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Glacier-bed characteristics that are poorly known and modeled are important in projected sea-level rise from ice-sheet changes under strong warming, especially in the Thwaites Glacier drainage of West Antarctica. Ocean warming may induce ice-shelf thinning or loss, or thinning of ice in estuarine zones, reducing backstress on grounded ice. Models indicate that, in response, more-nearly-plastic beds favor faster ice loss by causing larger flow acceleration, but more-nearly-viscous beds favor localized near-coastal thinning that could speed grounding-zone retreat into interior basins where marine-ice-sheet instability or cliff instability could develop and cause very rapid ice loss. Interpretation of available data indicates that the bed is spatially mosaicked, with both viscous and plastic regions. Flow against bedrock topography removes plastic lubricating tills, exposing bedrock that is eroded on up-glacier sides of obstacles to form moats with exposed bedrock tails extending downglacier adjacent to lee-side soft-till bedforms. Flow against topography also generates high-ice-pressure zones that prevent inflow of lubricating water over distances that scale with the obstacle size. Extending existing observations to sufficiently large regions, and developing models assimilating such data at the appropriate scale, present large, important research challenges that must be met to reliably project future forced sea-level rise. [ABSTRACT FROM AUTHOR]

Glacial hydraulic tremor (GHT) can be monitored to observe changes in location and distribution of water flow beneath glacial ice, allowing the spatiotemporal evolution of subglacial hydrology to be studied continuously and remotely. We use frequency-dependent polarization analysis (FDPA) to classify types of GHT and assess its spatio-temporal extent beneath Rhonegletscher, Switzerland, in a continuous seismic record through 2018 and 2019 at three ice-proximal bedrock-based seismometers. We determine the frequency bands composing the GHT and calculate back azimuth angles pointing to a previously known subglacial channel. We also inspect the relationship between GHT seismic power and water discharge from the glacier to compare daily and seasonal shifts with the observed GHT. We observed the seasonal shift from a distributed system to a channelized system, and our dataset allowed comparison of channel locations within and across seasons, with implications for sediment evacuation and bed erosion. The successful use of this method to assess GHT previously on Taku glacier (the methods of which this project follows) and now Rhonegletscher shows that existing ice-proximal passive seismic installations can be used to easily and continuously monitor subglacial hydraulic activity. [ABSTRACT FROM AUTHOR]

Glacier-bed characteristics that are poorly known and modeled are important in projected sea-level rise from ice-sheet changes under strong warming, especially in the Thwaites Glacier drainage of West Antarctica. Ocean warming may induce ice-shelf thinning or loss, or thinning of ice in estuarine zones, reducing backstress on grounded ice. Models indicate that, in response, more-nearly-plastic beds favor faster ice loss by causing larger flow acceleration, but more-nearly-viscous beds favor localized near-coastal thinning that could speed grounding-zone retreat into interior basins where marine-ice-sheet instability or cliff instability could develop and cause very rapid ice loss. Interpretation of available data indicates that the bed is spatially mosaicked, with both viscous and plastic regions. Flow against bedrock topography removes plastic lubricating tills, exposing bedrock that is eroded on up-glacier sides of obstacles to form moats with exposed bedrock tails extending downglacier adjacent to lee-side soft-till bedforms. Flow against topography also generates high-ice-pressure zones that prevent inflow of lubricating water over distances that scale with the obstacle size. Extending existing observations to sufficiently large regions, and developing models assimilating such data at the appropriate scale, present large, important research challenges that must be met to reliably project future forced sea-level rise. [ABSTRACT FROM AUTHOR]

Marine-terminating glaciers lose mass through melting and iceberg calving, and we find that meltwater drainage systems influence calving timing at Helheim Glacier, a tidewater glacier in East Greenland. Meltwater feeds a buoyant subglacial discharge plume at the terminus of Helheim Glacier, which rises along the glacial front and surfaces through the mélange. Here, we use high-resolution satellite and time-lapse imagery to observe the surface expression of this meltwater plume and how plume timing and location compare with that of calving and supraglacial meltwater pooling from 2011 to 2019. The plume consistently appeared at the central terminus even as the glacier advanced and retreated, fed by a well-established channelized drainage system with connections to supraglacial water. All full-thickness calving episodes, both tabular and non-tabular, were separated from the surfacing plume by either time or by space. We hypothesize that variability in subglacial hydrology and basal coupling drive this inverse relationship between subglacial discharge plumes and full-thickness calving. Surfacing plumes likely indicate a low-pressure subglacial drainage system and grounded terminus, while full-thickness calving occurrence reflects a terminus at or close to flotation. Our records of plume appearance and full-thickness calving therefore represent proxies for the grounding state of Helheim Glacier through time. [ABSTRACT FROM AUTHOR]

