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- Thomas, R., Frederick, E., Li, J., Krabill, W., Manizade, S., Paden, J., Sonntag, J., Swift, R., and Yungel, J. Accelerating ice loss from the fastest Greenland and Antarctic glaciers. Geophysical Research Letters 38(10): art. L10502, 2011.
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Ice discharge from the fastest glaciers draining the Greenland and Antarctic ice sheets – Jakobshavn Isbrae (JI) and Pine Island Glacier (PIG) – continues to increase, and is now more than double that needed to balance snowfall in their catchment basins. Velocity increase probably resulted from decreased buttressing from thinning (and, for JI, breakup) of their floating ice tongues, and from reduced basal drag as grounding lines on both glaciers retreat. JI flows directly into the ocean as it becomes afloat, and here creep rates are proportional to the cube of bed depth. Rapid thinning of the PIG ice shelf increases the likelihood of its breakup, and subsequent rapid increase in discharge velocity. Results from a simple model indicate that JI velocities should almost double to >20 km a-1 by 2015, with velocities on PIG increasing to >10 km a-1 after breakup of its ice shelf. These high velocities would probably be sustained over many decades as the glaciers retreat within their long, very deep troughs. Resulting sea-level rise would average about 1.5 mm a-1.
- Ren, D. and Leslie, L.M. Three positive feedback mechanisms for ice-sheet melting in a warming climate. Journal of Glaciology 57(206): 1057-1066, 2011.
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Three positive feedback mechanisms that accelerate ice-sheet melting are assessed in a warming climate, using a numerical ice model driven by atmospheric climate models. The Greenland ice sheet (GrIS) is the modeling test-bed under accelerated melting conditions. The first feedback is the interaction of sea water with ice. It is positive because fresh water melts ice faster than salty water, owing primarily to the reduction in water heat capacity by solutes. It is shown to be limited for the GrIS, which has only a small ocean interface, and the grounding line of some fast glaciers becomes land-terminating during the 21st century. The second positive feedback, strain heating, is positive because it produces further ice heating inside the ice sheet. The third positive feedback, granular basal sliding, applies to all ice sheets and becomes the dominant feedback during the 21st century. A numerical simulation of Jakobshavn Isbræ over the 21st century reveals that all three feedback processes are active for this glacier. Compared with the year 2000 level, annual ice discharge into the ocean could increase by ~1.4 km³ a-1 (~5% of the present annual rate) by 2100. Granular basal sliding contributes ~40% of this increase.
- Barry, R.G. The cryosphere – past, present, and future: a review of the frozen water resources of the world. Polar Geography 34(4): 219-227, 2011.
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The paper addresses the frozen water resources of the world based on a literature review. The components of the global cryosphere are first summarized. Then the accessible frozen water resources of the world are examined – seasonal snow cover and its regional variations – freshwater ice, and mountain glaciers. Mountains play a dominant role in discharge by virtue of their seasonal snow cover. Current trends in seasonal snow cover and mountain glacier ice volume are discussed. Earlier snowmelt has led to advances in the timing of peak runoff. Due to warming trends, the proportion of precipitation falling as snow is decreasing at lower elevations. Model predictions for this century indicate that the greatest changes will occur in snow-dominated basins in mid- to high-latitudes. Future water resources from glaciers, associated with glacier retreat, are projected to lead to increased river runoff in the near term and then significant reductions with a shift in the timing of peak flow. The long-term changes associated with ice sheet mass loss will lead to sea level rise of between 0.9 and 1.6 m by 2100.
- Fountain, A.G., Campbell, J.L., Schuur, E.A.G., Stammerjohn, S.E., Williams, M.W., and Ducklow, H.W. The disappearing cryosphere: impacts and ecosystem responses to rapid cryosphere loss. BioScience 62(4): 405-415, 2012.
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The cryosphere – the portion of the Earth's surface where water is in solid form for at least one month of the year – has been shrinking in response to climate warming. The extents of sea ice, snow, and glaciers, for example, have been decreasing. In response, the ecosystems within the cryosphere and those that depend on the cryosphere have been changing. We identify two principal aspects of ecosystem-level responses to cryosphere loss: (1) trophodynamic alterations resulting from the loss of habitat and species loss or replacement and (2) changes in the rates and mechanisms of biogeochemical storage and cycling of carbon and nutrients, caused by changes in physical forcings or ecological community functioning. These changes affect biota in positive or negative ways, depending on how they interact with the cryosphere. The important outcome, however, is the change and the response the human social system (infrastructure, food, water, recreation) will have to that change.
- Jacob, T., Wahr, J., Pfeffer, W.T., and Swenson, S. Recent contributions of glaciers and ice caps to sea level rise. Nature 482(7386): 514-518, 2012.
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Glaciers and ice caps (GICs) are important contributors to present-day global mean sea level rise. Most previous global mass balance estimates for GICs rely on extrapolation of sparse mass balance measurements representing only a small fraction of the GIC area, leaving their overall contribution to sea level rise unclear. Here we show that GICs, excluding the Greenland and Antarctic peripheral GICs, lost mass at a rate of 148 ± 30 Gt yr-1 from January 2003 to December 2010, contributing 0.41 ± 0.08 mm yr-1 to sea level rise. Our results are based on a global, simultaneous inversion of monthly GRACE-derived satellite gravity fields, from which we calculate the mass change over all ice-covered regions greater in area than 100 km2. The GIC rate for 2003–2010 is about 30 per cent smaller than the previous mass balance estimate that most closely matches our study period. The high mountains of Asia, in particular, show a mass loss of only 4 ± 20 Gt yr-1 for 2003–2010, compared with 47–55 Gt yr-1 in previously published estimates. For completeness, we also estimate that the Greenland and Antarctic ice sheets, including their peripheral GICs, contributed 1.06 ± 0.19 mm yr-1 to sea level rise over the same time period. The total contribution to sea level rise from all ice-covered regions is thus 1.48 ± 0.26 mm-1, which agrees well with independent estimates of sea level rise originating from land ice loss and other terrestrial sources.
