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The out‐of‐phase mode of winter surface air temperature anomalies (SATAs) over northern Central Asia (NCA; 45°–65°N, 40°–100°E) between January and February is investigated in this study. This mode corresponds to warm (cold) SATAs in January (February) over NCA and is mainly modulated by the enhanced tropical convection anomalies over the Maritime Continent in previous late December, associated with MJO phase 4. These tropical convection anomalies can induce anomalous tropospheric Rossby‐wave sources over the North Pacific in late December. The eastward‐propagating Rossby‐wave train, triggered by these anomalous Rossby‐wave sources, can cause negative and positive tropospheric geopotential height anomalies over the Greenland–Scandinavia region and NCA in the following early–mid‐January, subsequently leading to warm SATAs over NCA in January. The negative geopotential height anomalies over the Greenland–Scandinavia region in early–mid‐January can trigger upward‐propagating wave activity fluxes (WAFs) into the stratosphere, resulting in negative stratospheric geopotential height anomalies in late January–early February. These stratospheric anomalies, by triggering downward‐propagating WAFs, can in turn lead to positive tropospheric geopotential height anomalies over the Greenland–Scandinavia region in early February. These anomalies over the Greenland–Scandinavia region can maintain themselves in the following mid‐ and late February by feedback of anomalous storm tracks, and cause negative geopotential height anomalies and subsequently cold SATAs over NCA in February by triggered southeastward‐propagating Rossby‐wave train. [ABSTRACT FROM AUTHOR]

The Greenland Ice Sheet is a leading contributor to global sea‐level rise because climate warming has enhanced surface meltwater runoff. Melt rates are particularly sensitive to air temperatures due to feedbacks with albedo. The primary melt‐albedo feedback, fluctuation of seasonal snowlines, however, is determined not only by melt but also by antecedent snowfall which could delay the onset of dark glacier ice exposure. Here we investigate the role of snowfall versus air temperatures on ice sheet melt‐albedo feedbacks using satellite remote sensing and atmospheric reanalysis data. We find several lines of evidence that snowline fluctuations are driven primarily by air temperatures and that snowfall is a secondary control. First, standardized linear regressions indicate that the timing of glacier ice exposure is nearly twice as sensitive to air temperatures than antecedent snowfall. Second, in 74% of the ablation zone by area, winter snowfall rates are not significantly correlated with winter air temperatures. This relationship implies that ice sheet melt due to climate warming is unlikely to be compensated by higher snowfall rates in the ablation zone. Third, we find no significant change in snowfall rates in the ablation zone during our 1981–2021 study period. Our findings demonstrate that snowfall is unlikely to reduce future ice sheet melt and that ice sheet meltwater runoff should be accurately predicted by air temperatures. Although given the importance of melt‐albedo feedbacks, ice sheet models that parameterize albedo or are coupled with regional climate models are likely to provide the most accurate projections of mass loss. Plain Language Summary: The Greenland Ice Sheet is currently losing mass because rates of mass loss (mainly due to surface meltwater runoff and iceberg discharge into the ocean) are higher than rates of mass gain (mainly due to snowfall). As the climate warms, rates of mass loss are expected to increase non‐linearly because rising air temperatures darken the surface, leading to more solar energy absorption, and more melting. The main process responsible for surface darkening is the exposure of dark glacier ice due to Greenland's seasonally evolving snowline. The position of the snowline is determined not only by summer melt but also by the thickness of the snowpack that accumulates during winter. In this study, we investigated whether summer melt or snowpack thickness was more important for determining the timing of glacier ice exposure using satellite remote sensing and climate model outputs. We found that the glacier ice exposure is much more sensitive to air temperatures than snowfall and that snowfall did not change in the ablation zone during the 1981–2021 study period. Our findings imply that snowfall is unlikely to reduce ice sheet mass loss and that meltwater runoff from the ice sheet should be accurately predicted by air temperatures. Key Points: We investigate the extent to which antecedent snowfall versus summer air temperatures controls melt‐albedo feedbacksWe find that melt‐albedo feedbacks are primarily driven by air temperatures and that antecedent snowfall is of secondary importanceWe find little evidence that snowfall will compensate enhanced ice sheet melt due to climate warming [ABSTRACT FROM AUTHOR]

Sudden Stratospheric Warming events (SSW) are extreme phenomena during which stratospheric temperature can increase by tens of degrees in a few days. They are due to the propagation and breaking of the planetary waves, leading to a perturbation of the polar vortex. SSWs also influence polar ozone concentrations and midlatitude weather. The Infrared Atmospheric Sounding Interferometers (IASI) monitor atmospheric composition and temperature globally since 2007, and they are ideal to observe the changes of temperature and ozone during SSWs. Since the launch of the first IASI, there have been several SSWs in the Northern Hemisphere, including eight major events that are investigated in this study. We find that during major SSWs, the temperature anomaly propagates from 10 hPa to the lower stratosphere and the maximum anomaly at 200 hPa is correlated to the maximum anomaly at 10 hPa. During these events, negative anomalies of temperature in Europe and Russia and positive anomalies in Canada and Greenland are often observed at 750 hPa. The cold air outbreaks that usually follow major SSWs are responsible for anomalies of −15 K. Finally, we look at the evolution of the total ozone column following major events. Major SSWs lead to higher springtime ozone concentrations and the ozone anomaly in March is correlated to the duration of the positive temperature anomaly at 10 hPa. These results show the potential of the IASI mission and its successors, IASI‐New Generation, for the study of SSWs and their effects on weather and atmospheric composition. Plain Language Summary: Sudden Stratospheric Warming (SSW) events are extreme events during which stratospheric temperature can increase by tens of degrees in a few days. Although they happen in the stratosphere, they have effects on midlatitude weather and ozone concentrations. With observation from the Infrared Atmospheric Sounding Interferometers, we find that temperature anomalies associated with SSW propagate from the upper to the lower stratosphere. They are also responsible for cold air outbreaks in North America and Europe. Finally, we find that SSWs lead to larger ozone concentrations in the spring. Key Points: Sudden Stratospheric Warmings impact temperature at all altitudes and can cause cold air outbreaks at midlatitudesOzone springtime concentrations are strongly linked to the occurrence of major Sudden Stratospheric WarmingsThe Infrared Atmospheric Sounding Interferometers are efficient tools to study temperature and ozone during Sudden Stratospheric Warmings [ABSTRACT FROM AUTHOR]

Rainfall high on the Greenland Ice Sheet is an emerging phenomenon with consequences for the thermal and structural makeup of the surface layer. We document changes to Greenland's firn column due to a 4‐day cold‐season warm/rain event. Heavy precipitation occurred with a sudden 30°C increase in air temperature, reaching 0°C at 2,000 m elevation. Thermistor strings within the firn layer across a 35 km transect show rapid warming of 6°–23°C reaching depths of 2–10 m. Antecedent conditions governed the magnitude, duration, and depth‐distribution of sensible and latent heat added to the firn column. Heat fluxes from the firn layer required up to 8 weeks to recover to baseline, a significant fraction of the winter period. The amount of liquid water refrozen in the firn column was ∼20%–100% of the prior summer demonstrating the impact of extreme weather events on the ice sheet's evolution and runoff characteristics. Plain Language Summary: Based on measurements from a network of sensors installed on the Greenland Ice Sheet, we document changes in the firn layer (snow in transition to ice) due to a late Autumn extraordinary warm/rain event. After an extended period of cold‐season cooling of the firn, the event caused the firn layer to warm by 6°–23°C, mainly in one day, as rainwater and surface melt penetrated up to 6 m depth and refroze. The water added to the firn layer during this extreme weather event was ∼25%–100% of what was added during the entire previous summer melt season. The unusual timing of water input during the cold‐season likely altered the firn differently than summer input from melt or rain. Our results demonstrate that only a minor increase in the frequency and magnitude of similar rain events has potential for outsized impact on the thermal and density transformation of Greenland's firn layer which has implications for future runoff processes. Key Points: A cold‐season rainstorm on the Greenland Ice Sheet demonstrates outsized impact of extreme weather events on evolution of the firn layerFirn layer temperature quickly increased by up to 23°C, altering heat fluxes and requiring up to 2 months to thermally recoverThe liquid water refrozen within the firn layer was ∼20%–100% of the amount of water refrozen during the previous summer [ABSTRACT FROM AUTHOR]