Recent seismic measurements from upper Thwaites Glacier indicate that the bed-type variability is closely related to the along-flow basal topography. In high-relief subglacial highlands, stoss sides of topographic highs have a relatively higher acoustic impedance ('hard' bed) with lower acoustic impedance ('soft' till) on lee sides. This pattern is similar to observations of many deglaciated terrains. Subglacial hydraulic-potential gradient and its divergence show a tendency for water to diverge over the stoss sides and converge into the lee sides. Convergence favors a thicker or more widespread water system, which can more efficiently decouple ice from the underlying till. Under such circumstances, till deformation does occur but, fluxes are relatively small. Till carried from the lee sides onto stoss sides of downstream bumps should couple to the ice more efficiently, increasing the ability for transport by till deformation. In turn, this suggests that steady-state till transport can be achieved if the stoss-side till layer is thin or discontinuous. In addition, the large basal shear stress generated in the highlands seems too high for a bed lubricated by a continuous although thin deforming till, suggesting till discontinuity, which would allow debris-laden ice to erode bedrock on stoss sides, supplying additional till for transport. [ABSTRACT FROM AUTHOR]

The article informs about polar regions outpacing the global average Antarctic temperatures. Topics discussed include threats to wildlife and traditional human livelihoods, and extreme weather at lower latitudes; mitigation efforts can slow or reduce warming; and international cooperation will be crucial to foreseeing and adapting to changes.

Recent seismic measurements from upper Thwaites Glacier indicate that the bed-type variability is closely related to the along-flow basal topography. In high-relief subglacial highlands, stoss sides of topographic highs have a relatively higher acoustic impedance ('hard' bed) with lower acoustic impedance ('soft' till) on lee sides. This pattern is similar to observations of many deglaciated terrains. Subglacial hydraulic-potential gradient and its divergence show a tendency for water to diverge over the stoss sides and converge into the lee sides. Convergence favors a thicker or more widespread water system, which can more efficiently decouple ice from the underlying till. Under such circumstances, till deformation does occur but, fluxes are relatively small. Till carried from the lee sides onto stoss sides of downstream bumps should couple to the ice more efficiently, increasing the ability for transport by till deformation. In turn, this suggests that steady-state till transport can be achieved if the stoss-side till layer is thin or discontinuous. In addition, the large basal shear stress generated in the highlands seems too high for a bed lubricated by a continuous although thin deforming till, suggesting till discontinuity, which would allow debris-laden ice to erode bedrock on stoss sides, supplying additional till for transport. [ABSTRACT FROM AUTHOR]

Large-ensemble perturbed-parameter forward ice-flow modeling can provide useful insights to uncertainties in inversions for basal drag or other ice-flow parameters. Inversion and data assimilation provide estimates of poorly known parameters that are essential for accurate prognostic modeling. Because ice flow depends on many such parameters with their associated uncertainties, which may interact in nonlinear ways, full uncertainty assessment for parameter estimates is challenging. With rising computational power, it is increasingly practicable to explore co-dependencies and sensitivities. Here, we use a well-characterized higher-order flowline model configured for a well-lubricated ("shelfy") ice stream to run large ensembles, perturbing the magnitude and spatial pattern of basal drag, basal topography, and input flux. We find for steady state that ice-stream velocity and thickness along the entire domain are especially correlated to drag at the downstream end, but with greater local correlation during transients. The modeled effects of basal topographic perturbations on velocity and ice thickness are primarily local. Perturbations of input ice fluxes interact with the others in interesting ways. These insights point to the value of inversions informed by concentrated observations during forced transients such as lake-drainage events, accumulation-rate fluctuations or ice-shelf losses, and to the care needed when interpreting local results of some inversions for basal-drag parameters. • Perturbed-parameter ensembles reveal ice-flow-model uncertainties and interactions. • Basal-friction perturbations at the downstream end especially affect ice-stream flow. • During transients, basal-friction perturbations influence local ice-stream flow more. • Basal-topography perturbations mainly affect ice-stream thickness locally. • Large uncertainties in input flux can obscure other ice-flow controls. [ABSTRACT FROM AUTHOR]