- Perovich, D.K. The changing Arctic sea ice cover. Oceanography 24(3): 162-173, 2011.
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Arctic sea ice cover has declined over the past few decades. The end of summer September ice extent reached a record minimum in 2007. While there has been a modest recovery since then, the past four years (2007–2010) show the lowest sea ice extent in the 30-year satellite record. Submarine and satellite ice thickness measurements show a factor of two decrease (3 m to 1.4 m) from 1957–1976 to 2003–2007. There has been a shift from sea ice cover consisting mainly of ice more than a year old to ice less than a year old. These changes have resulted in a less robust ice cover that is more sensitive to dynamic and thermodynamic forcing. Changes in atmospheric pressure fields in recent years have affected the distribution of ice in the Arctic Basin. Increases in advected ocean heat through Bering Strait may serve as a trigger for ice retreat in the Chukchi and Beaufort Seas. More open water has led to enhanced solar heat input and warming of the upper ocean and greater ice melt. While there may not be a tipping point for Arctic sea ice cover, positive feedbacks do contribute to rapid changes. The declining Arctic sea ice cover is affecting human activities.
- Pisaric, M.F.J., Thienpont, J.R., Kokelj, S.V., Nesbitt, H., Lantz, T.C., Solomon, S., and Smol, J.P. Impacts of a recent storm surge on an Arctic delta ecosystem examined in the context of the last millennium. Proceedings of the National Academy of Sciences [USA] 108(22): 8960-8965, 2011.
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One of the most ominous predictions related to recent climatic warming is that low-lying coastal environments will be inundated by higher sea levels. The threat is especially acute in polar regions because reductions in extent and duration of sea ice cover increase the risk of storm surge occurrence. The Mackenzie Delta of northwest Canada is an ecologically significant ecosystem adapted to freshwater flooding during spring breakup. Marine storm surges during the open-water season, which move saltwater into the delta, can have major impacts on terrestrial and aquatic systems. We examined growth rings of alder shrubs (Alnus viridis subsp. fruticosa) and diatoms preserved in dated lake sediment cores to show that a recent marine storm surge in 1999 caused widespread ecological changes across a broad extent of the outer Mackenzie Delta. For example, diatom assemblages record a striking shift from freshwater to brackish species following the inundation event. What is of particular significance is that the magnitude of this recent ecological impact is unmatched over the >1,000-year history of this lake ecosystem. We infer that no biological recovery has occurred in this lake, while large areas of terrestrial vegetation remain dramatically altered over a decade later, suggesting that these systems may be on a new ecological trajectory. As climate continues to warm and sea ice declines, similar changes will likely be repeated in other coastal areas of the circumpolar Arctic. Given the magnitude of ecological changes recorded in this study, such impacts may prove to be long lasting or possibly irreversible.
- Antoniades, D., Francus, P., Pienitz, R., St-Onge, G., and Vincent, W.F. Holocene dynamics of the Arctic's largest ice shelf. Proceedings of the National Academy of Sciences [USA] 108(47): 18899-18904, 2011.
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Ice shelves in the Arctic lost more than 90% of their total surface area during the 20th century and are continuing to disintegrate rapidly. The significance of these changes, however, is obscured by the poorly constrained ontogeny of Arctic ice shelves. Here we use the sedimentary record behind the largest remaining ice shelf in the Arctic, the Ward Hunt Ice Shelf (Ellesmere Island, Canada), to establish a long-term context in which to evaluate recent ice-shelf deterioration. Multiproxy analysis of sediment cores revealed pronounced biological and geochemical changes in Disraeli Fiord in response to the formation of the Ward Hunt Ice Shelf and its fluctuations through time. Our results show that the ice shelf was absent during the early Holocene and formed 4,000 years ago in response to climate cooling. Paleoecological data then indicate that the Ward Hunt Ice Shelf remained stable for almost three millennia before a major fracturing event that occurred ~1,400 years ago. After reformation ~800 years ago, freshwater was a constant feature of Disraeli Fiord until the catastrophic drainage of its epishelf lake in the early 21st century.
- Gardner, A.S., Moholdt, G., Wouters, B., Wolken, G.J., Burgess, D.O., Sharp, M.J., Cogley, J.G., Braun, C., and Labine, C. Sharply increased mass loss from glaciers and ice caps in the Canadian Arctic Archipelago. Nature 473(7347): 357-360, 2011.
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Mountain glaciers and ice caps are contributing significantly to present rates of sea level rise and will continue to do so over the next century and beyond. The Canadian Arctic Archipelago, located off the northwestern shore of Greenland, contains one-third of the global volume of land ice outside the ice sheets, but its contribution to sea-level change remains largely unknown. Here we show that the Canadian Arctic Archipelago has recently lost 61 ± 7 gigatonnes per year (Gt yr-1) of ice, contributing 0.17 ± 0.02 mm yr-1 to sea-level rise. Our estimates are of regional mass changes for the ice caps and glaciers of the Canadian Arctic Archipelago referring to the years 2004 to 2009 and are based on three independent approaches: surface mass-budget modelling plus an estimate of ice discharge (SMB+D), repeat satellite laser altimetry (ICESat) and repeat satellite gravimetry (GRACE). All three approaches show consistent and large mass-loss estimates. Between the periods 2004–2006 and 2007–2009, the rate of mass loss sharply increased from 31 ± 8 Gt yr-1 to 92 ± 12 Gt yr-1 in direct response to warmer summer temperatures, to which rates of ice loss are highly sensitive (64 ± 14 Gt yr-1 per 1 K increase). The duration of the study is too short to establish a long-term trend, but for 2007–2009, the increase in the rate of mass loss makes the Canadian Arctic Archipelago the single largest contributor to eustatic sea-level rise outside Greenland and Antarctica.