Changes in ice‐sheet size impact atmospheric circulation, a phenomenon documented by models but constrained by few paleoclimate records. We present sub‐centennial‐scale records of summer temperature and summer precipitation hydrogen isotope ratios (δ2H) spanning 12–7 ka from a lake on Baffin Island. In a transient model simulation, winds in this region were controlled by the relative strength of the high‐pressure systems and associated anticyclonic circulation over the retreating Greenland and Laurentide ice sheets. The correlation between summer temperature and precipitation δ2H proxy records changed from negative to positive at 9.8 ka. This correlation structure indicates a shift from alternating local and remote moisture, governed by the two ice‐sheet high‐pressure systems, to only remote moisture after 9.8 ka, governed by the strong Greenland high‐pressure system after the Laurentide Ice Sheet retreated. Such rapid atmospheric circulation changes may also occur in response to future, gradual ice‐sheet retreat. Plain Language Summary: Continental ice sheets alter atmospheric circulation, influencing global heat and moisture distribution. Records of atmospheric circulation during previous periods of ice‐sheet retreat can provide insights into the changes that are possible in the future. This study examines summer atmospheric circulation in Baffin Bay from 12,000 to 7,000 years ago, a period of dramatic ice‐sheet retreat. Precipitation isotopes reflect moisture source, which responds to changes in air temperature and atmospheric circulation. This study uses records of temperature and precipitation isotopes from the same sediment archive to tease apart the influence of temperature from that of atmospheric circulation. The precipitation isotopes in this record distinct changes in moisture sources, which a climate model simulation suggests was caused by retreating ice sheets. Before about 10 ka, when the Laurentide Ice Sheet (LIS) covered eastern Canada, summer winds in Baffin Bay shifted regularly between south and north, carrying air with unique temperature and precipitation isotope signatures. As the LIS retreated, the Greenland Ice Sheet (GIS) remained relatively large and dominated atmospheric circulation, causing a rapid shift to southeasterly winds. As the GIS retreats in the future, atmospheric circulation may undergo similar rapid changes. Key Points: Summer precipitation isotopes in the Arctic reflect temperature and moisture transport historyPaired records of summer temperature and precipitation isotopes isolate the influence of temperature from that of transportMoisture delivery to the eastern Canadian Arctic changed dramatically due to Early Holocene Laurentide Ice Sheet retreat [ABSTRACT FROM AUTHOR]

Quantitative temperature reconstructions from lacustrine organic geochemical proxies including branched glycerol dialkyl glycerol tetraethers (brGDGTs) and alkenones provide key constraints on past continental climates. However, estimation of air temperatures from proxies can be impacted by non‐stationarity in the relationships between seasonal air and water temperatures, a factor not yet examined in strongly seasonal high‐latitude settings. We pair downcore analyses of brGDGTs and alkenones measured on the same samples through the Holocene with forward‐modeled proxy values based on thermodynamic lake model simulations for a western Greenland lake. The measured brGDGT distributions suggest that stable autochthonous (aquatic) production overpowers allochthonous inputs for most samples, justifying the use of the lake model to interpret temperature‐driven changes. Conventional calibration of alkenones (detected only after 5.5 thousand years BP) suggests substantially larger temperature variations than conventional calibration of brGDGTs. Comparison of proxy measurements to forward‐modeled values suggests variations in brGDGT distributions monotonically reflect multi‐decadal summer air temperatures changes, although the length of the ice‐free season dampens the influence of air temperatures on water temperatures. Drivers of alkenone variability remain less clear; potential influences include small changes in the seasonality of proxy production or biases toward specific years, both underlain by non‐linearity in water‐air temperature sensitivity during relevant seasonal windows. We demonstrate that implied temperature variability can differ substantially between proxies because of differences in air‐water temperature sensitivity during windows of proxy synthesis without necessitating threshold behavior in the lake or local climate, and recommend that future studies incorporate lake modeling to constrain this uncertainty. Plain Language Summary: Reconstructions of past temperature change from the Arctic are necessary to constrain long‐term sensitivity of the region and the Greenland Ice Sheet to climate changes. We collected sediments from a western Greenland lake, which continuously accumulated for the past 9,000 years. In these sediments, we analyzed the chemical compositions of two temperature‐sensitive classes of molecules produced by microorganisms within the lake. Surprisingly, the two proxies suggest different magnitudes of temperature changes when compared using conventional calibration approaches. Although changes in both proxies are often linked to air temperatures, they are likely more directly sensitive to water temperatures, which can differ from air temperatures—particularly in strongly seasonal environments like the Arctic. We use a lake model to simulate changes in lake water temperatures and convert to proxy units, enabling us to compare air and water temperatures to proxy measurements within a common frame of reference. We show that proxy sensitivity to air temperature changes can differ substantially depending on exactly when the proxies are produced. Because air and lake water temperatures during likely periods of production rarely change at the same rate, we suggest that future studies interpreting water temperature‐sensitive proxies should incorporate lake modeling to constrain this uncertainty. Key Points: Downcore branched glycerol dialkyl glycerol tetraethers and alkenone measurements imply different magnitudes of Holocene air temperature variability on western GreenlandLake water temperatures during likely windows of biomarker production do not change at the same rate as air temperaturesLake water temperature‐sensitive proxies are impacted by lake dynamics in seasonal environments [ABSTRACT FROM AUTHOR]

The accelerating flux of glacial meltwater to the oceans due to global warming is a potential trigger for future climate disturbance. Past disruption of Atlantic Ocean circulation, driven by melting of land‐based ice, is linked in models to reduced ocean‐atmosphere heat transfer and abrupt cooling during stadial events. The most recent stadial, the Younger Dryas (YD), is traditionally viewed as a severe cooling centered on the North Atlantic but with hemispheric influence. However, indications of summer warmth question whether YD cooling was truly year‐round or restricted to winter. Here, we present a beryllium‐10‐dated glacier record from the north‐east North Atlantic, coupled with 2‐D glacier‐climate modeling, to reconstruct Lateglacial summer air temperature patterns. Our record reveals that, contrary to the prevailing model, the last glacial advance in Scotland did not occur during the YD but predated the stadial, while the YD itself was characterized by warming‐driven deglaciation. We argue that these apparently paradoxical findings can be reconciled with regional and global climate events by invoking enhanced North Atlantic seasonality—with anomalously cold winters but warming summers—as an intrinsic response to globally increased poleward heat fluxes. Plain Language Summary: The accelerating melting of the Greenland Ice Sheet has key ramifications for North Atlantic Ocean circulation and heat transport. In the paleoclimate record, discrete periods of ocean current weakening are linked to increased meltwater input from collapsing ice sheets and are believed to have driven severe year‐round cooling of adjacent landmasses; these periods are known as stadials. Yet, research on the Younger Dryas (YD) stadial suggests that summers may have remained warm. We present a new glacier record from northwest Scotland to reconstruct patterns of stadial summer air temperature in the North Atlantic. Our data reveal that the last glacial advance did not occur during the YD, as widely assumed, but predated the stadial. The YD itself was a period of deglaciation indicative of summer climate warming. We propose that stadials are anomalous periods of high seasonality in the North Atlantic region, with cold winters but warming summers, and might be a persistent feature of AMOC disruption, both past and future. Key Points: Lateglacial fluctuations of Scottish glaciers are dated with 10Be. Equilibrium line altitudes are resolved via glacier‐climate modelingThe last pulse of glaciation culminated prior to the Younger Dryas (YD), while the stadial was characterized by deglaciation and summer warmingThe YD stadial was defined by anomalous thermal seasonality in the North Atlantic region, rather than by year‐round cooling [ABSTRACT FROM AUTHOR]

