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The stability of the East Antarctic Ice Sheet and its contribution to past sea-level rise are not well defined; in this paper, airborne geophysical data and ice-sheet models are used to show that the Totten Glacier has undergone large-scale retreats and advances, and that it could contribute several metres of sea-level rise in a fully retreated scenario. East Antarctic Ice Sheet stability Much of West Antarctica is known to be susceptible to rapid ice sheet retreat, but the stability of the East Antarctic Ice Sheet — the largest potential contributor to sea-level change — is less understood. Alan Aitken et al . use a suite of remote sensing data to show that Totten Glacier in East Antarctica eroded deep swaths in two distinct sub-basins. This suggests that the erosive portion of the glacier tends to switch between its present location and one that is hundreds of kilometres farther inland. Although this study cannot establish the timing of past or future retreats, the results do establish that this sector of the East Antarctic Ice Sheet could contribute several metres of sea-level rise in a fully retreated scenario driven by atmospheric and oceanic warming. Climate variations cause ice sheets to retreat and advance, raising or lowering sea level by metres to decametres. The basic relationship is unambiguous, but the timing, magnitude and sources of sea-level change remain unclear; in particular, the contribution of the East Antarctic Ice Sheet (EAIS) is ill defined, restricting our appreciation of potential future change. Several lines of evidence suggest possible collapse of the Totten Glacier into interior basins during past warm periods, most notably the Pliocene epoch 1 , 2 , 3 , 4 , causing several metres of sea-level rise. However, the structure and long-term evolution of the ice sheet in this region have been understood insufficiently to constrain past ice-sheet extents. Here we show that deep ice-sheet erosion—enough to expose basement rocks—has occurred in two regions: the head of the Totten Glacier, within 150 kilometres of today’s grounding line; and deep within the Sabrina Subglacial Basin, 350–550 kilometres from this grounding line. Our results, based on ICECAP aerogeophysical data, demarcate the marginal zones of two distinct quasi-stable EAIS configurations, corresponding to the ‘modern-scale’ ice sheet (with a marginal zone near the present ice-sheet margin) and the retreated ice sheet (with the marginal zone located far inland). The transitional region of 200–250 kilometres in width is less eroded, suggesting shorter-lived exposure to eroding conditions during repeated retreat–advance events, which are probably driven by ocean-forced instabilities. Representative ice-sheet models indicate that the global sea-level increase resulting from retreat in this sector can be up to 0.9 metres in the modern-scale configuration, and exceeds 2 metres in the retreated configuration.

We present two narratives on the future of Antarctica and the Southern Ocean, from the perspective of an observer looking back from 2070. In the first scenario, greenhouse gas emissions remained unchecked, the climate continued to warm, and the policy response was ineffective; this had large ramifications in Antarctica and the Southern Ocean, with worldwide impacts. In the second scenario, ambitious action was taken to limit greenhouse gas emissions and to establish policies that reduced anthropogenic pressure on the environment, slowing the rate of change in Antarctica. Choices made in the next decade will determine what trajectory is realized. The future of Antarctica and the Southern Ocean by 2070 is described under two scenarios, one in which action is taken to limit greenhouse gas emissions, and one in which no action is taken.

Radar inference of the bulk properties of glacier beds, most notably identifying basal melting, is, in general, derived from the basal reflection coefficient. On the scale of an ice sheet, unambiguous determination of basal reflection is primarily limited by uncertainty in the englacial attenuation of the radio wave, which is an Arrhenius function of temperature. Existing bed-returned power algorithms for deriving attenuation assume that the attenuation rate is regionally constant, which is not feasible at an ice-sheet-wide scale. Here we introduce a new semi-empirical framework for deriving englacial attenuation, and, to demonstrate its efficacy, we apply it to the Greenland Ice Sheet. A central feature is the use of a prior Arrhenius temperature model to estimate the spatial variation in englacial attenuation as a first guess input for the radar algorithm. We demonstrate regions of solution convergence for two input temperature fields and for independently analysed field campaigns. The coverage achieved is a trade-off with uncertainty and we propose that the algorithm can be \"tuned\" for discrimination of basal melt (attenuation loss uncertainty  ∼ 5 dB). This is supported by our physically realistic ( ∼ 20 dB) range for the basal reflection coefficient. Finally, we show that the attenuation solution can be used to predict the temperature bias of thermomechanical ice sheet models and is in agreement with known model temperature biases at the Dye 3 ice core.