- Kinnard, C., Zdanowicz, C.M., Fisher, D.A., Isaksson, E., de Vernal, A., and Thompson, L.G. Reconstructed changes in Arctic sea ice over the past 1,450 years. Nature 479(7374): 509-512, 2011.
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Arctic sea ice extent is now more than two million square kilometres less than it was in the late twentieth century, with important consequences for the climate, the ocean and traditional lifestyles in the Arctic. Although observations show a more or less continuous decline for the past four or five decades, there are few long-term records with which to assess natural sea ice variability. Until now, the question of whether or not current trends are potentially anomalous has therefore remained unanswerable. Here we use a network of high-resolution terrestrial proxies from the circum-Arctic region to reconstruct past extents of summer sea ice, and show that-although extensive uncertainties remain, especially before the sixteenth century-both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years. Enhanced advection of warm Atlantic water to the Arctic seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming.
- Rampal, P., Weiss, J., Dubois, C., and Campin, J.-M. IPCC climate models do not capture Arctic sea ice drift acceleration: Consequences in terms of projected sea ice thinning and decline. Journal of Geophysical Research 116: art. C00D07, 2011.
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IPCC climate models underestimate the decrease of the Arctic sea ice extent. The recent Arctic sea ice decline is also characterized by a rapid thinning and by an increase of sea ice kinematics (velocities and deformation rates), with both processes being coupled through positive feedbacks. In this study we show that IPCC climate models underestimate the observed thinning trend by a factor of almost 4 on average and fail to capture the associated accelerated motion. The coupling between the ice state (thickness and concentration) and ice velocity is unexpectedly weak in most models. In particular, sea ice drifts faster during the months when it is thick and packed than when it is thin, contrary to what is observed; also models with larger long-term thinning trends do not show higher drift acceleration. This weak coupling behavior (1) suggests that the positive feedbacks mentioned above are underestimated and (2) can partly explain the models' underestimation of the recent sea ice area, thickness, and velocity trends. Due partly to this weak coupling, ice export does not play an important role in the simulated negative balance of Arctic sea ice mass between 1950 and 2050. If we assume a positive trend on ice speeds at straits equivalent to the one observed since 1979 within the Arctic basin, first-order estimations give shrinking and thinning trends that become significantly closer to the observations.
- Mahlstein, I. and Knutti, R. September Arctic sea ice predicted to disappear near 2 °C global warming above present. Journal of Geophysical Research 117(D06): art. D06104, 2012.
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The decline of Arctic sea ice is one of the most visible signs of climate change over the past several decades. Arctic sea ice area shows large interannual variability due to the numerous factors, but on longer time scales the total sea ice area is approximately linearly related to Arctic surface air temperature in models and observations. Overall, models however strongly underestimate the recent sea ice decline. Here we show that this can be explained with two interlinked biases. Most climate models simulate a smaller sea ice area reduction per degree local surface warming. Arctic polar amplification, the ratio between Arctic and global temperature, is also underestimated but a number of models are within the uncertainty estimated from natural variability. A recalibration of an ensemble of global climate models using observations over 28 years provides a scenario independent relationship and yields about 2 °C change in annual mean global surface temperature above present as the most likely global temperature threshold for September sea ice to disappear, but with substantial associated uncertainty. Natural variability in the Arctic is large and needs to be considered both for such recalibrations as well as for model evaluation, in particular when observed trends are relatively short.
- Zdanowicz, C., Smetny-Sowa, A., Fisher, D., Schaffer, N., Copland, L., Eley, J., and Dupont, F. Summer melt rates on Penny Ice Cap, Baffin Island: Past and recent trends and implications for regional climate. Journal of Geophysical Research 117(F2): art. F02006, 2012.
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At latitude 67°N, Penny Ice Cap on Baffin Island is the southernmost large ice cap in the Canadian Arctic, yet its past and recent evolution is poorly documented. Here we present a synthesis of climatological observations, mass balance measurements and proxy climate data from cores drilled on the ice cap over the past six decades (1953 to 2011). We find that starting in the 1980s, Penny Ice Cap entered a phase of enhanced melt rates related to rising summer and winter air temperatures across the eastern Arctic. Presently, 70 to 100% (volume) of the annual accumulation at the ice cap summit is in the form of refrozen meltwater. Recent surface melt rates are found to be comparable to those last experienced more than 3000 years ago. Enhanced surface melt, water percolation and refreezing have led to a downward transfer of latent heat that raised the subsurface firn temperature by 10 °C (at 10 m depth) since the mid-1990s. This process may accelerate further mass loss of the ice cap by pre-conditioning the firn for the ensuing melt season. Recent warming in the Baffin region has been larger in winter but more regular in summer, and observations on Penny Ice Cap suggest that it was relatively uniform over the 2000-m altitude range of the ice cap. Our findings are consistent with trends in glacier mass loss in the Canadian High Arctic and regional sea-ice cover reduction, reinforcing the view that the Arctic appears to be reverting back to a thermal state not seen in millennia.
- Howat, I.M., Ahn, Y., Joughin, I., van den Broeke, M.R., Lenaerts, J.T.M., and Smith, B. Mass balance of Greenland's three largest outlet glaciers, 2000–2010. Geophysical Research Letters 38(12): art. 12501, 2011.