This study presents an approach to provide seamless climate information by concatenating decadal climate predictions and climate projections in time. Results for near‐surface air temperature over 29 regions indicate that such an approach has potential to provide meaningful information but can also introduce significant inconsistencies. Inconsistencies are often most pronounced for relatively extreme quantiles of the CMIP6 multi‐model ensemble distribution, whereas they are generally smaller and mostly insignificant for quantiles close to the median. The regions most affected are the North Atlantic, Greenland and Northern Europe. Two potential ways to reduce inconsistencies are discussed, including a simple calibration method and a weighting approach based on model performance. Calibration generally reduces inconsistencies but does not eliminate all of them. The impact of model weighting is minor, which is found to be linked to the small size of the decadal climate prediction ensemble, which in turn limits the applicability of that method. Plain Language Summary: Continuous and consistent information about the evolution of climate in next 1–40 years is crucial for the development of mitigation and adaptation strategies. Historically, scientific products providing weather and climate information have been designed for specific time scales, for example, seasonal or decadal climate predictions (1–10 years), or long‐term climate projections (1–100 + years). As a consequence, currently only climate projections can be used to provide continuous information beyond the 10 years time scale. However, as climate projections are not initialized using observations, they are likely – depending on the variable and region – less skillful than initialized predictions for shorter lead times. In this study we assess if meaningful continuous climate information can be obtained by concatenating decadal climate predictions and climate projections in time. Results suggest that significant inconsistencies (materialized in jumps in the resulting time series) can be introduced by this approach, however, for some regions temporal concatenation might provide meaningful climate information beyond decadal time scales. Furthermore, potential ways to reduce inconsistencies when concatenating are discussed. Key Points: Concatenating climate predictions and projections can introduce inconsistenciesConcatenation is less problematic for quantiles close to the median compared to more extreme quantiles of the ensemble distributionA simple calibration method reduces but does not eliminate inconsistencies [ABSTRACT FROM AUTHOR]

The North Atlantic Oscillation (NAO) index is often characterized by independent positive and negative NAO events with a characteristic spatial pattern and a typical lifetime of around 1 week. These events are separated by periods of near‐neutral NAO conditions. Here, we challenge this view by showing in reanalysis and observed data that the strength and spatial shape of NAO events depends on the NAO index prior to the window of 1 week and this dependency is most pronounced for negative NAO events. The influence is seen in the mean sea level pressure, and in other important features, including blocking frequency and jet stream characteristics, and also in air surface temperature and precipitation in parts of Europe. This new appreciation is important for efforts to improve methods for subseasonal‐to‐seasonal predictions of NAO. Plain Language Summary: Northern European winters differ. Some are mild and moist with prevailing westerly winds and some are cold and dry, dominated by the Siberian air masses from East. When the Northern European winters are mild, the winter in Greenland is cold, and vice versa. This is the North Atlantic Oscillation (NAO) and its companion, the Europe–Greenland temperature seesaw. They were described in qualitative terms already in the late eighteenth century. Nowadays, the NAO is often described as a series of independent positive or negative events with a duration of around 1 week with intermittent periods of near‐neutral conditions and variable duration. The net result of these events determines the NAO strength of the winter. We show that this picture is too simplified because the NAO outside the time horizon of 1 week influences the strength of the individual NAO events. This is most pronounced for the negative events where this influence can both distort and amplify or diminish the pattern of the event. Also the typical temperatures and precipitation patterns associated with NAO are influenced significantly by this effect. One important consequence for our discovery is the improved prospects of doing subseasonal‐to‐seasonal predictions. Key Points: The daily North Atlantic Oscillation (NAO) index consists of positive and negative events of duration around 1 week separated by periods of near‐neutral conditionsThe strength and spatial shape of negative NAO events depends on NAO‐index values prior to the eventBlocking frequencies, jet strength and position, and surface temperature and precipitation have similar dependencies [ABSTRACT FROM AUTHOR]

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) from lacustrine sediments have been widely used to reconstruct mean annual air temperature (MAAT). Although many proxy calibrations relating brGDGT characteristics have been put forth, these calibrations may produce warm biases when applied to lakes in cold regions. We present an expanded Chinese lake surface sediment brGDGT‐MAAT calibration with 29 new surface samples from cold regions along with 39 previously published from Chinese lakes. We deployed sediment traps in a meromictic lake, Dagze Co, and compared results with previously published data from a dimictic lake, Lake 578 in Greenland, to determine potential seasonal and depth‐dependence of brGDGTs. In the meromictic lake, brGDGTs are primarily produced in the lake bottom water, whereas in the dimictic lake, the brGDGTs are produced throughout the water column and mainly reflect the annual bottom water temperature or mixing season water column temperature. We applied our refined calibration to a sediment core from Western Tibet to examine how fluctuations in temperature influenced the Guge Kingdom over the last 2,000 years. Our record reveals relatively warm temperatures during the Medieval Climate Anomaly, cooling of 2°C to −2°C during the Little Ice Age, warming into the eighteenth century, and stabilization after 1800 CE. The temperature variations coincided with a transition of dynasties in Western Tibet. Temperature sensitivity tests on barley distribution, the principal cultivated cereal in Tibet, suggest that a decline in temperature led to a decreased crop yield that may have factored into the disappearance of the Guge Kingdom. Plain Language Summary: Paleoclimate reconstructions from lake sediments can provide a wealth of information on past climate changes. The accuracy of these reconstructions depends on a modern "translation" between proxies and climate variables. Lacustrine branched glycerol dialkyl glycerol tetraethers (brGDGTs) are viewed as a promising tool for reconstructing air temperatures. However, there is not a one‐to‐one correlation between air temperatures and lacustrine brGDGT‐based temperatures; therefore, we have to conduct calibration studies to understand how they are related. In this study, we compared sediment traps in a meromictic and dimictic lake to determine niche partitioning of brGDGTs. Comparisons between core‐top reconstructed and observed water column temperatures show that in meromictic lakes, brGDGTs are primarily produced in the lake bottom water, whereas in the dimictic lakes, they are produced throughout the entire water column and reflect annual bottom water temperatures or water column temperatures during the mixing season. We then establish a new calibration and apply it to a lake sediment record, which indicates that temperatures cooled during 1630 CE. This time period coincides with the disappearance of the Guge Kingdom. Using ecological niche modeling, we confirm that cooling led to a decline in barley, which has likely contributed to the collapse of the Kingdom. Key Points: Different lake mixing types profoundly impact brGDGT‐reconstructed temperaturesAir temperatures in Western Tibet were 2–3°C cooler than present during the LIATemperature variability influenced crop yield. A decreased crop yield likely contributed to the collapse of the Guge Kingdom [ABSTRACT FROM AUTHOR]

A period of 18 years of infrasonic recordings was analyzed from a microbarometer array (I18DK) in northwestern Greenland, near Qaanaaq. A huge number of infrasonic detections, over 700,000, have been made in I18DKs soundscape during the Arctic summers. Simultaneously identified were both calving events from marine‐terminating glaciers and discharge related acoustics from a land‐terminating glacier. This infrasonic activity is correlated to sea‐surface and atmospheric temperature, respectively. Inter‐yearly to daily variations were retrieved showing a strong variability in infrasonic detection rates and hence glacier activity. The highest number of infrasonic detections were found in recent years from the land‐terminating glacier. The latter is supported by actual discharge measurements and partly by a discharge model. It is concluded that monitoring infrasound from glaciers can complement other techniques to remotely and passively get insights into glacier dynamics with high temporal and spatial resolution. Plain Language Summary: Infrasound is inaudible sound and created when large volumes of air are displaced in the atmosphere. Such infrasound is continuously measured in northwestern Greenland, near the village of Qaanaaq. Recordings over 18 years have been analyzed. Both land and marine‐terminating glaciers were identified, being active at the same time. Variations in activity of the glaciers were found between different years and also on a daily basis. The activity is related to calving of pieces of ice from the glacier and material running off the glacier. In recent years, the highest activity in discharge was observed. The latter is support by modeling and measurements of discharge which have been taken locally over 3 years. It is concluded, that infrasound measurements can help in gaining more insights into glacier dynamics, with high temporal and spatial resolution. As such, the detection and identification of inaudible sound complements other (remote) monitoring techniques. Key Points: Infrasound monitoring of multiple glaciers simultaneously revealed details on glacier dynamics with a high temporal and spatial scaleInter‐yearly to daily variations for marine (calving) and land‐terminating glaciers (melt water run‐off) were observedover 18 yearsInfrasound activity related to glacier discharge was highest in recent years, supported by both modeling and actual discharge measurements [ABSTRACT FROM AUTHOR]