Observations at the West Antarctic Ice Sheet (WAIS) Divide site show that near-surface snow is strongly altered by weather-related processes such as strong winds and temperature fluctuations, producing features that are recognizable in the deep ice core. Prominent "glazed" surface crusts develop frequently at the site during summer seasons. Surface, snow pit, and ice core observations made in this study during summer field seasons from 2008-2009 to 2012-2013, supplemented by automated weather station (AWS) data with short- and longwave radiation sensors, revealed that such crusts formed during relatively low-wind, low-humidity, clear-sky periods with intense daytime sunshine. After formation, such glazed surfaces typically developed cracks in a polygonal pattern likely from thermal contraction at night. Cracking was commonest when several clear days occurred in succession and was generally followed by surface hoar growth; vapor escaping through the cracks during sunny days may have contributed to the high humidity that favored nighttime formation of surface hoar. Temperature and radiation observations show that daytime solar heating often warmed the near-surface snow above the air temperature, contributing to upward mass transfer, favoring crust formation from below, and then surface hoar formation. A simple surface energy calculation supports this observation. Subsequent examination of the WDC06A deep ice core revealed that crusts are preserved through the bubbly ice, and some occur in snow accumulated during winters, although not as commonly as in summertime deposits. Although no one has been on site to observe crust formation during winter, it may be favored by greater wintertime wind packing from stronger peak winds, high temperatures and steep temperature gradients from rapid midwinter warmings reaching as high as -15 °C, and perhaps longer intervals of surface stability. Time variations in crust occurrence in the core may provide paleoclimatic information, although additional studies are required. Discontinuity and cracking of crusts likely explain why crusts do not produce significant anomalies in other paleoclimatic records. [ABSTRACT FROM AUTHOR]

Thwaites Glacier (TG) plays an important role in future sea-level rise (SLR) contribution from the West Antarctic Ice Sheet. Recent observations show that TG is losing mass, and its grounding zone is retreating. Previous modeling has produced a wide range of results concerning whether, when, and how rapidly further retreat will occur under continued warming. These differences arise at least in part from ill-constrained processes, including friction from the bed, and future atmosphere and ocean forcing affecting ice-shelf and grounding-zone buttressing. Here, we apply the Ice Sheet and Sea-level System Model (ISSM) with a range of specifications of basal sliding behavior in response to varying ocean forcing. We find that basin-wide bed character strongly affects TG's response to sub-shelf melt by modulating how changes in driving stress are balanced by the bed as the glacier responds to external forcing. Resulting differences in dynamic thinning patterns alter modeled grounding-line retreat across Thwaites' catchment, affecting both modeled rates and magnitudes of SLR contribution from this critical sector of the ice sheet. Bed character introduces large uncertainties in projections of TG under equal external forcing, pointing to this as a crucial constraint needed in predictive models of West Antarctica. [ABSTRACT FROM AUTHOR]

This article discusses the development of advanced weather predicting and environmental forecasting techniques. The authors comment on the social impact of improved weather prediction, including earlier extreme weather warnings. They also examine the efforts of such organizations as the European Centre for Medium-Range Weather Forecasts and the National Oceanic and Atmospheric Administration's National Centers for Environmental Protection.

Basal channels, which form where buoyant plumes of ocean water and meltwater carve troughs upwards into ice-shelf bases, are widespread on Antarctic ice shelves. The formation of these features modulates ice-shelf basal melt by influencing the flow of buoyant plumes, and influences structural stability through concentration of strain and interactions with fractures. Because of these effects, and because basal channels can change rapidly, on timescales similar to those of ice-shelf evolution, constraining the impacts of basal channels on ice shelves is necessary for predicting future ice-shelf destabilization and retreat. We suggest that future research priorities should include constraining patterns and rates of basal channel change, determining mechanisms and detailed patterns of basal melt, and quantifying the influence that channel-related fractures have on ice-shelf stability. [ABSTRACT FROM AUTHOR]

Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence. [ABSTRACT FROM AUTHOR]

In the article, the authors discuss issues on climate change, particularly the expected mean sea level rise around the world due to temperature increase. Also cited are how to effectively manage coastal risks due to climate change, a study using a combination of ice-fracture processes and inferences from paleoclimatic data to calculate the Antarctic contribution to global sea level rise by 2100, and the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) on the issue.