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Acceleration of Greenland's three largest outlet glaciers, Helheim, Kangerdlugssuaq and Jakobshavn Isbrae, accounted for a substantial portion of the ice sheet's mass loss over the past decade. Rapid changes in their discharge, however, make their cumulative mass-change uncertain. We derive monthly mass balance rates and cumulative balance from discharge and surface mass balance (SMB) rates for these glaciers from 2000 through 2010. Despite the dramatic changes observed at Helheim, the glacier gained mass over the period, due primarily to the short-duration of acceleration and a likely longer-term positive balance. In contrast, Jakobshavn Isbrae lost an equivalent of over 11 times the average annual SMB and loss continues to accelerate. Kangerdlugssuaq lost over 7 times its annual average SMB, but loss has returned to the 2000 rate. These differences point to contrasts in the long-term evolution of these glaciers and the danger in basing predictions on extrapolations of recent changes.
- Kobashi, T., Kawamura, K., Severinghaus, J.P., Barnola, J.-M., Nakaegawa, T., Vinther, B.M., Johnsen, S.J., and Box, J.E. High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophysical Research Letters 38(21): art. L21501, 2011.
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Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise. Yet, it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability. To address this need, we reconstruct Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilises argon and nitrogen isotopic ratios from occluded air bubbles. The estimated average Greenland snow temperature over the past 4000 years was -30.7°C with a standard deviation of 1.0°C and exhibited a long-term decrease of roughly 1.5°C, which is consistent with earlier studies. The current decadal average surface temperature (2001–2010) at the GISP2 site is -29.9°C. The record indicates that warmer temperatures were the norm in the earlier part of the past 4000 years, including century-long intervals nearly 1°C warmer than the present decade (2001–2010). Therefore, we conclude that the current decadal mean temperature in Greenland has not exceeded the envelope of natural variability over the past 4000 years, a period that seems to include part of the Holocene Thermal Maximum. Notwithstanding this conclusion, climate models project that if anthropogenic greenhouse gas emissions continue, the Greenland temperature would exceed the natural variability of the past 4000 years sometime before the year 2100.
- Frey, K.E., Perovich, D.K., and Light, B. The spatial distribution of solar radiation under a melting Arctic sea ice cover. Geophysical Research Letters 38(22): art. L22501, 2011.
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The sea ice cover of the Chukchi and Beaufort Seas is currently undergoing a fundamental shift from multiyear ice to first-year ice. Field observations of sea ice physical and optical properties were collected in this region during June–July 2010, revealing unexpectedly complex spatial distributions of solar radiation under the melt-season ice cover. Based on our optical measurements of first-year ice, we found the under-ice light field in the upper ocean to be spatially heterogeneous and dependent on wavelength, ice thickness, and the areal and geometric distribution of melt ponded and bare ice surfaces. Much of the observed complexity in radiation fields arose because the transmission of light through ponded ice was generally an order of magnitude greater than through bare, unponded ice. Furthermore, while many sites exhibited a consistent, exponential decay in light transmission through both ponded and bare ice surfaces, light transmission under bare ice was also observed to increase with depth (reaching maximum values ~5–10 m below the bottom of the ice). A simple geometric model shows these transmission peaks are a result of scattering in the ice and the interspersion of bare and ponded sea ice surfaces. These new observations of complex radiation fields beneath melt-season first-year sea ice have significant implications for biological production, biogeochemical processes, and the heat balance of sea ice and under-ice ocean waters and should be carefully considered when modeling these sea ice-related phenomena.
- Jackson, J.M., Williams, W.J., and Carmack, E.C. Winter sea-ice melt in the Canada Basin, Arctic Ocean. Geophysical Research Letters 39(3): art. L03603, 2012.
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Recent warming and freshening of the Canada Basin has led to the year-round storage of solar radiation as the near-surface temperature maximum (NSTM). Using year-round ocean (from ice tethered profilers and autonomous ocean flux buoys), sea-ice (from ice mass balance buoys), and atmosphere (from NCEP/NCAR reanalysis) data from 2005–2010, we find that heat from the NSTM is entrained into the surface mixed layer (SML) during winter. Entrainment can only occur when the base of the SML reaches the top of the NSTM. If this condition is met, the surface forcing and stratification together determine whether the SML deepens into the NSTM. Heat transfer occurs by diffusion or by the erosion of the summer halocline. The average temperature of the SML warmed by as much as 0.06°C during storm events. Solar radiation began warming the SML about 1 month early during the winter of 2007–2008 and this can be explained by thin sea ice.
- Perovich, D.K. and Polashenski, C. Albedo evolution of seasonal Arctic sea ice. Geophysical Research Letters 39(8): art. L08501, 2012.
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There is an ongoing shift in the Arctic sea ice cover from multiyear ice to seasonal ice. Here we examine the impact of this shift on sea ice albedo. Our analysis of observations from four years of field experiments indicates that seasonal ice undergoes an albedo evolution with seven phases; cold snow, melting snow, pond formation, pond drainage, pond evolution, open water, and freezeup. Once surface ice melt begins, seasonal ice albedos are consistently less than albedos for multiyear ice resulting in more solar heat absorbed in the ice and transmitted to the ocean. The shift from a multiyear to seasonal ice cover has significant implications for the heat and mass budget of the ice and for primary productivity in the upper ocean. There will be enhanced melting of the ice cover and an increase in the amount of sunlight available in the upper ocean.
- Andresen, C.S. et al. Rapid response of Helheim Glacier in Greenland to climate variability over the past century. Nature Geoscience 5(1): 37-41, 2012.
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During the early 2000s the Greenland Ice Sheet experienced the largest ice-mass loss of the instrumental record, largely as a result of the acceleration, thinning and retreat of large outlet glaciers in West and southeast Greenland. The quasi-simultaneous change in the glaciers suggests a common climate forcing. Increasing air and ocean temperatures have been indicated as potential triggers. Here, we present a record of calving activity of Helheim Glacier, East Greenland, that extends back to about AD 1890, based on an analysis of sedimentary deposits from Sermilik Fjord, where Helheim Glacier terminates. Specifically, we use the annual deposition of sand grains as a proxy for iceberg discharge. Our record reveals large fluctuations in calving rates, but the present high rate was reproduced only in the 1930s. A comparison with climate indices indicates that high calving activity coincides with a relatively strong influence of Atlantic water and a lower influence of polar water on the shelf off Greenland, as well as with warm summers and the negative phase of the North Atlantic Oscillation. Our analysis provides evidence that Helheim Glacier responds to short-term fluctuations of large-scale oceanic and atmospheric conditions, on timescales of 3–10 years.