We provide the northernmost permafrost thermal analysis in Greenland through the ground temperature time series (2014–2021) and cryostratigraphy of two 20‐m deep boreholes (SN1 and SN2) at Villum Research Station, Station Nord (81°N). Three sedimentary units are identified in the stratigraphy: glacial, glaciomarine, and beach deposits. These sandy and gravelly deposits are interpreted to comprise a deglaciation and isostatic emergence sequence. Ice‐poor epigenetic permafrost grew downwards into the deposits following subaerial exposure. Relatively high salinity values (up to 70 ppt) are observed in the glaciomarine unit, resulting in freezing point depressions between 0 and −4°C. The Prinsesse Ingeborg Halvø study area has a polar tundra climate and is unique compared to other high Arctic areas because of a thick (>1 m), long‐lasting snow cover. This snow cover results in relatively high permafrost temperatures for the climate and latitude and the warmest known permafrost north of 80°N. Over the seven‐year study period, average 20 m permafrost temperature was −7.87°C at SN1 and −7.06°C at SN2. The warming rate at 20 m depth was 0.07°C/year at SN1 and 0.05°C/year at SN2, rates which are similar to those of other high Arctic sites. Active layer thickness, extrapolated from the temperature measurements, varied between 0.5 and 1.16 m. The interplay between snow dynamics and seasonal air temperature controls ground thermal regime in the study area. Air temperatures during autumn and midwinter, the fastest warming seasons, influence the ground thermal regime through the gradually developing, dense snowpack. Plain Language Summary: Permafrost, ground that is frozen for at least two consecutive years, is commonly found in high mountains and polar environments. Permafrost samples and long‐term permafrost temperature records are necessary to fully understand permafrost and how it is changing with a warming climate. We present results of permafrost research from Villum Research Station at Station Nord, the northernmost permafrost monitoring site in Greenland and second northernmost worldwide. We collected core samples from two different 20‐m deep boreholes and measured temperature in the boreholes for over seven years to understand the permafrost's physical and thermal properties, and how the permafrost reacts to the weather and climate. The permafrost began to form more than 8,000 years ago, after the study area was deglaciated and the sea level lowered. A major reason this permafrost is warmer than other sites in the high Arctic is because the area has more than 1 m of snow in the winter, which is unusual for the polar tundra and acts as an insulating layer. Permafrost at 20 m depth has warmed 0.05–0.07°C per year from 2014 to 2021; these rates are similar to those at other high Arctic sites. Key Points: The presented permafrost thermal record is the second northernmost in the Arctic and reveals the warmest known permafrost north of 80°NThe warming rate at 20 m depth was 0.07°C/year and 0.05°C/year at the two sites from 2014 to 2021 despite thick and long‐lasting snow coverEpigenetic permafrost grew into sands and gravels deposited during deglaciation and isostatic emergence of the Prinsesse Ingeborg Halvø [ABSTRACT FROM AUTHOR]

Dramatic increases in air temperature and precipitation are occurring in the High Arctic (>70°N), yet few studies have characterized the long‐term responses of High Arctic ecosystems to the interactive effects of experimental warming and increased rain. Beginning in 2003, we applied a factorial summer warming and wetting experiment to a polar semidesert in northwest Greenland. In summer 2018, we assessed several metrics of ecosystem structure and function, including plant cover, greenness, ecosystem CO2 exchange, aboveground (leaf, stem) and belowground (litter, root, soil) carbon (C) and nitrogen (N) concentrations (%) and pools, as well as leaf and soil stable isotopes (δ13C and δ15N). Wetting induced the most pronounced changes in ecosystem structure, accelerating the expansion of Salix arctica cover by 370% and increasing aboveground C, N, and biomass pools by 94%–101% and root C, N, and biomass pools by 60%–122%, increases which coincided with enhanced net ecosystem CO2 uptake. Further, wetting combined with warming enhanced plot‐level greenness, whereas in isolation neither wetting nor warming had an effect. At the plant level, the effects of warming and wetting differed among species and included warming‐linked decreases in leaf N and δ15N in S. arctica, whereas leaf N and δ15N in Dryas integrifolia did not respond to the climate treatments. Finally, neither plant‐ nor plot‐level C and N allocation patterns nor soil C, N, δ13C, or δ15N concentrations changed in response to our manipulations, indicating that these ecosystem metrics may resist climate change, even in the longer term. In sum, our results highlight the importance of summer precipitation in regulating ecosystem structure and function in arid parts of the High Arctic, but they do not completely refute previous findings of resistance in some High Arctic ecosystem properties to climate change. [ABSTRACT FROM AUTHOR]

Helheim Glacier, one of the largest marine‐terminating outlet glaciers draining the Greenland Ice Sheet, underwent significant retreat and acceleration in the early 2000s, accounting for an appreciable proportion of the ice sheet's mass loss during that period. Using a range of remotely sensed datasets, we show that despite a subsequent readvance, the glacier has continued to lose mass and thin, and has retreated inland of the retracted position occupied in 2005. Critically, the near‐terminus is up to 100 m thinner than during 2005, and the front 5 km is within 25–50 m of flotation, with retrograde bed slopes extending several kilometers inland of the terminus. The neighboring Fenris and Midgard Glaciers have both undergone recent large‐scale and rapid retreat once their near‐terminus regions began to float, suggesting that under projected climate warming and associated glacier thinning, Helheim Glacier is poised to pass a threshold whereby the near‐terminus region will retreat rapidly. Plain Language Summary: A significant proportion of the Greenland Ice Sheet's contribution to global sea‐level rise is as a result of mass loss from its marine‐terminating glaciers. Helheim Glacier, located in southeast Greenland, is one of the largest and fastest‐flowing glaciers draining the Greenland Ice Sheet. During the early 2000s, Helheim Glacier underwent dramatic retreat and acceleration, but subsequently readvanced during relatively cooler conditions in 2006. However, persistently high ocean and atmospheric temperatures have meant that Helheim Glacier has in fact been continuously losing mass since 2003, despite this readvance. Here, we use a range of remotely sensed data to show that since 2014, Helheim Glacier has accelerated and retreated to a greater extent than occurred in 2003–2005. More importantly, as the glacier has been losing mass over the past two decades, it is currently much thinner than during its peak retreat in 2005. As the glacier continues to lose mass, it will pass a threshold whereby the ice will float and rapidly disintegrate. We observe that this process has already occurred at the glaciers neighboring Helheim Glacier, which are subject to the same climate forcing and thus provide an analogue for the future response of Helheim Glacier to continued warming. Key Points: Helheim Glacier is more retreated and the near‐terminus region is up to 100 m thinner than during its much‐reported dramatic retreat in 2005Helheim, one of Greenland's largest ice dischargers, is now more vulnerable than at any point since the Little Ice AgeHelheim's new configuration offers potential for sustained dynamic instability and a major contribution to global sea‐level rise [ABSTRACT FROM AUTHOR]

Climate change has ecosystem‐wide cascading effects. Little is known, however, about the resilience of Arctic marine ecosystems to environmental change. Here we quantify and compare large‐scale patterns in rocky intertidal biomass, coverage and zonation in six regions along a north‐south gradient of temperature and ice conditions in West Greenland (60–72°N). We related the level and variation in assemblage composition, biomass and coverage to latitudinal‐scale environmental drivers. Across all latitudes, the intertidal assemblage was dominated by a core of stress‐tolerant foundation species that constituted > 95% of the biomass. Hence, canopy‐forming macroalgae, represented by Fucus distichus subsp. evanescens and F. vesiculosus and, up to 69°N, also Ascophyllum nodosum, together with Semibalanus balanoides, occupied > 70% of the vertical tidal range in all regions. Thus, a similar functional assemblage composition occurred across regions, and no latitudinal depression was observed. The most conspicuous difference in species composition from south to north was that three common species (the macroalgae Ascophyllum nodosum, the amphipod Gammarus setosus and the gastropod Littorina obtusata) disappeared from the mid‐intertidal, although at different latitudes. There were no significant relationships between assemblage metrics and air temperature or sea ice coverage as obtained from weather stations and satellites, respectively. Although the mean biomass decreased > 50% from south to north, local biomass in excess of 10 000 g ww m−2 was found even at the northernmost site, demonstrating the patchiness of this habitat and the effect of small‐scale variation in environmental characteristics. Hence, using the latitudinal gradient in a space‐for‐time substitution, our results suggest that while climate modification may lead to an overall increase in the intertidal biomass in north Greenland, it is unlikely to drive dramatic functional changes in ecosystem structure in the near future. Our dataset provides an important baseline for future studies to verify these predictions for Greenland's intertidal zone. [ABSTRACT FROM AUTHOR]