Interactions between subglacial hydrology and the ocean make the existence of estuaries at the grounding zones of ice sheets likely. Here we present geophysical observations of an estuary at the downstream end of the hydrologic system that links the active subglacial lakes beneath Whillans Ice Stream to the ocean beneath the Ross Ice Shelf, Antarctica. This subglacial estuary consists of a hydropotential low upstream of the grounding zone, which is linked to the ocean by a hydropotential trough and a large subglacial channel. This subglacial channel, which is imaged using active source seismic methods, has an apparent width of 1 km and a maximum depth of 7 m. The hydropotential trough continues upstream of the grounding zone and results from an along-flow depression in surface elevations. Pressure differences along the trough axis are within a range that can be overcome by tidally induced processes, making the interaction of subglacial and ocean water likely. [ABSTRACT FROM AUTHOR]

The societal benefits of an educated citizenry may be lost if "customers" at tuition-driven universities demand less of what they pay for because they value a credential more than the education it represents. Insights from potential employers may help students see the value of education and demand their money's worth. [ABSTRACT FROM AUTHOR]

The article discusses research investigating the effect of oceanic forcing on the ice sheets in Greenland and Antarctica. The authors note that much of the ice loss in Greenland and Antarctica's ice sheets is in response to oceanic forcing, particularly the floating ice shelves. Topics include observations establishing a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss, scientists' lack of understanding of the processes by which warmer ocean temperatures drive the observed retreat in Greenland, and the difficulties of conducting experiments in Greenland and Antarctica which are designed to identify the relevant processes.

The article reviews aspects of ice-core science such as the reliability of ice-core results and key advances in the field. The history of ice-core science is provided starting with Ernst Sorge and the expedition led by Alfred Wegener at Eismitte in Greenland. Several indicators of climate and temperature are discussed including Borehole temperatures, thermal diffusion in firn, and number density of bubbles. The reliability of ice-core records is explained through ice-core measurements, samples of old air archived in ice, and ice retrieved from icebergs.

Abstract: Climate is continually changing on numerous time scales, driven by a range of factors. In general, longer-lived changes are somewhat larger, but much slower to occur, than shorter-lived changes. Processes linked with continental drift have affected atmospheric circulation, oceanic currents, and the composition of the atmosphere over tens of millions of years. A global cooling trend over the last 60 million years has altered conditions near sea level in the Arctic from ice-free year-round to completely ice covered. Variations in arctic insolation over tens of thousands of years in response to orbital forcing have caused regular cycles of warming and cooling that were roughly half the size of the continental-drift-linked changes. This “glacial-interglacial” cycling was amplified by the reduced greenhouse gases in colder times and by greater surface albedo from more-extensive ice cover. Glacial-interglacial cycling was punctuated by abrupt millennial oscillations, which near the North Atlantic were roughly half as large as the glacial-interglacial cycles, but which were much smaller Arctic-wide and beyond. The current interglaciation, the Holocene, has been influenced by brief cooling events from single volcanic eruptions, slower but longer lasting changes from random fluctuations in the frequency of volcanic eruptions, from weak solar variability, and perhaps by other classes of events. Human-forced climate changes appear similar in size and duration to the fastest natural changes of the past, but future changes may have no natural analog. [Copyright &y& Elsevier]

Abstract: Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic Ocean may become seasonally ice-free by the year 2040 or even earlier. Putting this into perspective requires information on the history of Arctic sea-ice conditions through the geologic past. This information can be provided by proxy records from the Arctic Ocean floor and from the surrounding coasts. Although existing records are far from complete, they indicate that sea ice became a feature of the Arctic by 47Ma, following a pronounced decline in atmospheric pCO2 after the Paleocene–Eocene Thermal Optimum, and consistently covered at least part of the Arctic Ocean for no less than the last 13–14 million years. Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth''s overall cooler climate. Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities. [Copyright &y& Elsevier]