- Box, J.E. and Decker, D.T. Greenland marine-terminating glacier area changes: 2000–2010. Annals of Glaciology 52(59): 91-98, 2011.
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Area changes at 39 of the widest Greenland marine-terminating glacier outlets are measured in consecutive annual end-of-melt-season Moderate Resolution Imaging Spectroradiometer (MODIS) scenes spanning ten annual intervals (2000–10). The rates of cumulative area change for glaciers and ice shelves are well represented by linear least-squares fits, R = -0.99 and R = -0.94, with average rates of -70 and -65 km² a-1, respectively. Collectively, during this decade, the 39 glaciers lost a cumulative area of 1368 km². More than three-quarters of the total area change occurred north of 72° N. The largest 11-year area change for a single glacier during the survey period is the 311 km² loss at Humboldt Glacier. The largest annual change for a single glacier was extreme compared with the others, where Petermann glacier retreated 17 km between 3 and 5 August 2010. For the 10 year sample, on average, the count of glaciers retreating is twice that advancing. A larger distinction is evident considering area change, with the ratio of retreat and advance, on average, nine times the gain. For glaciers with ice shelves, we find no year with collective area gain.
- Jiskoot, H., Juhlin, D., St. Pierre, H., and Citterio, M. Tidewater glacier fluctuations in central East Greenland coastal and fjord regions (1980s–2005). Annals of Glaciology 53(60): 35-44, 2012.
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Summer 2000/01 ASTER and Landsat 7 scenes and semi-automated digitization were used to compile a glacier inventory for local glaciers of the Geikie Plateau region, central East Greenland. Of the 332 glaciers (41 591 km²), 120 are tidewater-terminating and drain 90% of the glacierized area. Differencing the 2000/01 tidewater margins from the 1980s GEUS map database ice polygons (113 glaciers) and from 2004/05 ASTER tidewater margins (78 glaciers) shows a cumulative tidewater terminus width decrease from 196 km to 183 km between the 1980s and 2000s, with a corresponding areal loss of ~31 km² and an effective length change of -14.3 km. Between 2000/01 and 2004/05, areal loss was 26 km². Average margin retreat rate increased two- to threefold, from 1.7–2.1 km² a-1 (1980s–2000) to 3.9-5.7 km² a-1 (2000–05). Advances are negligible, apart from two surges, of which one was previously undetected. Coastal, 'outer' fjord-terminating, glaciers have two to three times larger areal and effective length retreat rates than 'inner' fjord-terminating glaciers. We investigate possible causes and hypothesize that, in addition to ocean temperature and sea ice, changes in sea fog may affect coastal-terminating more than inner fjord-terminating glaciers.
- Thomson, L.I., Osinski, G.R., and Ommanney, C.S.L. Glacier change on Axel Heiberg Island, Nunavut, Canada. Journal of Glaciology 57(206): 1079-1086, 2011.
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Historical records are valuable for assessing glacier change in the Canadian High Arctic. Ommanney's (1969) detailed inventory of Axel Heiberg Island glaciers, based on photography from 1958-59, has been revisited, converted into digital format and compared to glacier extents mapped from 1999–2000 satellite imagery. Our results show that the island-wide ice coverage decreased by 15.92 km² in the 42 year period, a loss of <1%. However, two trends are apparent: one of advance or minor retreat from basins hosting outlet glaciers from Müller and Steacie Ice Caps, and one of significant retreat, on the order of 50–80%, for independent ice masses, which include valley glaciers, mountain glaciers, glacierets, and ice caps smaller than 25 km². If the contributions to ice advance of only three surging glaciers are removed, then the island-wide ice loss approaches 60 km². Furthermore, it is notable that 90% of ice masses smaller than 0.2 km² disappeared entirely during the 42 year study period, an observation confirmed by field studies. Successful predictions from the original inventory are highlighted and the likely mechanisms driving the observed advances and retreats are discussed.
- Nick, F.M. et al. The response of Petermann Glacier, Greenland, to large calving events, and its future stability in the context of atmospheric and oceanic warming. Journal of Glaciology 58(208): 229-239, 2012.
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This study assesses the impact of a large 2010 calving event on the current and future stability of Petermann Glacier, Greenland, and ascertains the glacier's interaction with different components of the climate and ocean system. We use a numerical ice-flow model that captures the major aspects of the glacier's mass budget, the resistive forces controlling glacier flow, and includes dynamic calving. Satellite observations and model results show that the recent break-off of 25% of the floating tongue did not result in a significant glacier speed-up due to the low lateral resistance of this relatively wide and thin ice tongue. We demonstrate that seasonal speed-up at Petermann Glacier is mainly driven by meltwater lubrication rather than freeze-up conditions in the fjord. Results also show that sub-shelf ocean melt may have a profound effect on the future stability of Petermann Glacier, emphasizing the urgent need for more observations, and a better understanding of fjord temperature variability and circulation.
- Polyakov, I.V., Walsh, J.E., and Kwok, R. Recent changes of arctic multiyear sea ice coverage and the likely causes. Bulletin of the American Meteorological Society 93(2): 145-151, 2012.
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- Walsh, J.E., Overland, J.E., Groisman, P.Y., and Rudolf, B. Ongoing climate change in the Arctic. Ambio 40(Suppl. 1): 6-16, 2011.