We analyze the past 67,000 years of climate using Antarctic ice‐core records to constrain the mechanisms involved in (a) the "bipolar seesaw" relationship between Greenland and Antarctic surface temperature variations, and (b) mechanisms of millennial‐scale atmospheric CO2 concentration variations. Specifically, we determine for each Greenland Stadial the rate of Antarctic temperature and atmospheric CO2 rise. We find that Antarctic warming rates significantly decrease as the climate cools during the glacial period, whereas the rate of atmospheric CO2 rise does not significantly change. Also, we find that the rates of Antarctic warming and atmospheric CO2 rise are both insensitive to whether a given stadial contains a Heinrich event. These results challenge the view that a single Southern‐Ocean‐based mechanism dominates the observed glacial variability in Antarctic temperature and atmospheric CO2. Instead, our results are consistent with an important contribution of low‐ and mid‐latitude processes to millennial‐scale atmospheric CO2 changes. Plain Language Summary: Glacial climate is characterized by millennial‐scale variations in polar temperature and atmospheric CO2 concentration. The observed similarities between the shape of atmospheric CO2 and Antarctic temperature records, derived from Antarctic ice cores, have led to a common view that variations in both are dominated by a common Southern Ocean mechanism. However, a systematic comparison of the rates of change of Antarctic temperature and atmospheric CO2 during these millennia‐scale events has not previously been conducted. Furthermore, it is not clearly demonstrated if the rate of change of the Antarctic temperature and atmospheric CO2 were sensitive to changes in the mean climate state or the occurrence of massive iceberg discharge into the North Atlantic. Here, using ice‐core data, we show that the rate of Antarctic warming reduces as the glacial climate cools. In contrast, the rate of atmospheric CO2 rise is insensitive to glacial cooling. Neither the warming rate nor the rate of atmospheric CO2 rise are affected by iceberg discharge events. Our result is in contrast with a simple common Southern Ocean control on atmospheric CO2 and Antarctic temperature and suggests that the millennial‐scale atmospheric CO2 changes include significant contributions from mid‐to‐low latitudes and processes independent of the background climate. Key Points: The Antarctic warming rate during Antarctic Isotope Maxima significantly decreased as the climate cooled toward the glacial maximumIn contrast, the rate of millennial‐scale CO2 rise is insensitive to the varying background climateThe Antarctic warming rate and the rate of millennial‐scale CO2 rise were insensitive to whether the stadial contained a Heinrich event [ABSTRACT FROM AUTHOR]

Three clearly defined abrupt cooling events (ACEs) can be observed within Greenland Interstadial (GI)‐1 in the Greenland ice‐core records. However, the spatial variation in amplitude and timing of these ACEs is poorly understood due to the paucity of well‐dated records with quantified temperature reconstructions. This study presents high‐resolution chironomid‐inferred July air temperature (TJul) and oxygen isotope (δ18O) records from Crudale Meadow (Orkney Isles, UK). Three centennial‐scale ACEs punctuate the Windermere Interstadial at Crudale Meadow. The largest ACE shows an amplitude of 5.4 °C and a 1% isotopic decline and is centred on ~14.0 ka bp, consistent with the timing of the GI‐1d event in the Greenland stratigraphy. The two other observed ACEs are of smaller magnitude and are centred on ~13.6 ka bp and ~13.2 ka bp, with these smaller magnitude events tentatively correlated with the GI‐1cii and GI‐1b events, respectively, but lack sufficient chronological constraint to fully assess their timing. When comparing the Crudale Meadow record with other locations in the British Isles a strong relationship can be observed between the magnitude of TJul cooling and latitude, with a reduced signal in more southerly locations, indicating that oceanic forcing may be a key driver of the ACEs. [ABSTRACT FROM AUTHOR]

Questions: Shrub vegetation has been expanding across much of the rapidly changing Arctic. Yet, there is still uncertainty about the underlying drivers of shrub community composition. Here, we use extensive vegetation surveys and a trait-based approach to answer the following questions: which abiotic and biotic factors explain abundance of shrub species and functional groups in the Arctic tundra, and can we interpret these relationships using plant traits related to resource acquisition? Location: Nuup Kangerlua (Godthåbsfjord), western Greenland. Methods: We tested the power of nine climatic, topographic and biotic variables to explain the abundances of nine shrub species using a Bayesian hierarchical modelling framework. Results: We found highly variable responses among species and functional groups to both abiotic and biotic environmental variation. The overall most important abiotic explanatory variable was annual air temperature variability, which was highly correlated with winter minimum air temperature. Functional community composition and graminoid abundance were the most influential biotic factors. While we did not find systematic patterns between shrub abundances and abiotic variables with regard to resource acquisition traits, these traits did explain relationships between shrub abundances and biotic variables. Conclusions: Shrub abundance responses to abiotic variables rarely aligned with expectations based on plants’ resource acquisition traits or functional groups. Our results, therefore, indicate that approaches exclusively based on resource acquisition traits might be limited in their ability to predict abundances of individual groups and species, particularly in response to complex abiotic environments. However, integrating community theory and functional trait concepts represents a promising pathway to better predict biotic interactions and ultimately responses of dominant shrub vegetation to rapid environmental changes across the arctic tundra biome. [ABSTRACT FROM AUTHOR]

A World Meteorological Organization (WMO) Extremes Evaluation Committee investigated an observation of −69.6 °C by Klinck Automatic Weather Station (AWS) in Greenland on 22 December 1991 as the lowest temperature observed in Greenland, thereby making it the lowest recorded near‐surface air temperature for the Northern and Western Hemispheres and for WMO Region VI. The committee examined the metadata and observations of the station as well as the regional synoptic circulation. The committee concluded that the observation is credible in terms of instrument calibration, monitoring of the station and the synoptic situation. Consequently, the WMO Rapporteur accepted the observation as the officially lowest observed near‐surface air temperature for Greenland, the Northern and Western Hemisphere and for WMO Region VI. As a supplement to this investigation, the committee also recommends that opportunities be investigated such that AWS data from Greenland can be efficiently incorporated into real‐time weather forecasts and hence into reanalysis datasets. [ABSTRACT FROM AUTHOR]

Substantial marine, terrestrial, and atmospheric changes have occurred over the Greenland region during the last century. Several studies have documented record‐levels of Greenland Ice Sheet (GrIS) summer melt extent during the 2000s and 2010s, but relatively little work has been carried out to assess regional climatic changes in other seasons. Here, we focus on the less studied cold‐season (i.e., autumn and winter) climate, tracing the long‐term (1873–2013) variability of Greenland's air temperatures through analyses of coastal observations and model‐derived outlet glacier series and their linkages with North Atlantic sea ice, sea surface temperature (SST), and atmospheric circulation indices. Through a statistical framework, large amounts of west and south Greenland temperature variance (up to r2 ~ 50%) can be explained by the seasonally‐contemporaneous combination of the Greenland Blocking Index (GBI) and the North Atlantic Oscillation (NAO; hereafter the combination of GBI and NAO is termed GBI). Lagged and concomitant regional sea‐ice concentration (SIC) and the Atlantic Multidecadal Oscillation (AMO) seasonal indices account for small amounts of residual air temperature variance (r2 < ~10%) relative to the GBI. The correlations between GBI and cold‐season temperatures are predominantly positive and statistically‐significant through time, while regional SIC conditions emerge as a significant covariate from the mid‐20th century through the conclusion of the study period. The inclusion of the cold‐season Pacific Decadal Oscillation (PDO) in multivariate analyses bolsters the air temperature variance explained by the North Atlantic regional predictors, suggesting the remote, background climate state is important to long‐term Greenland temperature variability. These findings imply that large‐scale tropospheric circulation has a strong control on surface temperature over Greenland through dynamic and thermodynamic impacts and stress the importance of understanding the evolving two‐way linkages between the North Atlantic marine and atmospheric environment in order to more accurately predict Greenland seasonal climate variability and change through the 21st century. [ABSTRACT FROM AUTHOR]

