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The ice sheets of Greenland and Antarctica are losing ice at accelerating rates, much of which is a response to oceanic forcing, especially of the floating ice shelves. Recent observations establish a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss. In Antarctica, most of these processes are reasonably well understood but have not been rigorously quantified. In Greenland, an understanding of the processes by which warmer ocean temperatures drive the observed retreat remains elusive. Experiments designed to identify the relevant processes are confounded by the logistical difficulties of instrumenting ice-choked fjords with actively calving glaciers. For both ice sheets, multiple challenges remain before the fully coupled ice-ocean-atmosphere models needed for rigorous sea-level projection are available.

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

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.

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]

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]

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]

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

Rapid retreat of Antarctic ice shelves in a warmer climate remains challenging to predict, contributing to uncertainties in projections of sea level rise. The collapse of the Larsen B Ice Shelf on the Antarctic Peninsula in 2002 was perhaps the most dramatic example of ice shelf retreat on record. Despite a general consensus that regional climate warming is associated with ice shelf collapse, knowledge of the precise details leading to and following collapse is limited. Here we examine a comprehensive set of satellite observations, modeling experiments, and climate reanalysis data to elucidate calving behavior, flow dynamics, and mechanical conditions of the northern Larsen Ice Shelf since the mid-1960s. Our analysis reveals that the Larsen B collapse in March 2002 was the last phase of a calving sequence that began in 1998, and was controlled by both atmospheric and oceanic anomalies that weakened the ice shelf structure over time. The collapse was preceded by a transition from infrequent calving of large tabular-icebergs and localized flow acceleration to frequent calving of small-icebergs and ice-shelf-wide acceleration. The intermittent pulses of the calving sequence were triggered by failure of the northern shear margin, most likely due to ocean-driven weakening by incision of sub-ice-shelf channels, and were further controlled by the location of effective buttressing sources and preexisting crevasses. The limited observational data indicate a similar process for the Larsen A ice shelf. These calving events corresponded with warm anomalies associated with a La Niña / positive Southern Annular Mode teleconnection pattern. The results suggest that warm climate anomalies control the occurrence of calving, while the extent and speed of calving are governed by ice shelf geometry and mechanical conditions, in particular, the sturdiness of the "weakest" shear margin. Sudden widespread flow acceleration and frequent small-iceberg calving may serve as quantifiable precursors for ice shelf destabilization. • A transition in calving and acceleration style preceded collapse of Larsen Ice shelf. • Destabilization of the ice shelves was controlled by the weakest shear margin(s). • A La Niña / positive Southern Annular Mode teleconnection pattern promotes calving. • Sturdiness of shear margins is critical in ice shelf vulnerability to climate change. [ABSTRACT FROM AUTHOR]

Abstract: The initial retreat of ice shelf grounding lines stabilized on seaward-sloping beds is influenced by the rheology of these beds, according to new model results. We apply a fully-coupled process model to investigate how the response of an ice stream to increased ocean temperature beneath its ice shelf depends on the assumed form of its basal flow law. For the same applied oceanic warming, the increase in grounding-line flux can be twice as great for an effectively-plastic bed as for a linear-viscous bed, suggesting that improved knowledge of the basal flow law of ice streams is necessary for predicting ice-sheet response to climatic forcing. [Copyright &y& Elsevier]

Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought. [ABSTRACT FROM AUTHOR]