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During the past decade, the Arctic has experienced its highest temperatures of the instrumental record, even exceeding the warmth of the 1930s and 1940s. Recent paleo-reconstructions also show that recent Arctic summer temperatures are higher than at any time in the past 2000years. The geographical distribution of the recent warming points strongly to an influence of sea ice reduction. The spatial pattern of the near-surface warming also shows the signature of the Pacific Decadal Oscillation in the Pacific sector as well as the influence of a dipole-like circulation pattern in the Atlantic sector. Areally averaged Arctic precipitation over the land areas north of 55°N shows large year-to-year variability, superimposed on an increase of about 5% since 1950. The years since 2000 have been wetter than average according to both precipitation and river discharge data. There are indications of increased cloudiness over the Arctic, especially low clouds during the warm season, consistent with a longer summer and a reduction of summer sea ice. Storm events and extreme high temperature show signs of increases. The Arctic Ocean has experienced enhanced oceanic heat inflows from both the North Atlantic and the North Pacific. The Pacific inflows evidently played a role in the retreat of sea ice in the Pacific sector of the Arctic Ocean, while the Atlantic water heat influx has been characterized by increasingly warm pulses. Recent shipboard observations show increased ocean heat storage in newly sea-ice-free ocean areas, with increased influence on autumn atmospheric temperature and wind fields.
- Callaghan, T.V., Johansson, M., Key, J., Prowse, T., Ananicheva, M., and Klepikov, A. Feedbacks and interactions: from the Arctic cryosphere to the climate system. Ambio 40(Suppl. 1): 75-86, 2011.
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Changes in the Arctic's climate are a result of complex interactions between the cryosphere, atmosphere, ocean, and biosphere. More feedbacks from the cryosphere to climate warming are positive and result in further warming than are negative, resulting in a reduced rate of warming or cooling. Feedbacks operate at different spatial scales; many, such as those operating through albedo and evapotranspiration, will have significant local effects that together could result in global impacts. Some processes, such as changes in carbon dioxide (CO2) emissions, are likely to have very small global effects but uncertainty is high whereas others, such as subsea methane (CH4) emissions, could have large global effects. Some cryospheric processes in the Arctic have teleconnections with other regions and major changes in the cryosphere have been largely a result of large-scale processes, particularly atmospheric and oceanic circulation. With continued climate warming it is highly likely that the cryospheric components will play an increasingly important climatic role. However, the net effect of all the feedbacks is difficult to assess because of the variability in spatial and temporal scales over which they operate. Furthermore, general circulation models (GCMs) do not include all major feedbacks while those included may not be accurately parameterized. The lack of full coupling between surface dynamics and the atmosphere is a major gap in current GCMs.
- Vincent, W.F. et al. Ecological implications of changes in the Arctic cryosphere. Ambio 40(Suppl. 1): 87-99, 2011.
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Snow, water, ice, and permafrost are showing evidence of substantial change in the Arctic, with large variations among different geographical areas. As a result of these changes, some habitats and their associated ecosystems are expanding, while others are undergoing rapid contraction. The warming of the Arctic cryosphere is limiting the range for cold-adapted biota, and less specialized taxa including invasive species from the south are likely to become increasingly common. Extreme climate events such as winter thawing are likely to become more frequent, and may accelerate shifts in community structure and processes. Many Arctic ecosystems are interdependent, and changes in the cryosphere are altering physical, biogeochemical, and biological linkages, as well as causing positive feedback effects on atmospheric warming. All of these climate-related effects are compounded by rapid socio-economic development in the North, creating additional challenges for northern communities and indigenous lifestyles that depend on Arctic ecosystem services.
- Olsen, M.S. et al. The changing Arctic cryosphere and likely consequences: an overview. Ambio 40(Suppl. 1): 111-118, 2011.
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The Arctic cryosphere is a critically important component of the earth system, affecting the energy balance, atmospheric and ocean circulation, freshwater storage, sea level, the storage, and release of large quantities of greenhouse gases, economy, infrastructure, health, and indigenous and non-indigenous livelihoods, culture and identity. Currently, components of the Arctic cryosphere are subjected to dramatic change due to global warming. The need to document, understand, project, and respond to changes in the cryosphere and their consequences stimulated a comprehensive international assessment called "SWIPA": Snow, Water, Ice, Permafrost in the Arctic. Some of the extensive key SWIPA chapters have been summarized and made more widely available to a global audience with multi-disciplinary interests in this Special Report of Ambio. In this article, an overview is provided of this Special Report in the context of the more detailed and wider scope of the SWIPA Report. Accelerated changes in major components of the Arctic cryosphere are documented. Evidence of feedback mechanisms between the cryosphere and other parts of the climate system are identified as contributing factors to enhanced Arctic warming while the growing importance of Arctic land-based ice as a contributor to global sea-level rise is quantified. Cryospheric changes will result in multifaceted and cascading effects for people within and beyond the Arctic presenting both challenges and opportunities.
- Hughes, A.L.C., Rainsley, E., Murray, T., Fogwill, C.J., Schnabel, C., and Xu, S. Rapid response of Helheim Glacier, southeast Greenland, to early Holocene climate warming. Geology 40(5): 427-430, 2012.
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Recent changes in speed, thinning, and retreat rates of marine-terminating outlet glaciers have raised concerns about the future stability of the Greenland Ice Sheet. Establishing a longer term record of outlet glacier retreat rates is essential to provide a context for present-day observations and to improve and constrain numerical models of outlet glacier behavior. New exposure dating (10Be) of streamlined bedrock surfaces and glacial erratic boulders of Sermilik Fjord, southeast Greenland, the present-day drainage route of Helheim Glacier, documents rapid retreat (~80 m a-1) of this major marine-terminating outlet glacier at the close of the last glaciation. The glacier front retreated ~80 km to within 20 km of the present-day (2010) position of Helheim Glacier in <1 ka, ca. 10.8 ± 0.3 ka ago. Retreat followed rapidly rising air temperatures at the start of the Holocene, and at this temporal resolution there is no evidence that fjord geometry influenced glacier behavior. The significant response to climatic amelioration at the end of the last glacial suggests a high sensitivity to abrupt temperature increases, which has major implications for the future stability of present-day Greenlandic outlet glaciers in a warming climate.