We present two narratives on the future of Antarctica and the Southern Ocean, from the perspective of an observer looking back from 2070. In the first scenario, greenhouse gas emissions remained unchecked, the climate continued to warm, and the policy response was ineffective; this had large ramifications in Antarctica and the Southern Ocean, with worldwide impacts. In the second scenario, ambitious action was taken to limit greenhouse gas emissions and to establish policies that reduced anthropogenic pressure on the environment, slowing the rate of change in Antarctica. Choices made in the next decade will determine what trajectory is realized.

Issue Title: Life in Extreme Environments, Part II Antarctic subglacial lakes have, over the past few years, been hypothesised to house unique forms of life and hold detailed sedimentary records of past climate change. Testing this hypothesis requires in situ examinations. The direct measurement of subglacial lakes has been considered ever since the largest and best-known lake, named Lake Vostok, was identified as having a deep water-column. The Subglacial Antarctic Lake Environments (SALE) programme, set up by the Scientific Committee on Antarctic Research (SCAR) to oversee subglacial lakes research, state that prior exploration of smaller lakes would be a \"prudent way forward\". Over 145 subglacial lakes are known to exist in Antarctica, but one lake in West Antarctica, officially named Ellsworth Subglacial Lake (referred to hereafter as Lake Ellsworth), stands out as a candidate for early exploration. A consortium of over 20 scientists from seven countries and 14 institutions has been assembled to plan the exploration of Lake Ellsworth. An eight-year programme is envisaged: 3 years for a geophysical survey, 2 years for equipment development and testing, 1 year for field planning and operation, and 2 years for sample analysis and data interpretation. The science experiment is simple in concept but complex in execution. Lake Ellsworth will be accessed using hot water drilling. Once lake access is achieved, a probe will be lowered down the borehole and into the lake. The probe will contain a series of instruments to measure biological, chemical and physical characteristics of the lake water and sediments, and will utilise a tether to the ice surface through which power, communication and data will be transmitted. The probe will pass through the water column to the lake floor. The probe will then be pulled up and out of the lake, measuring its environment continually as this is done. Once at the ice surface, any water samples collected will be taken from the probe for laboratory analysis (to take place over subsequent years). The duration of the science mission, from deployment of the probe to its retrieval, is likely to take between 24 and 36 h. Measurements to be taken by the probe will provide data about the following: depth, pressure, conductivity and temperature; pH levels; biomolecules (using life marker chips); anions (using a chemical analyzer); visualisation of the environment (using cameras and light sources); dissolved gases (using chromatography); and morphology of the lake floor and sediment structures (using sonar). After the probe has been retrieved, a sediment corer may be dropped into the lake to recover material from the lake floor. Finally, if time permits, a thermistor string may be left in the lake water to take time-dependent measurements of the lake's water column over subsequent years. Given that the comprehensive geophysical survey of the lake will take place in two seasons during 2007-2009, a two-year instrument and logistic development phase from 2008 (after the lake's bathymetry has been assessed) makes it possible that the exploration of Lake Ellsworth could take place at the beginning of the next decade.[PUBLICATION ABSTRACT]

Airborne radio-echo sounding (RES) data display layering within the Antarctic ice sheet. At ice depths below 1000m these layers are caused by horizons of ice with relatively high acidity which were originally deposited on the ice surface after large volcanic events. Layering which is less than 1000 m from the ice surface can also be due to variation in ice density. Theoretically, therefore, internal RES layering below 1000 m should represent isochronous planes. This theory is upheld under examination of existing RES data where internal layers have been observed to follow the rules of superposition. For example, RES layers are deposited as discrete bands, fold and fault in a manner analogous to geological features, never cross over each other and, in an undisturbed deposit, have a depth-age relationship which means that the oldest layers are located at the lowest level. Moreover, the location of internal layering is independent of radiowave receiver altitude, the frequency of the radiowave does not affect layer depth, and the pulse width of the e/m wave does not affect identification of layers. Thus, RES reflects actual dielectric layering within the ice sheet. Glaciologists use RES layering for a number of reasons, including: (1) correlating ice cores; (2) as boundary conditions for numerical models to help determine the direction of ice flow; and (3) as a means of identifying the three-dimensional ice-sheet geometry and architecture.