satellite images (IKONOS/QuickBird) in three peat plateau complexes, spread out across the northern circumpolar region along a climatic and permafrost gradient. From the mid-1970s until the mid-2000s there has been an increase in mean annual air temperature, winter precipitation, and ground temperature in all three study areas. The two peat plateaus located in the continuous and discontinuous permafrost zones, respectively, where mean annual air temperatures are below -5 degrees C and ground temperatures are -2 degrees C or colder, have experienced small changes in thermokarst lake extent. In the peat plateau located in the sporadic permafrost zone where the mean annual air temperature is around -3 degrees C, and the ground temperature is close to 0 degrees C, lake drainage and infilling with fen vegetation has been extensive and many new thermokarst lakes have formed. In a future progressively warmer and wetter climate permafrost degradation can cause significant impacts on landscape composition and greenhouse gas exchange also in areas with extensive peat plateaus, which presently still experience stable permafrost conditions.

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• High-resolution remote sensing identification of thermokarst lake dynamics in a subarctic peat plateau complex

  2010. A. Britta K. Sannel, Ian A. Brown. Canadian journal of remote sensing 36 (1), S26-S40

  Artikel

  Methods to quantify the dynamics of thermokarst lake margins in subarctic permafrost peatlands have been examined using historical aerial photographs and QuickBird imagery from the Hudson Bay Lowlands in west-central Canada, spanning a time period of 52 years (1954-2006). The goal of this study was to develop a method for detection of metre-scale changes in thermokarst lake extent using a time series of high resolution imagery. The method should be applicable to a variety of lake forms, transferable to other locations and sufficiently robust as to support different data types. Semi-automatic remote sensing techniques such as unsupervised and supervised classification and texture and high-pass filtering were tested, evaluated and rejected. According to an experiment of manual digitalization of shorelines by multiple operators the relative uncertainty for lakes surrounded by peat plateau was ±1.5 m. The uncertainty was reduced to ±0.6 m when binary encoding of transects perpendicular to the shoreline was used to refine the manual delineation. This proved to be the most accurate method to detect small-scale changes in lake extent. An increased understanding and quantification of thermokarst dynamics in permafrost peatlands is important for predicting future scenarios of greenhouse gas emissions from these ecosystems under changing climatic conditions and our method supports such goals.

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• Long-term climate variability in continental subarctic Canada: A 6200-year record derived from stable isotopes in peat

  2010. Päivi Kaislahti Tillman (et al.). Palaeogeography, Palaeoclimatology, Palaeoecology 298 (3), 235-246

  Artikel

  The rapid warming of arctic regions during recent decades has been recorded by instrumental monitoring, but the natural climate variability in the past is still sparsely reconstructed across many areas. We have reconstructed past climate changes in subarctic west-central Canada. Stable carbon and oxygen isotope ratios (δ¹³C, δ¹⁸O) were derived from a single Sphagnum fuscum plant component; α-cellulose isolated from stems. Periods of warmer and cooler conditions identified in this region, described in terms of a “Mediaeval Climatic Anomaly” and “Little Ice Age” were registered in the temperature reconstruction based on the δ¹³C record. Some conclusions could be drawn about wet/dry shifts during the same time interval from the δ¹⁸O record, humification indices and the macrofossil analysis. The results were compared with other proxy data from the vicinity of the study area. The amplitude of the temperature change was similar to that in chironomid based reconstructions, showing c. 6.5±2.3°C variability in July temperatures during the past 6.2 ka.