- Fisher, D., Zheng, J., Burgess, D., Zdanowicz, C., Kinnard, C., Sharp, M., and Bourgeois, J. Recent melt rates of Canadian arctic ice caps are the highest in four millennia. Global and Planetary Change 84-85: 3-7, 2012.
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There has been a rapid acceleration in ice-cap melt rates over the last few decades across the entire Canadian Arctic. Present melt rates exceed the past rates for many millennia. New shallow cores at old sites bring their melt series up-to-date. The melt-percentage series from the Devon Island and Agassiz (Ellesmere Island) ice caps are well correlated with the Devon net mass balance and show a large increase in melt since the middle 1990s. Arctic ice core melt series (latitude range of 67 to 81 N) show the last quarter century has had the highest melt in two millennia and The Holocene-long Agassiz melt record shows that the last 25 years has the highest melt in 4200 years. The Agassiz melt rates since the middle 1990s resemble those of the early Holocene thermal maximum over 9000 years ago.
- van As, D., Hubbard, A.L., Hasholt, B., Mikkelsen, A.B., van den Broeke, M.R., and Fausto, R.S. Large surface meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during the record-warm year 2010 explained by detailed energy balance observations. Cryosphere 6(1): 199-209, 2012.
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This study uses data from six on-ice weather stations, calibrated MODIS-derived albedo and proglacial river gauging measurements to drive and validate an energy balance model. We aim to quantify the record-setting positive temperature anomaly in 2010 and its effect on mass balance and runoff from the Kangerlussuaq sector of the Greenland ice sheet. In 2010, the average temperature was 4.9 °C (2.7 standard deviations) above the 1974–2010 average in Kangerlussuaq. High temperatures were also observed over the ice sheet, with the magnitude of the positive anomaly increasing with altitude, particularly in August. Simultaneously, surface albedo was anomalously low in 2010, predominantly in the upper ablation zone. The low albedo was caused by high ablation, which in turn profited from high temperatures and low winter snowfall. Surface energy balance calculations show that the largest melt excess (~170%) occurred in the upper ablation zone (above 1000 m), where higher temperatures and lower albedo contributed equally to the melt anomaly. At lower elevations the melt excess can be attributed to high atmospheric temperatures alone. In total, we calculate that 6.6 ± 1.0 km3 of surface meltwater ran off the ice sheet in the Kangerlussuaq catchment in 2010, exceeding the reference year 2009 (based on atmospheric temperature measurements) by ~150%. During future warm episodes we can expect a melt response of at least the same magnitude, unless a larger wintertime snow accumulation delays and moderates the melt-albedo feedback. Due to the hypsometry of the ice sheet, yielding an increasing surface area with elevation, meltwater runoff will be further amplified by increases in melt forcings such as atmospheric heat.
- Walsh, K.M., Howat, I.M., Ahn, Y., and Enderlin, E.M. Changes in the marine-terminating glaciers of central east Greenland, 2000–2010. Cryosphere 6(1): 211-220, 2012.
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Marine-terminating outlet glaciers of the Greenland Ice Sheet have undergone substantial changes over the past decade. The synchronicity of these changes suggest a regional external forcing, such as changes in coastal ocean heat transport and/or increased surface melt and subglacial runoff. A distinct contrast in rates of ice front retreat has been observed between glaciers north and south of 69° N latitude on along the East Greenland coast. This latitude corresponds with the northward limit of subtropical waters carried by the Irminger Current, suggesting variability in ocean heat transport as the dominant forcing. Glacier surging, however, is yet another mechanism of change in this region. In order to provide further spatial and temporal constraint on glacier change across this important oceanographic transition zone, we construct time series of thinning, retreat and flow speed of 37 marine-terminating glaciers along the central east Greenland coast from 2000 to 2010. We assess this dataset for spatial and temporal patterns that may elucidate the mechanisms of glacier change. We confirm that glacial retreat, dynamical thinning, and acceleration have been more pronounced south of 69° N, with a high degree of variability along the Blosseville Coast and little inter-annual change in Scoresby Sound. Our results support the conclusion that variability in coastal ocean heat transport is the primary driver of regional glacier change, but that local factors, such as surging and/or individual glacier morphology, are overprinted on this regional signal.
- Comiso, J.C. Large decadal decline of the Arctic multiyear ice cover. Journal of Climate 25(4): 1176-1193, 2012.
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The perennial ice area was drastically reduced to 38% of its climatological average in 2007 but recovered slightly in 2008, 2009, and 2010 with the areas being 10%, 24%, and 11% higher than in 2007, respectively. However, trends in extent and area remained strongly negative at -12.2% and -13.5% decade-1, respectively. The thick component of the perennial ice, called multiyear ice, as detected by satellite data during the winters of 1979–2011 was studied, and results reveal that the multiyear ice extent and area are declining at an even more rapid rate of -15.1% and -17.2% decade-1, respectively, with a record low value in 2008 followed by higher values in 2009, 2010, and 2011. Such a high rate in the decline of the thick component of the Arctic ice cover means a reduction in the average ice thickness and an even more vulnerable perennial ice cover. The decline of the multiyear ice area from 2007 to 2008 was not as strong as that of the perennial ice area from 2006 to 2007, suggesting a strong role of second-year ice melt in the latter. The sea ice cover is shown to be strongly correlated with surface temperature, which is increasing at about 3 times the global average in the Arctic but appears weakly correlated with the Arctic Oscillation (AO), which controls the atmospheric circulation in the region. An 8-9-yr cycle is apparent in the multiyear ice record, which could explain, in part, the slight recovery in the last 3 yr.