We provide an updated analysis of instrumental Greenland monthly temperature data to 2019, focusing mainly on coastal stations but also analysing ice‐sheet records from Swiss Camp and Summit. Significant summer (winter) coastal warming of ~1.7 (4.4)°C occurred from 1991–2019, but since 2001 overall temperature trends are generally flat and insignificant due to a cooling pattern over the last 6–7 years. Inland and coastal stations show broadly similar temperature trends for summer. Greenland temperature changes are more strongly correlated with Greenland Blocking than with North Atlantic Oscillation changes. In quantifying the association between Greenland coastal temperatures and Greenland Ice Sheet (GrIS) mass‐balance changes, we show a stronger link of temperatures with total mass balance rather than surface mass balance. Based on Greenland coastal temperatures and modelled mass balance for the 1972–2018 period, each 1°C of summer warming corresponds to ~(91) 116 Gt·yr−1 of GrIS (surface) mass loss and a 26 Gt·yr−1 increase in solid ice discharge. Given an estimated 4.0–6.6°C of further Greenland summer warming according to the regional model MAR projections run under CMIP6 future climate projections (SSP5‐8.5 scenario), and assuming that ice‐dynamical losses and ice sheet topography stay similar to the recent past, linear extrapolation gives a corresponding GrIS global sea‐level rise (SLR) contribution of ~10.0–12.6 cm by 2100, compared with the 8–27 cm (mean 15 cm) "likely" model projection range reported by IPCC in 2019 (SPM.B1.2). However, our estimate represents a lower limit for future GrIS change since fixed dynamical mass losses and amplified melt arising from both melt‐albedo and melt‐elevation positive feedbacks are not taken into account here. [ABSTRACT FROM AUTHOR]

Active layer probing in northern Sweden, northeast Greenland, and central Svalbard indicates active layer thickening has occurred at Circumpolar Active Layer Monitoring (CALM) sites with long‐term, continuous observations, since the sites were established at these locations in 1978, 1996, and 2000, respectively. The study areas exhibit a reverse latitudinal gradient in average active layer thickness (ALT), which is explained by site geomorphology and climate. Specifically, Svalbard has a more maritime climate and thus the thickest active layer of the study areas (average ALT = 99 cm, 2000–2018). The active layer is thinnest at the northern Sweden sites because it is primarily confined to superficial peat. Interannual variability in ALT is not synchronous across this Nordic Arctic region, but study sites in the same area respond similarly to local meteorology. ALT correlates positively with thawing degree days in Sweden and Greenland, as has been observed in other Arctic regions. However, ALT in Svalbard correlates with freezing degree days, where the maritime Arctic climate results in relatively high and variable winter air temperatures. The difference in annual ALT at adjacent sites is attributed to differences in snow cover and geomorphology. From 2000 to 2018, the average rate of active layer thickening at the Nordic Arctic CALM probing sites was 0.5 cm/yr. The average rate was 1 cm/yr for Nordic Arctic CALM database sites with significant trends, which includes a borehole in addition to probing sites. This range is in line with the circum‐Arctic average of 0.8 cm/yr from 2000 to 2018. [ABSTRACT FROM AUTHOR]

Anthropogenic climate disruption, including temperature and precipitation regime shifts, has been linked to animal population declines since the mid‐20th century. However, some species, such as Arctic‐breeding geese, have thrived during this period. An increased understanding of how climate disruption might link to demographic rates in thriving species is an important perspective in quantifying the impact of anthropogenic climate disruption on the global state of nature. The Greenland barnacle goose (Branta leucopsis) population has increased tenfold in abundance since the mid‐20th century. A concurrent weather regime shift towards warmer, wetter conditions occurred throughout its range in Greenland (breeding), Ireland and Scotland (wintering) and Iceland (spring and autumn staging). The aim of this study was to determine the relationship between weather and demographic rates of Greenland barnacle geese to discern the role of climate shifts in the population trend. We quantified the relationship between temperature and precipitation and Greenland barnacle goose survival and productivity over a 50 year period from 1968 to 2018. We detected significant positive relationships between warmer, wetter conditions on the Icelandic spring staging grounds and survival. We also detected contrasting relationships between warmer, wetter conditions during autumn staging and survival and productivity, with warm, dry conditions being the most favourable for productivity. Survival increased in the latter part of the study period, supporting the possibility that spring weather regime shifts contributed to the increasing population trend. This may be related to improved forage resources, as warming air temperatures have been shown to improve survival rates in several other Arctic and northern terrestrial herbivorous species through indirect bottom‐up effects on forage availability. [ABSTRACT FROM AUTHOR]

The Community Earth System Model version 2.1 (CESM2.1) is used to investigate the evolution of the Greenland ice sheet (GrIS) surface mass balance (SMB) under an idealized CO2 forcing scenario of 1% increase until stabilization at 4× pre‐industrial at model year 140. In this simulation, the SMB calculation is coupled with the atmospheric model, using a physically based surface energy balance scheme for melt, explicit calculation of snow albedo, and a realistic treatment of polar snow and firn compaction. By the end of the simulation (years 131–150), the SMB decreases with 994 Gt yr−1 with respect to the pre‐industrial SMB, which represents a sea‐level rise contribution of 2.8 mm yr−1. For a threshold of 2.7‐K global temperature increase with respect to pre‐industrial, the rate of expansion of the ablation area increases, the mass loss accelerates due to loss of refreezing capacity and accelerated melt, and the SMB becomes negative 6 years later. Before acceleration, longwave radiation is the most important contributor to increasing energy for melt. After acceleration, the large expansion of the ablation area strongly reduces surface albedo. This and much increased turbulent heat fluxes as the GrIS‐integrated summer surface temperature approaches melt point become the major sources of energy for melt. Plain Language Summary: The Greenland ice sheet (GrIS) is the largest freshwater reservoir in the Northern Hemisphere, with the potential of raising sea levels with 7.1 m if fully melted. Here, we investigate the evolution of the GrIS under elevated levels of CO2 in a model with an advanced and coupled surface mass balance (SMB; the difference between mass gain and mass loss at the ice sheet surface) calculation. By the end of our simulation, the global mean temperature increases with 5.3 K, contributing to a sea‐level rise rate of 2.8 mm yr−1 from the GrIS surface. At a global temperature increase of 2.7 K, the rate of mass loss from the GrIS accelerates due to a darker ice sheet surface and much increased atmospheric temperatures over the GrIS. Key Points: GrIS surface mass loss accelerates for a global warming of 2.7 K through increased surface melt and loss of refreezing capacityLongwave radiation is the main contributor to melt increase before acceleration; albedo and turbulent fluxes add major contributions afterAnthropogenic‐forced atmospheric circulation changes (North Atlantic Oscillation and Greenland blocking index) partially reduce melt [ABSTRACT FROM AUTHOR]