Abstract: Paleoclimatic records show that the Greenland Ice Sheet consistently has lost mass in response to warming, and grown in response to cooling. Such changes have occurred even at times of slow or zero sea-level change, so changing sea level cannot have been the cause of at least some of the ice-sheet changes. In contrast, there are no documented major ice-sheet changes that occurred independent of temperature changes. Moreover, snowfall has increased when the climate warmed, but the ice sheet lost mass nonetheless; increased accumulation in the ice sheet''s center has not been sufficient to counteract increased melting and flow near the edges. Most documented forcings and ice-sheet responses spanned periods of several thousand years, but limited data also show rapid response to rapid forcings. In particular, regions near the ice margin have responded within decades. However, major changes of central regions of the ice sheet are thought to require centuries to millennia. The paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2°C or more than 7°C). Paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming, or changes may have been induced by factors other than temperature, without having been recorded. [Copyright &y& Elsevier]

Abstract: Arctic amplification, the observation that surface air temperature changes in the Arctic exceed those of the Northern Hemisphere as a whole, is a pervasive feature of climate models, and has recently emerged in observational data relative to the warming trend of the past century. The magnitude of Arctic amplification is an important, but poorly constrained variable necessary to estimate global average temperature change over the next century. Here we evaluate the mechanisms responsible for Arctic amplification on Quaternary timescales, and review evidence from four intervals in the past 3 Ma for which sufficient paleoclimate data and model simulations are available to estimate the magnitude of Arctic amplification under climate states both warmer and colder than present. Despite differences in forcings and feedbacks for these reconstructions compared to today, the Arctic temperature change consistently exceeds the Northern Hemisphere average by a factor of 3–4, suggesting that Arctic warming will continue to greatly exceed the global average over the coming century, with concomitant reductions in terrestrial ice masses and, consequently, an increasing rate of sea level rise. [Copyright &y& Elsevier]

Abstract: New model results show that a glacier advance caused by deposition of a rock avalanche on the ice will be followed by stagnation of the advanced ice lobe, producing distributed, hummocky deposits quite different from the single moraine ridges typically dated in paleoclimatic reconstructions. We obtain these results using a numerical flow-line glacier model with superimposed rock debris. In particular, our results imply that the single moraine ridge known as the Waiho Loop of the Franz Josef Glacier, New Zealand, is not the result of an advance triggered by a rock avalanche, else there would be a widespread, hummocky, glacial stagnation deposit associated with it. [Copyright &y& Elsevier]

Abstract: Stagnation of the terminal region of a glacier occurs in response to sufficiently large and rapid climatic warming, so the presence of stagnation deposits provides quantitative information on the climate change that forced retreat. Here we use a simple flow-line glacier model to investigate the relationship between stagnation, climate forcing and aspects of the glacier bed. For climatic warming greater than the threshold to cause stagnation, larger or faster warming events cause longer regions of a glacier to stagnate. Smaller or slower warming episodes, below the threshold for stagnation, cause retreat while active flow persists along the entire glacier length. The threshold for stagnation depends not only on the climatic forcing but also on many other aspects of the glacier, with stagnation favored by factors including a lower mean bed slope with greater roughness. Quantitative determination of the climatic forcing consistent with the occurrence or absence of stagnation deposits requires that these site-specific characteristics be incorporated in modeling. [Copyright &y& Elsevier]

Abstract: A simple shallow-ice flowline glacier model coupled to a model of sediment transport and deposition is used to simulate formation and preservation of moraines. The number, positions, and volume of moraines formed all are sensitive to the climate history assumed. We drive the model with the GISP2 central-Greenland temperature record, and with reduced-millennial-amplitude versions of that record, to test the hypothesis that the Younger Dryas and other millennial oscillations were primarily wintertime events and thus had less influence on glacier behavior than did the Last Glacial Maximum with its strong summertime as well as wintertime signal. We find that forcing the model by GISP2, with dampened strength of the millennial-scale signal, provides modeled moraine configurations that reflect observed moraine records in the Northern Hemisphere. [Copyright &y& Elsevier]

Abstract: The great ice sheets covering Antarctica and Greenland were, traditionally, believed to take thousands of years to respond to external forcing. Recent observations suggest, however, that major changes in the dynamics of parts of the ice sheets are taking place over timescales of years. These changes were not predicted by numerical models, and the underlying cause(s) remains uncertain. It has been suggested that regional oceanic and/or atmospheric warming are responsible but separating the influence and importance of these two forcings has not been possible. In most cases, the role of atmospheric versus oceanic control remains uncertain. Here, we review the observations of rapid change and discuss the possible mechanisms, in the light of advances in numerical modelling and our understanding of the processes that may be responsible. [Copyright &y& Elsevier]

