Changes in atmospheric circulation over the past five decades have enhanced the wind-driven inflow of warm ocean water onto the Antarctic continental shelf, where it melts ice shelves from below 1-3 . Atmospheric circulation changes have also caused rapid warming 4 over the West Antarctic Ice Sheet, and contributed to declining sea-ice cover in the adjacent Amundsen-Bellingshausen seas 5 . It is unknown whether these changes are part of a longer-term trend. Here, we use waterisotope (δ 18 O) data from an array of ice-core records to place recent West Antarctic climate changes in the context of the past two millennia. We find that the δ 18 O of West Antarctic precipitation has increased significantly in the past 50 years, in parallel with the trend in temperature, and was probably more elevated during the 1990s than at any other time during the past 200 years. However, δ 18 O anomalies comparable to those of recent decades occur about 1% of the time over the past 2,000 years. General circulation model simulations suggest that recent trends in δ 18 O and climate in West Antarctica cannot be distinguished from decadal variability that originates in the tropics. We conclude that the uncertain trajectory of tropical climate variability represents a significant source of uncertainty in projections of West Antarctic climate and ice-sheet change.The West Antarctic Ice Sheet (WAIS), which is grounded largely below sea level, is potentially unstable. Mass loss from the WAIS is contributing to present sea-level rise, owing to the widespread thinning of ice shelves and the acceleration of the large outlet glaciers that drain the ice sheet into the ocean 1 . Contemporaneous with the loss of mass from the WAIS, air temperatures over the WAIS have increased significantly in the past 50 years 4,6,7 .Climate and ice-sheet changes in West Antarctica are closely linked with one another by changes in regional atmospheric circulation 8 . Observations beneath the floating ice shelf of Pine Island Glacier, a major drainage system for the flow of the WAIS into the Amundsen Sea, show that the primary cause of ice-shelf thinning is the presence of warm Circumpolar Deep Water on
In recent years, ground-penetrating radar (GPR) has been increasingly used for characterization of subglacial and englacial environments at polythermal glaciers. The geophysical method is able to exploit the dielectric difference between water, air, sediment and ice, allowing delineation of subsurface hydrological, thermal and structural conditions. More recent GPR research has endeavoured to examine temporal change in glaciers, in particular the distribution of the cold ice zone at polythermal glaciers. However, the exact nature of temporal change that can be identified using GPR has not been fully examined. This research presents the results of three GPR surveys conducted over the course of a summer ablation season at a polythermal glacier in the Canadian Arctic. A total of approximately 30 km of GPR profiles were collected in 2002 repeatedly covering the lower 2 km of Stagnation Glacier, Bylot Island (72?58? N 78?22? W). Comparison between profiles indicated changes in the radar signature, including increased noise, appearance and disappearance of englacial reflections, and signal attenuation in the latter survey. Further, an area of chaotic returns in up-glacier locations, which was interpreted to be a wet temperate ice zone, showed marked recession over the course of the ablation season. Combining all the temporal changes that were detected by GPR, results indicate that a polythermal glacier may exhibit strongly seasonal changes in hydrological and thermal characteristics throughout the ice body, including the drainage of 17 000 m3 of temporarily stored intra-glacial meltwater. It is also proposed that the liquid water content in the temperate ice zone of polythermal glaciers can be described as a fraction of a specific retention capacity. Copyright ? 2006 John Wiley & Sons, Ltd.Peer reviewe
Recent research has identified differences in processes contributing to suspended sediment concentration (SSC) dynamics in proglacial streams between High Arctic and alpine catchments, but does not examine processes explicitly linked to the periglacial environment. Three glacierized basins were studied: Austre Br?ggerbreen and Midre Lov?nbreen, Svalbard (79?N, 12?E) and Glacier B28, unofficially named Stagnation Glacier, Bylot Island, Nunavut (73?N, 78?W). SSC variations were modelled from continuous turbidity, discharge and meteorological data throughout the summer months. Three statistical tools were utilized: principal component analysis, change-point analysis and multivariate regression. These are shown to be effective in identifying subperiods of distinctive geocryological and glaciofluvial characteristics. Multivariate regression for the subseasons included autoregressive integrated moving-average modelling, and showed that SSC variations were related not only to discharge variability, but also to fluctuations in energy fluxes. The results are interpreted in terms of spatio-temporal changes in sediment mobilization and supply associated with changes in the relative importance of fluvial, glacial and periglacial processes. This evidence supports the notion of important linkages between glacial, fluvial and periglacial systems, but exemplifies distinct variability between High Arctic glaciers. Copyright ? 2005 John Wiley & Sons, Ltd.Peer reviewe
[1] Observations of snow accumulation rates from five new firn cores show a negative trend that is statistically significant over the past several decades across the central West Antarctic ice sheet (WAIS). A negative temporal trend in accumulation rates is unexpected in light of rising surface temperatures as well as model simulations predicting higher accumulation rates for the region. Both the magnitude of the mean accumulation rates and the range of interannual variability observed in the new records compare favorably to older records collected from a broad area of the WAIS, suggesting that the new data may serve as a regional proxy for recent temporal trends in West Antarctic accumulation rates. The observed negative trend in accumulation is likely the result of a shift in low-pressure systems over the Amundsen Sea region, dominated by changes in the austral fall season. Regional-scale climate models and reanalysis data do not capture the negative trend in accumulation rate observed in these firn cores. Nevertheless the models and reanalyses agree well in both accumulation-rate means and interannual variability, with no single model or dataset standing out as significantly more skilled at capturing the observed magnitude of and trend in accumulation rates in this region of the WAIS.
In high latitudes, recent research has demonstrated that both thermo-erosion and temperature dependence influence sediment release into fluvial systems. An analysis of proglacial suspended sediment concentration (SSC) dynamics is presented for three glacierized basins: cold-based Austre Br?ggerbreen (Svalbard), polythermal Midre Lov?nbreen (Svalbard) and polythermal Glacier B28 (Bylot Island). The temporal variation in processes dominating SSC patterns is assessed using stepwise multivariate regression following the subdivision of the time series. Partitioning of the time series is achieved through principal components and change point analyses. The regression models use discharge and surrogate predictor variables to model SSC, while improvements are made by using air temperature and radiation terms as independent variables. Comparisons are drawn between two sets of models with contrasting subseasonal division. By interpretation of the regression model characteristics, temporal changes in physical processes are implied over the course of the time periods. Numerical analyses suggest there is a trend for changes between fluvial, glacial and periglacial factors forcing responses in SSC. Therefore, it is conjectured that glaciofluvial sediment transfer at high latitudes is influenced by periglacial processes and conditions. This has implications for the predictions of fluvial sediment loads in a changing environment, and the use of sedimentary records for environmental reconstruction.Peer reviewe
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