- Khazendar, A., Rignot, E., and Larour, E. Acceleration and spatial rheology of Larsen C Ice Shelf, Antarctic Peninsula. Geophysical Research Letters 38(9): art. L09502, 2011.
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The disintegration of several Antarctic Peninsula ice shelves has focused attention on the state of the Larsen C Ice Shelf. Here, we use satellite observations to map ice shelf speed from the years 2000, 2006 and 2008 and apply inverse modeling to examine the spatial pattern of ice-shelf stiffness. Results show that the northern half of the ice shelf has been accelerating since 2000, speeding up by 15% between 2000 and 2006 alone. The distribution of ice stiffness exhibits large spatial variations that we link to tributary glacier flow and fractures. Our results reveal that ice down-flow from promontories is consistently softer, with the exception of Churchill Peninsula where we infer a stabilizing role for marine ice. We conclude that although Larsen C is not facing imminent collapse, it is undergoing significant change in the form of flow acceleration that is spatially related to thinning and fracture.
- Thomas, I.D. et al. Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations. Geophysical Research Letters 38(22): art. L22302, 2011.
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Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over-estimated, particularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root-mean-squares of observation-model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.
- Stearns, L.A. Dynamics and mass balance of four large East Antarctic outlet glaciers. Annals of Glaciology 52(59): 116-125, 2011.
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The East Antarctic ice sheet (EAIS) is Earth's largest reservoir of fresh water and has the potential to raise sea level by ~50 m. A significant amount of the ice sheet's mass is discharged by outlet glaciers draining through the Transantarctic Mountains, the balance characteristics of which are largely unknown. Here the mass balance is estimated for four glaciers draining ice from the EAIS through the Transantarctic Mountains into the Ross Sea embayment: David, Mulock, Byrd and Nimrod glaciers. Remote-sensing observations are used to map changes in ice flow and surface elevation, and ultimately to compute the mass balance of each glacier using the input-output method and three separate estimates for accumulation rate. Results computed using this method indicate small positive balances for David (2.41 ± 1.31 Gt a-1), Mulock (1.91 ± 0.84 Gt a-1) and Nimrod (0.88 ± 0.39 Gt a-1) glaciers, and a large positive imbalance for Byrd Glacier (21.67 ± 4.04 Gt a-1). This large imbalance for Byrd Glacier is inconsistent with other observations, and is likely due to an overestimation of accumulation rates across large regions of the interior catchment.
- Levermann, A., Albrecht, T., Winkelmann, R., Martin, M.A., Haseloff, M., and Joughin, I. Kinematic first-order calving law implies potential for abrupt ice-shelf retreat. Cryosphere 6(2): 273-286, 2012.
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Recently observed large-scale disintegration of Antarctic ice shelves has moved their fronts closer towards grounded ice. In response, ice-sheet discharge into the ocean has accelerated, contributing to global sea-level rise and emphasizing the importance of calving-front dynamics. The position of the ice front strongly influences the stress field within the entire sheet-shelf-system and thereby the mass flow across the grounding line. While theories for an advance of the ice-front are readily available, no general rule exists for its retreat, making it difficult to incorporate the retreat in predictive models. Here we extract the first-order large-scale kinematic contribution to calving which is consistent with large-scale observation. We emphasize that the proposed equation does not constitute a comprehensive calving law but represents the first-order kinematic contribution which can and should be complemented by higher order contributions as well as the influence of potentially heterogeneous material properties of the ice. When applied as a calving law, the equation naturally incorporates the stabilizing effect of pinning points and inhibits ice shelf growth outside of embayments. It depends only on local ice properties which are, however, determined by the full topography of the ice shelf. In numerical simulations the parameterization reproduces multiple stable fronts as observed for the Larsen A and B Ice Shelves including abrupt transitions between them which may be caused by localized ice weaknesses. We also find multiple stable states of the Ross Ice Shelf at the gateway of the West Antarctic Ice Sheet with back stresses onto the sheet reduced by up to 90 % compared to the present state.
- Pritchard, H.D., Ligtenberg, S.R.M., Fricker, H.A., Vaughan, D.G., van den Broeke, M.R., and Padman, L. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature 484(7395): 502-505, 2012.
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Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.
- Fricker, H.A. and Padman, L. Thirty years of elevation change on Antarctic Peninsula ice shelves from multimission satellite radar altimetry. Journal of Geophysical Research 117(C2): art. C02026, 2012.
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We use data acquired between 1978 and 2008 by four satellite radar altimeter missions (Seasat, ERS-1, ERS-2 and Envisat) to determine multidecadal elevation change rates (dhi/dt) for six major Antarctic Peninsula (AP) ice shelves. In areas covered by the Seasat orbit (to 72.16°S), regional-averaged 30-year trends were negative (surface lowering), with rates between -0.03 and -0.16 m a-1. Surface lowering preceded the start of near-continuous radar altimeter operations that began with ERS-1 in 1992. The average rate of lowering for the first 14 years of the period was typically smaller than the 30-year average; the exception was the southern Wilkins Ice Shelf, which experienced negligible lowering between 2000 and 2008, when a series of large calving events began. Analyses of the continuous ERS/Envisat time series (to 81.5°) for 1992–2008 reveal a period of strong negative dhi/dt on most ice shelves between 1992 and 1995. Based on prior studies of regional atmospheric and oceanic conditions, we hypothesize that the observed elevation changes on Larsen C Ice Shelf are driven primarily by firn compaction while the western AP ice shelves are responding to changes in both surface mass balance and basal melt rates. Our time series also show that large changes in dhi/dt can occur on interannual time scales, reinforcing the importance of long time series altimetry to separate long-term trends associated with climate change from interannual to interdecadal natural variability.
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