Linked, abrupt changes of North Atlantic deep water formation, North Atlantic sea ice extent, and widespread climate occurred repeatedly during the last ice age cycle and beyond in response to changing freshwater fluxes and perhaps other causes. This paradigm, developed and championed especially by W.S. Broecker, has repeatedly proven to be successfully predictive as well as explanatory with high confidence. Much work remains to fully understand what happened and to assess possible implications for the future, but the foundations for this work are remarkably solid. [ABSTRACT FROM AUTHOR]

Sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding-line retreat in response to a rise in relative sea level of at least several meters. Recent Antarctic changes thus cannot be attributed to sea-level rise, strengthening earlier interpretations that warming has driven ice-sheet mass loss. Large sea-level rise, such as the ≈100-meter rise at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to have synchronized the behavior of ice sheets in the past. [ABSTRACT FROM AUTHOR]

Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level ∼70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered. [ABSTRACT FROM AUTHOR]

Abstract: Ice-sheet modeling to understand past changes and project future ones remains limited by uncertainties in key parameters as well as by shortage of computational resources. A fast two-dimensional dynamic/thermodynamic flowline ice-sheet model has been developed and benchmarked to complement three-dimensional models by allowing a more-thorough exploration of parameter space. A nonlinear Glen flow law with an exponent equal to 3 is assumed for ice. A diffusion formulation of the continuity equation for mass balance of ice sheets is employed. The ice-sheet model is coupled to an elastic lithosphere/relaxed asthenosphere isostatic bedrock model. Snow and superimposed ice thicknesses are determined using advective continuity equations. These thicknesses are then used in the parameterization of the surface accumulation and ablation rates under specified meteorological conditions constrained by ice- and ocean-core data. Heat-flow continuity is maintained by the time-dependent advection/diffusion equation in the ice and time-dependent diffusion equation in the underlying rock. We conduct the first benchmarking of which we are aware of a 2-D model against the European Ice-Sheet Modeling Initiative''s (EISMINT) Level 2 and Level 3 intercomparison “Greenland” experiments. Appropriate behavior is simulated, with deviations from results of a three-dimensional model that are fully explainable as arising only from the change in dimensionality. Basal temperatures are quite accurate when compared to data and results from 3-D models. [Copyright &y& Elsevier]

Abstract: A case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the North Atlantic region during the last glaciation and into the Holocene. The primary evidence comes from mismatches between mean annual temperatures from Greenland ice cores in comparison with snowline changes in East Greenland, northern Europe, and North America. The most likely explanation is a shutdown (or reduction in strength) of the conveyor. This allows the spread of winter sea ice across the North Atlantic, thus causing the northern region to experience much colder winters. Because they mimic the Greenland temperature rather than the snowline signal, changes in the Atlantic Intertropical Convergence Zone and the Asian monsoon may also share a winter linkage with Greenland. Thus the paleoclimate record is consistent with the notion that a huge continental sector of the Northern Hemisphere, stretching from Greenland to Asia, was close to an extreme winter threshold during much of the last glaciation. Winter climate crossed this threshold repeatedly, with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics. [Copyright &y& Elsevier]

Abstract: A prominent, abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern-hemisphere regions especially in wintertime, in response to a very large outburst flood that freshened the North Atlantic. Changes were much larger than typical climate variability before and after the event, with anomalies up to many degrees contributing to major displacement of vegetative patterns. This “8k” event provides a clear case of cause and effect in the paleoclimatic realm, and so offers an excellent opportunity for model testing. The response to North Atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following North Atlantic freshening, and so greatly strengthens the case that those older events also reflect North Atlantic changes. The North Atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming-caused ice-melt and hydrologic-cycle intensification at high latitudes lead to major changes in North Atlantic circulation. Few model experiments have directly addressed the 8k event, and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies, so much work remains to be done. [Copyright &y& Elsevier]

Rapid flow of ice streams is caused either by great thickness, or by effective basal lubrication especially from deforming tills. Competing thermal processes act to stabilize and to destabilize the well-lubricated ice streams, and may contribute to their observed short-term variability yet long-term persistence. Increasing evidence indicates that ice streams are subject to speed-up in response to warming, through thinning or loss of ice shelves, and possibly in response to meltwater penetration to ice-stream beds. To cite this article: R.B. Alley et al., C. R. Physique 5 (2004). [Copyright &y& Elsevier]