Recent winters have been unique due to the rapid and extreme cooling of the subpolar North Atlantic. Here, we present a novel view on its causes and consequences. Combining in‐situ observations with remote sensing and atmospheric reanalysis data, we show that increased freshening of the subpolar region gives rise to a faster surface cooling in fall and winter. Large freshwater events, in particular, result in pronounced cold anomalies with sharp temperature gradients that promote an enhanced storminess. The storms reinforce the cooling by driving stronger heat losses and modulating the surface flow. Consistent with this mechanism, past freshwater events have been followed by cold anomalies in winter of approximately −2°C and increases in the North Atlantic Oscillation index of up to ∼0.6 within 3 years. We expect that future freshwater discharges into the North Atlantic will amplify the cold anomaly and trigger an enhanced wintertime storminess with far‐reaching climatic implications. Plain Language Summary: Recent winters have been unique due to a rapid and extreme cooling of the subpolar North Atlantic. Combining ocean and atmospheric data, we show that increased freshwater in this region leads to shallower surface layers that adjust faster to the lower air temperature in fall and winter. The faster surface cooling increases the south‐north temperature gradient, which promotes the development of storms. The storms, in turn, reinforce the cooling by modulating the surface flow. Accordingly, past freshwater events have been followed by an extremely cold ocean surface in the subpolar North Atlantic in winter and major changes in large‐scale weather patterns. We expect that future freshwater discharges from Greenland and the Arctic will amplify the cooling and trigger an enhanced wintertime storminess with far‐reaching implications for the climate. Key Points: Large freshwater events result in distinct cold anomalies with sharp temperature gradients in the subpolar North Atlantic in winterA strong, freshwater‐induced cold anomaly promotes an enhanced storminess, which reinforces the anomaly by modulating the surface flowConsistent with this mechanism, large freshwater events in the past have preceded positive North Atlantic Oscillation periods in winter [ABSTRACT FROM AUTHOR]

Over the past few decades the Arctic has warmed up more than the lower latitudes. Soil organic carbon (SOC) in the Arctic is vulnerable to climate change, and carbon dioxide (CO2) produced via SOC decomposition can amplify atmospheric temperature increase. Although SOC composition is relevant to decomposability, studies on its compositional changes with warming are scarce, particularly in the Arctic. Therefore, we investigated the responses of SOC and the bacterial community to climate manipulation under Cassiope and Salix heath vegetation communities in permafrost‐affected soil in Zackenberg, Greenland. After 8–9 years of experimental warming, we evaluated changes in SOC quantity and quality of three density fractions of soil: free light fraction (FLF), occluded light fraction (OLF) and heavy fraction (HF). The SOC content at 0–5‐cm depth was significantly reduced with warming under Cassiope, and it was accompanied by decreased FLF content, attributed to accelerated decomposition of the FLF by warming. However, SOC molecular composition and bacterial community composition were not affected by warming. By contrast, there was no warming effect on SOC under Salix, which could be partially due to smaller temperature increases caused by higher moisture levels associated with larger silt and clay contents, or to different responses of the dominant plant species to temperature. In both soils, more than 55% of SOC was associated with minerals, and its molecular composition indicated microbial decomposition. Our results suggested that long‐term warming in the high Arctic could induce the loss of SOC, particularly in the FLF; however, the response could vary with vegetation type and/or soil properties, that is, soil texture. Highlights: We show decreased SOC with long‐term (8‐year) warming of heath soils in the high ArcticParticularly, the free light fraction of SOC in topsoil decreased with warming in a Cassiope heath siteMineral‐associated SOC content was more than 55% and showed signs of microbial processingEffects of warming differed according to soil properties and/or plant community composition [ABSTRACT FROM AUTHOR]

From the onset of melt in early June, corresponding declines in Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) freeboard and surface albedo can be seen over the entire Arctic sea ice cover. In the 2019 summer, area‐averaged freeboard decreased from 34 cm prior to melt to a minimum of 12 cm in August while the area‐averaged albedo decreased from ~0.7 to 0.38 for the same period. Calculations using ICESat‐2 freeboards and modeled ice thickness from Pan‐Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) give area‐averaged snow depths ranging from 17 cm prior to melt to 3 cm in August over seasonal ice and from 34 to 4 cm over multiyear ice. Mean rates of snow ablation (including evaporation) in mid‐June were as high as 2 cm/day, comparable to field records from other years. Increases in freeboard after mid‐August in the high latitude (>80°N) multiyear ice cover, north of the Greenland coast, are likely due to earlier freeze‐up and snow accumulation in these regions with shorter melt seasons. Plain Language Summary: As the air temperature warms with the onset of summer, the Arctic Ocean ice cover undergoes its annual transformation from a highly reflective snow‐covered surface in winter to a darker, more absorbent surface during summer composed of bare ice, melt ponds, and open water. Several weeks after the onset of melt, much of the snow layer disappears as the snow is converted into meltwater. Over the entire ice cover, this loss of the snow layer can be seen in the lowering of surface heights measured by the lidar on NASA's Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2). In this paper, we describe the changes in surface height and the corresponding changes in snow depth from the satellite observations and compare them with existing measurements from field programs, which are limited due to challenging logistics during the summer. We show, for the first time, that we can observe corresponding changes in surface height and reflectivity as well as the different rates of melt in the decay of the snow cover. These results will allow for an improved understanding of the impact of the snow layer on the fate of the ice cover at the end of the summer. Key Points: We provide a first look at the evolution of the summer ice cover in ICESat‐2 sea ice productsObservations show a clear lowering of the freeboard associated with snow melt over the entire Arctic Ocean ice coverArea‐averaged snow depths ranged from 17 cm prior to melt to 3 cm in August over seasonal ice and from 34 to 4 cm over multiyear ice [ABSTRACT FROM AUTHOR]

Greenland ice sheet melt is a large contributor to rising global sea level and melt is dependent on surface air temperature. Arctic temperatures are strongly coupled to clouds but spatial connections between clouds and temperature have yet to be established across Greenland. By mapping spaceborne lidar measurements and surface observations, it is shown that radiatively opaque clouds generally coincide with anomalously warm near‐surface temperatures at Greenland sites. These results indicate that both temperatures over 0 °C as well as positive daily temperature anomalies relate to spatially extensive opaque cloud cover. While prior studies indicate that clouds enhance extreme melt events, this research shows that opaque cloud cover and surface warming are closely related across the Greenland ice sheet, particularly in the ablation region. These findings establish broadly the spatial relationships between opaque clouds and temperatures and demonstrate the importance of direct observations across Greenland. Plain Language Summary: Greenland ice sheet melt linked to temperatures is a large contributor to rising global sea level. Arctic temperatures are related to cloud cover but spatial changes in clouds and temperature for Greenland have yet to be quantified. Here we show that a specific type of clouds, determined by a measurement signature, coincide with warm temperatures at the Greenland surface. These results indicate that unusual occurrences of temperatures above 0 °C relate closely to cloud cover. While prior work has shown that clouds enhance extreme melt events, this research shows that cloud cover and surface warming are closely tied across the Greenland ice sheet, particularly in the regions of high mass loss. These findings establish the variability in clouds and demonstrate the importance of direct observations across Greenland. Key Points: Daily variability of opaque cloud cover and temperatures are closely linkedAnomalies in opaque clouds and temperatures are tied to circulation regimeThe largest positive anomalies in both opaque clouds and temperatures relate to days with southerly transport [ABSTRACT FROM AUTHOR]

In recent years, several large outlet glaciers in Greenland lost their floating ice tongue, yet little is known regarding their stability over a longer timescale. Here we compile historical documents to demonstrate a major ice tongue collapse of Kangerlussuaq Glacier between 1932 and 1933. This event resulted in a 9‐km retreat, exceeding any of the glacier's recent major retreat events. Sediment cores from the fjord are used to reconstruct sea surface temperatures and to investigate a potential sedimentological trace of the collapse. During the 1920s, local and regional sea surface temperatures and air temperatures increased rapidly, suggesting a climatic trigger for the collapse. Fjord bathymetry played an important role too, as the (partially) pinned ice tongue retreated off a submarine moraine during the event. This historical analogue of a glacier tongue collapse emphasizes the fragility of remaining ice tongues in North Greenland within a warming climate. Plain Language Summary: In the past two decades, multiple Greenlandic glaciers retreated because their floating part (="ice tongue") melted and broke off. While it is believed that such events are the result of a warming climate, not much is known about how often or when such events have occurred in the past. In this study, we compiled multiple historical sources to show that Kangerlussuaq Glacier, one of Greenland's largest glaciers, retreated drastically between 1932 and 1933. During this event the ice tongue collapsed, leading to a 9‐km retreat, which is more than during any of the glacier's recent retreat events. By studying fjord sediments we show that the ocean temperatures increased prior to the event, as did air temperatures. Thus, climatic warming likely triggered the collapse. While other glaciers had already started their retreat decades earlier, Kangerlussuaq Glacier had been stable until 1932, probably due to stabilizing effect of an underwater moraine. Overall, this study emphasizes that ice tongues are sensitive to climatic warming and highlights the precarious position of current ice tongues in Northern Greenland. Key Points: Historical evidence reveals a major collapse of Kangerlussuaq Glacier's ice tongue between 1932 and 1933The collapse was likely triggered by increasing air and ocean temperatures during the late 1920s/early 1930sCompared to other glaciers in Greenland this retreat event occurred relatively late, probably due to its pinning to a large submarine moraine [ABSTRACT FROM AUTHOR]

