M. R. van den Broeke

One-to-one coupling of glacial climate variability in Greenland and Antarctica.

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth’s climate dynamics. For the last glacial period, ice core studies have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard–Oeschger events in Greenland through the Atlantic meridional overturning circulation. It has been unclear, however, whether the shorter Dansgaard–Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard–Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

Antarctic ice-sheet loss driven by basal melting of ice shelves

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.

Large and rapid melt-induced velocity changes in the ablation zone of the Greenland Ice Sheet.

Continuous Global Positioning System observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the Greenland Ice Sheet. Within days, ice velocity reacts to increased meltwater production and increases by a factor of 4. Such a response is much stronger and much faster than previously reported. Over a longer period of 17 years, annual ice velocities have decreased slightly, which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years. The positive-feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades.

Calving fluxes and basal melt rates of Antarctic ice shelves.

Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 gigatonnes per year. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of −1,454 ± 174 gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering and enhanced discharge. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.

A Review of Antarctic Surface Snow Isotopic Composition: Observations, Atmospheric Circulation, and Isotopic Modeling*

A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

A new, high-resolution surface mass balance map of Antarctica (1979–2010) based on regional atmospheric climate modeling

A new, high resolution (27 km) surface mass balance (SMB) map of the Antarctic ice sheet is presented, based on output of a regional atmospheric climate model that includes snowdrift physics and is forced by the most recent reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-Interim (1979–2010). The SMB map confirms high accumulation zones in the western Antarctic Peninsula (>1500 mm y!1) and coastal West Antarctica (>1000 mm y!1), and shows low SMB values in large parts of the interior ice sheet (

The role of albedo and accumulation in the 2010 melting record in Greenland

Analyses of remote sensing data, surface observations and output from a regional atmosphere model point to new records in 2010 for surface melt and albedo, runoff, the number of days when bare ice is exposed and surface mass balance of the Greenland ice sheet, especially over its west and southwest regions. Early melt onset in spring, triggered by above-normal near-surface air temperatures, contributed to accelerated snowpack metamorphism and premature bare ice exposure, rapidly reducing the surface albedo. Warm conditions persisted through summer, with the positive albedo feedback mechanism being a major contributor to large negative surface mass balance anomalies. Summer snowfall was below average. This helped to maintain low albedo through the 2010 melting season, which also lasted longer than usual.

Retreating alpine glaciers: increased melt rates due to accumulation of dust (Vadret da Morteratsch, Switzerland)

The automatic weather station (AWS) on the snout of the Vadret da Morteratsch, Switzerland, has delivered a unique 12 year meteorological dataset from the ablation zone of a temperate glacier. This dataset can be used to study multi-annual trends in the character of the surface energy budget. Since 2003 there has been a substantial darkening of the glacier tongue due to the accumulation of mineral and biogenic dust. The typical surface albedo in summer has dropped from 0.32 to 0.15. We have analysed the implications of the lowered albedo for the energy balance and the annual ablation. For the 4 year period 2003-06, the decreased albedo caused an additional removal of about 3.5 m of ice. Calculations with an energy-balance model show that the same increase in ablation is obtained by keeping the ice albedo fixed to 0.32 and increasing the air temperature by 1.7 K. Our analysis confirms that for retreating glaciers the deposition of dust from exposed side moraines on the glacier surface constitutes an important feedback mechanism. The mineral dust stimulates the growth of algae, lowers the surface albedo, enhances the melt rates, and thereby facilitates the further retreat of the glacier snout.

Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data

We use Gravity Recovery and Climate Experiment (GRACE) monthly gravity fields to determine the regional acceleration in ice mass loss in Greenland and Antarctica for 2003-2013. We find that the total mass loss is controlled by only a few regions. In Greenland, the southeast and northwest generate 70% of the loss (280 ± 58 Gt/yr) mostly from ice dynamics, the southwest accounts for 54% of the total acceleration in loss (25.4 ± 1.2 Gt/yr 2 ) from a decrease in surface mass balance (SMB), followed by the northwest (34%), and we find no significant acceleration in the northeast. In Antarctica, the Amundsen Sea (AS) sector and the Antarctic Peninsula account for 64% and 17%, respectively, of the total loss (180 ± 10 Gt/yr) mainly from ice dynamics. The AS sector contributes most of the acceleration in loss (11 ± 4 Gt/yr 2 ), and Queen Maud Land, East Antarctica, is the only sector with a significant mass gain due to a local increase in SMB (63 ± 5 Gt/yr).

Dynamic thinning of glaciers on the Southern Antarctic Peninsula

Increasingly rapid ice sheet melting Glaciers on the Southern Antarctic Peninsula have begun losing mass at a rapid and accelerating rate. Wouters et al. documented the dramatic thinning of the land-based ice, which began in 2009, using satellite altimetry and gravity observations. The melting and weakening of ice shelves reduce their buttressing effect, allowing the glaciers to flow more quickly to the sea. Science, this issue p. 899 Glaciers on the Southern Antarctic Peninsula are disappearing at increasing rates. Growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice, especially if it is resting on bedrock below sea level. Much of the Southern Antarctic Peninsula satisfies this condition and also possesses a bed slope that deepens inland. Such ice sheet geometry is potentially unstable. We use satellite altimetry and gravity observations to show that a major portion of the region has, since 2009, destabilized. Ice mass loss of the marine-terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of –56 ± 8 gigatons per year, constituting a major fraction of Antarctica’s contribution to rising sea level. The widespread, simultaneous nature of the acceleration, in the absence of a persistent atmospheric forcing, points to an oceanic driving mechanism.