Thermomechanical flowline simulations indicate that the Siple Coast ice streams of West Antarctica have experienced only small deglacial thickness changes, are thinning more rapidly than their beds are rising isostatically, and can continue to retreat. Thickness changes of O(100)m are modelled at the modern grounding line through the last glacial cycle. The accumulation-rate increase accompanying warming out of the last glacial maximum (LGM) leads to a maximum simulated thickness change at the modern grounding line approximately 8 ka.Idealized isostatic simulations support coupling the ice-sheet model to an underlying elastic-lithosphere and relaxed-asthenosphere bedrock model. Dynamic interactions between ice and bedrock over the last glacial cycle indicate that isostatic rebound is raising the ice sheet at the modern grounding line faster than the rising sea level is submerging it. While, in and of itself, this could potentially lead to a grounding-line re-advance, ice flow is modelled to respond to recent changes in temperature, accumulation rate, and basal processes more rapidly than it does to bedrock-elevation and/or sea-level fluctuations. Previous results based on thermal controls on ice-stream behavior [Parizek et al., 2002: Geophysical Research Letters 29 (2002); Parizek et al., 2003: Annals of Glaciology 36 (2003) 251] support the view that thinning of the ice streams at the retreating grounding line will likely continue. While results indicate an additional few tens of meters of rebound remaining, land- and air-based observations will help constrain this magnitude with potential implications for uncovering past ice-loading history and future ice-sheet stability. [Copyright &y& Elsevier]

The Greenland ice sheet is likely to make a faster contribution to sea-level rise in a warming world than previously believed, based on numerical modelling using a parameterization of recent results showing surface-meltwater lubrication of ice flow. Zwally et al. (Science 297(557) (2002) 218) documented correlation between increased ice velocity and increased surface melt (as parameterized by positive degree days). They argued that surface water is piped directly to the bed with little delay, causing increased basal-water pressures and basal-sliding velocities, an effect not included in recent Greenland ice-sheet models known to the authors.Using the Pennsylvania State University/University of Chicago thermomechanical flowline model, numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the Zwally et al. data under three different global warming scenarios (2×CO2, 4×CO2, and 8×CO2). Comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates, additional thinning and retreat of the margin, and a reduction in ice volume leading to an increased contribution to global sea-level rise. [Copyright &y& Elsevier]

Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important sources of information on many biogeochemical cycles involving carbon, nitrogen, and oxygen. Here, we present a 106,000-year record of atmospheric nitrous oxide (N[sub2]O) along with corresponding isotopic records spanning the last 30,000 years, which together suggest minimal changes in the ratio of marine to terrestrial N[sub2]O production. During the last glacial termination, both marine and oceanic N[sub2]O emissions increased by 40 ± 8%. We speculate that our records do not support those hypotheses that invoke enhanced export production to explain low carbon dioxide values during glacial periods. [ABSTRACT FROM AUTHOR]

Automatic weather stations (AWSs) currently provide the only year-round, continuous direct measurements of near-surface weather on the West Antarctic ice sheet away from the coastal manned stations. Improved interpretation of the ever-growing body of ice-core-based paleoclimate records from this region requires a deeper understanding of Antarctic meteorology. As the spatial coverage of the AWS network has expanded year to year, so has the meteorological database. Unfortunately, many of the records are relatively short (less than 10 yr) and/or incomplete (to varying degrees) due to the vagaries of the harsh environment. Climate downscaling work in temperate latitudes suggests that it is possible to use GCM-scale meteorological datasets (e.g., ECMWF reanalysis products) to address these problems in the AWS record and create a uniform and complete database of West Antarctic surface meteorology (at AWS sites). Such records are highly relevant to the improved interpretation of the expanding library of snow-pit and ice-core datasets. Artificial neural network (ANN) techniques are used to predict 6-hourly AWS surface data (temperature, pressure) using large-scale features of the atmosphere (e.g., 500-mb geopotential height) from a region around the AWS. ANNs are trained with a calendar year of observed AWS data (possibly incomplete) and corresponding GCM-scale data. This methodology is sufficient both for high quality predictions within the training set and for predictions outside the training set that are at least comparable to the state of the art. For example, the results presented herein for temperature prediction are approximately equal to those from a satellite-based methodology but with no exposure to problems from surface melt events or sensor changes. Similarly, the significant biases seen in ECMWF surface temperatures are absent from the predictions here, resulting in an rms error that is half as large with respect to the original AWS observations. These... [ABSTRACT FROM AUTHOR]