Climate change in the Arctic is expected to have a major impact on stream ecosystems, affecting hydrological and thermal regimes. Although temperature is important to a range of in‐stream processes, previous Arctic stream temperature research is limited—focused on glacierised headwaters in summer—with limited attention to snowmelt streams and winter. This is the first high‐resolution study on stream temperature in north‐east Greenland (Zackenberg). Data were collected from five streams from September 2013 to September 2015 (24 months). During the winter, streams were largely frozen solid and water temperature variability low. Spring ice‐off date occurred simultaneously across all streams, but 11 days earlier in 2014 compared with 2015 due to thicker snow insulation. During summer, water temperature was highly variable and exhibited a strong relationship with meteorological variables, particularly incoming shortwave radiation and air temperature. Mean summer water temperature in these snowmelt streams was high compared with streams studied previously in Svalbard, yet was lower in Swedish Lapland, as was expected given latitude. With global warning, Arctic stream thermal variability may be less in summer and increased during the winter due to higher summer air temperature and elevated winter precipitation, and the spring and autumn ice‐on and ice‐off dates may extend the flowing water season—in turn affecting stream productivity and diversity. [ABSTRACT FROM AUTHOR]

The Arctic region has experienced significant warming during the past two decades with major implications on the cryosphere. The causes of Arctic amplification are still an open question within the scientific community, attracting recent interest. The goal of this study is to quantify the contribution of atmospheric circulation on temperature variability in the Atlantic–Arctic region at decadal to intra‐annual timescales from 1951 to 2014. Daily 20th Century reanalyses geopotential height anomalies at 500 hPa were clustered into different weather regimes to assess their contribution to observed temperature variability. The results show that in winter, 25% of the warming (cooling) in the North Atlantic Ocean (northeastern Canada) is due to temporal decreases of high geopotential anomalies in Greenland. This regime influences air mass migration patterns, bringing less cold (warm) air masses into these regions. Additionally, atmospheric warming or cooling has been attributed to a change in nearby oceanic basin surface conditions because of sea ice decline. In summer, about 15% of the warming observed in Norwegian/Greenland Seas is related to an increase in temporal anticyclonic patterns. This ratio reaches 37% in Norway due to an amplification from downwards solar radiation. This study allows for better understanding how natural climate variability modulates the regional signature of climate change and estimating the uncertainties in climate projections. [ABSTRACT FROM AUTHOR]

Records of ice‐rafted debris (IRD) in sediments are commonly used as a proxy for iceberg production and to reconstruct past changes of glacier stability. However, the interpretation of IRD is complex as multiple processes modulate its variability. This study investigates the relationship between IRD variability and glaciological change by measuring IRD records from Upernavik Fjord and comparing these to frontal positions of Upernavik Isstrøm during the past century. Results show that the spatial variability of IRD deposition throughout the fjord is high, indicating that randomness inherent to IRD distorts the calving signal. However, we investigate whether IRD records can be combined to improve the reconstruction, as previously suggested, and show the importance of core site selection and number of cores on this approach. The outer‐fjord core compares relatively well to the observed front positions and this is reflected in the composite record: increased IRD deposition in 1937–1946, 1968–1980, and 1996–1999 occurred during periods of faster retreat. Comparison with climatic records shows that the calving episodes in the late '30 s/early '40 s and late '90 s are related to warm ocean and air temperatures, whereas intensified retreat and calving during the '70 s reflects partly an internal glacier response to the fjord geometry. [ABSTRACT FROM AUTHOR]

ABSTRACT We use observed air temperature data series from 14 meteorological stations in coastal Greenland (located all around the Greenland Ice Sheet) for 1960-2010, where long-term records for five of the stations extend back to 1890, to illustrate the annual and monthly temporal and spatial distribution of temperature extremes, with the main focus on the latest decade 2001-2010 (2000s). We find that the 2000s had the highest number of mean annual air temperature ( MAAT) warm extremes, and the 1890s the highest number of cold extremes, and that a high (low) positive North Atlantic Oscillation ( NAO) Index equals a high number of cold (warm) extreme events. For the 2000s the number of warm extremes was significantly higher by around 50% than the number in the 1940s (the early twentieth century warm period): the latter being the decade with the second highest occurrence of MAAT warm extremes. Since 1960, based on MAAT the number of cold extremes has overall decreased on the decadal timescale, besides a peak in 1980s, while warm extremes have increased, leading to a higher occurrence of extremes (cold plus warm extremes): an almost similar pattern occurred for monthly mean temperatures and monthly mean daily maximum and minimum temperature datasets. Furthermore, a division of Greenland into east and west sectors shows that the occurrence of cold (warm) extremes was more pronounced in the East than in the West in the 1960s and 1970s (mid-1980s to the 2000s). [ABSTRACT FROM AUTHOR]

ABSTRACT The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a 'heat dome' over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994-2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future 'average' summers projected this century. [ABSTRACT FROM AUTHOR]

Two approaches were used to qualify observed variability in Greenland cod ( Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self-sustaining cod stock off Greenland with high SSB that produced a series of above-average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’. [ABSTRACT FROM AUTHOR]

Establishing the timing of maximum Holocene warmth in the Arctic is critical for understanding global climate system response to external forcing. In Greenland, challenges in obtaining climate records that span the full Holocene have hampered efforts to robustly identify when the Holocene Thermal Maximum occurred. Reconstructing land‐based ice sheet history can fill this gap because these ice sheet regions respond sensitively to summer temperature. We synthesize new and published 10Be and 14C ages from southwest Greenland to map Greenland ice sheet margin positions from 12 to 7 ka and calculate retreat rates from 12 to 0 ka. We found that maximum Greenland ice sheet recession occurred between ~10.4 and 9.1 ka. Our reconstruction suggests that summer air temperatures in southwest Greenland were highest from ~10.4 to 9.1 ka, providing support for an early regional Holocene Thermal Maximum. These results can serve as benchmarks for comparison with ice sheet and climate model simulations. Plain Language Summary: As the climate warms, Arctic temperatures are expected to increase at a faster rate than the rest of the world. Previous research has determined that Arctic temperatures were several degrees Celsius higher than today at some point in the past 12,000 years, and understanding when this warm time occurred might give scientists important insights into Arctic climate conditions in the future. However, in Greenland, the timing of maximum warmth is not well defined because many existing climate reconstructions do not span all of the past 12,000 years. In this paper, we use the retreat rate of the western Greenland ice sheet to determine when the warmest time of the past 12,000 years occurred. This method is advantageous because (1) retreat rates of this part of the Greenland ice sheet are directly related to summer air temperature and (2) the retreat history of the western Greenland ice sheet retreat is well known. We bring together new and previously published data that relates to Greenland ice sheet change to calculate how quickly the ice sheet retreated between 12,000 years ago and the present. Using this information, we determined that the warmest time of the past 12,000 years occurred between about 10,400 and 9,100 years ago, which is several thousand years earlier than some previous estimates. These results can help improve computer simulations that are used to predict future changes in Arctic ice sheet extent, climate, and sea‐level. Key Points: We create a database of southwest Greenland ice sheet extents and retreat rates that can be used to assess ice sheet model performanceThe highest southwest Greenland ice sheet retreat rates occurred between 10,400 and 9,100 years agoOur record supports an early Holocene Thermal Maximum in southwest Greenland [ABSTRACT FROM AUTHOR]