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• Stable carbon and oxygen isotopes in Sphagnum fuscum peat from subarctic Canada: implications for palaeoclimate studies

  2010. Päivi Kaislahti Tillman (et al.). Chemical Geology 270 (1-4), 216-226

  Artikel

  Stable carbon and oxygen isotope ratios in single plant components in Sphagnum peat have a good potential to reveal environmental changes in peat archives. Two peat profiles, covering the past ~6000 years, and a Sphagnum hummock from a discontinuous permafrost area in west central Canada were studied in order to evaluate the effect of decomposition rate on isotope records and to assess which plant components are most suitable for climate reconstructions. The stable isotope values from the most recently forming Sphagnum tissues were compared with observational climate data to study the impact of variations in temperature and precipitation on the peat isotopes. Our results show that there is high correlation between δ¹³C values in α-cellulose isolated from Sphagnum fuscum stems and summer temperatures, whereas δ¹⁸O in the plant tissues is controlled by several factors, such as summer precipitation, summer temperature and evaporation. According to our results, decomposition as derived from C/N values and colorimetry does not seem to affect the oxygen and carbon isotope values of α-cellulose from Sphagnum fuscum peat significantly. There is, however, a (quasi-) constant offset between the isotope values of branches and stems and between whole plant material and  α-cellulose, which makes it crucial to select single moss-fractions when past climate and environmental changes are to be derived from the isotope record.

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• The thermal state of permafrost in the Nordic area during the International Polar Year 2007-2009

  2010. Hanne H. Christiansen (et al.). Permafrost and Periglacial Processes 21 (2), 156-181

  Artikel

  This paper provides a snapshot of the permafrost thermal state in the Nordic area obtained during the International Polar Year (IPY) 2007–2009. 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 0°C, 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.

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• Holocene peat growth and decay dynamics in sub-arctic peat plateaus, west-central Canada

  2009. Britta Sannel, Peter Kuhry. Boreas 38 (1), 13-24

  Artikel

  Peat and net carbon accumulation rates in two sub-arctic peat plateaus of west-central Canada have been studied through geochemical analyses and accelerator mass spectrometry (AMS) radiocarbon dating. The peatland sites started to develop around 6600–5900 cal. yr BP and the peat plateau stages are characterized by Sphagnum fuscum peat alternating with rootlet layers. The long-term peat and net carbon accumulation rates for both profiles are 0.30–0.31 mm/yr and 12.5–12.7 gC/m²yr, respectively. These values reflect very slow peat accumulation (0.04–0.09 mm/yr) and net carbon accumulation (3.7–5.2 gC/m²yr) in the top rootlet layers. Extensive AMS radiocarbon dating of one profile shows that accumulation rates are variable depending on peat plateau stage. Peat accumulation rates are up to six times higher and net carbon accumulation rates up to four times higher in S. fuscum than in rootlet stages. Local fires represented by charcoal remains in some of the rootlet layers result in very low accumulation rates. High C/N ratios throughout most of the peat profiles suggest low degrees of decomposition due to stable permafrost conditions. Hence, original peat accretion has remained largely unaltered, except in the initial stages of peatland development when permafrost was not yet present.

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• Long-term stability of permafrost in subarctic peat plateaus, west-central Canada

  2008. Britta Sannel, Peter Kuhry. The Holocene 18 (4), 589-601

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  Long-term vegetation succession and permafrost dynamics in subarctic peat plateaus of west-central Canada have been studied through detailed plant macrofossil analysis and extensive AMS radiocarbon dating of two peat profiles. Peatland inception at these sites occurred around 5800-5100 yr BP (6600-5900 cal. BP) as a result of paludification of upland forests. At the northern peat plateau site, located in the continuous permafrost zone, palaeobotanical evidence suggests that permafrost was already present under the forested upland prior to peatland development. Paludification was initiated by permafrost collapse, but re-aggradation of permafrost occurred soon after peatland inception. At the southern site, located in the discontinuous permafrost zone, the aggradation of permafrost occurred soon after peatland inception. In the peat plateaus, permafrost conditions have remained very stable until present. Sphagnum fuscum-dominated stages have alternated with more xerophytic communities characterized by ericaceous shrubs. Local peat fires have occurred, but most of these did not cause degradation of the permafrost. Starting from 2800-1100 yr BP (2900-1000 cal. BP) consistently dry surface conditions have prevailed, possibly related to continued frost heave or nearby polygon crack formation.

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