Re-analysis of existing data sets shows that they are fully consistent with northern-insolation control of Ice-Age cycles, and do not provide strong support for alternate models invoking tropical or southern controls. The coldest time of the last 100 ka in central Greenland was nearly synchronous with the minimum in summer insolation about 24 ka BP (thousand years before 1950). Approximately one-third of the total Termination-I warming since then was achieved over the almost 10 ka before the abrupt warming into the Bolling, which some workers have interpreted as the start of the warming. Southern warming as recorded in the Byrd Station, Antarctica ice-isotopic record lagged northern warming. CO2 rise was roughly synchronous with southern warming, methane rise may have been delayed a little as was sea-level rise, and the change in the isotopic composition of atmospheric oxygen was delayed significantly. The data for the extrema just before Termination II are consistent with occurrence of the same phasing as for Termination I, but with somewhat different progression of the two terminations once initiated, perhaps because of differences in the forcing. [Copyright &y& Elsevier]

Focuses on ice cores showing records of climate changes. Use of snow layers to determine age; Correlation technique used for dating; Ice cores as local paleothermometers; Ice-core records from Greenland; Implications of the events.

Orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming. Ice-albedo, sea-level, and greenhouse-gas feedbacks, together with tropical warming from weakening winds in response to polar amplification of warming, caused regional-to-global (near-) synchronization of deglaciation. Effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly. Ice-sheet–linked changes in freshwater delivery to the North Atlantic, and possibly free oscillations in the climate system, forced millennial climate oscillations associated with changes in North Atlantic deep water (NADW) flow. The North Atlantic typically operates in one of three modes: modern, glacial, and Heinrich. Deglaciation occurred from a glacial-mode ocean that, in comparison to modern, had shallower depth of penetration of NADW formed further south, causing strong northern cooling and the widespread cold, dry, and windy conditions associated with the glacial maximum and the cold phases of the millennial Dansgaard-Oeschger oscillations. The glacial mode was punctuated by meltwater-forced Heinrich conditions that caused only small additional cooling at high northern latitudes, but greatly reduced the formation of NADW and triggered an oceanic “seesaw” that warmed some high-latitude southern regions centered in the South Atlantic. [ABSTRACT FROM AUTHOR]

Focuses on a study which examined the influence of Northern Hemisphere ice sheets on the global climate system. Ice-sheet dynamics; Details on middle Pleistocene transition; Ice sheets at the last glacial maximum.

Presents letters to the editor about global warming in response to Raphael Kazmann's article in the July 1990 issue on page 13.

Studies the movement of ice streams in West Antarctica. Detection of sticky spots; Raised regions in the surface leading to sticky spots in the bed; Mechanisms causing sticky spots on the bed.

The grounding zone of Whillans Ice Stream, West Antarctica, exhibits an abrupt transition in basal properties from the grounded ice to the ocean cavity over distances of less than 0.5–1 km. Active-source seismic methods reveal the downglacier-most grounded portion of the ice stream is underlain by a relatively stiff substrate (relatively high shear wave velocities of 1100±430 ms-1) compared to the deformable till found elsewhere beneath the ice stream. Changes in basal reflectivity in our study area cannot be explained by the stage of the tide. Several kilometres upstream of the grounding zone, layers of subglacial water are detected, as are regions that appear to be water layers but are less than the thickness resolvable by our technique. The presence of stiff subglacial sediment and thin water layers upstream of the grounding zone supports previous studies that have proposed the dewatering of sediment within the grounding zone and the trapping of subglacial water upstream of the ocean cavity. The setting enables calibration of our methodology using returns from the floating ice shelf. This allows a comparison of different techniques used to estimate the sizes of the seismic sources, a constraint essential for the accurate recovery of subglacial properties. We find a strong correlation (coefficient of determination=0.46) between our calibrated method and a commonly used multiple-bounce method, but our results also highlight the incomplete knowledge of other factors affecting the amplitude of seismic sources and reflections in the cryosphere. [ABSTRACT FROM AUTHOR]