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We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered a few decades ago by increased ocean-induced thinning, which may have initiated marine ice-sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold-ocean period, the evolving glacier-bed/ice-shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin-wide thinning signal did not change. Thus, as predicted by theory, once marine ice-sheet instability is underway, a single transient high-amplitude ocean cooling has only a relatively minor effect on ice flow. The long-term effects of ocean-temperature variability on ice flow, however, are not yet known.

We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered a few decades ago by increased ocean-induced thinning, which may have initiated marine ice sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold-ocean period, the evolving glacier-bed/ice shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin-wide thinning signal did not change. Thus, as predicted by theory, once marine ice sheet instability is underway, a single transient high-amplitude ocean cooling has only a relatively minor effect on ice flow. The long-term effects of ocean temperature variability on ice flow, however, are not yet known.

We present an update of the ‘key points’ from the Antarctic Climate Change and the Environment (ACCE) report that was published by the Scientific Committee on Antarctic Research (SCAR) in 2009. We summarise subsequent advances in knowledge concerning how the climates of the Antarctic and Southern Ocean have changed in the past, how they might change in the future, and examine the associated impacts on the marine and terrestrial biota. We also incorporate relevant material presented by SCAR to the Antarctic Treaty Consultative Meetings, and make use of emerging results that will form part of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report

We present an update of the ‘key points’ from the Antarctic Climate Change and the Environment (ACCE) report that was published by the Scientific Committee on Antarctic Research (SCAR) in 2009. We summarise subsequent advances in knowledge concerning how the climates of the Antarctic and Southern Ocean have changed in the past, how they might change in the future, and examine the associated impacts on the marine and terrestrial biota. We also incorporate relevant material presented by SCAR to the Antarctic Treaty Consultative Meetings, and make use of emerging results that will form part of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report

We present an update of the 'key points' from the Antarctic Climate Change and the Environment (ACCE) report that was published by the Scientific Committee on Antarctic Research (SCAR) in 2009. We summarise subsequent advances in knowledge concerning how the climates of the Antarctic and Southern Ocean have changed in the past, how they might change in the future, and examine the associated impacts on the marine and terrestrial biota. We also incorporate relevant material presented by SCAR to the Antarctic Treaty Consultative Meetings, and make use of emerging results that will form part of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report.

The instrumental period began with the first voyages to the Southern Ocean during the Seventeenth and Eighteenth centuries when scientists such as Edmund Halley made observations of quantities such as geomagnetism. During the early voyages information was collected on the meteorological conditions across the Southern Ocean, ocean conditions, the sea ice extent and the terrestrial and marine biology. The continent itself was discovered in 1820, although the collection of data was sporadic through the remainder of the Nineteenth Century and it was not possible to venture into the inhospitable interior of Antarctica. At the start of the Twentieth Century stations were first operated year-round and this really began the period of organised scientific investigation in the Antarctic. Most of these stations were not operated for long periods, which is a handicap when trying to investigate climate change over the last century.

Results are presented from an isopycnic coordinate model of ocean circulation in the Amundsen Sea, focusing on the delivery of Circumpolar Deep Water (CDW) to the inner continental shelf around Pine Island Bay. The warmest waters to reach this region are channeled through a submarine trough, accessed via bathymetric irregularities along the shelf break. Temporal variability in the influx of CDW is related to regional wind forcing. Easterly winds over the shelf edge change to westerlies when the Amundsen Sea Low migrates west and south in winter/spring. This drives seasonal on-shelf flow, while inter-annual changes in the wind forcing lead to inflow variability on a decadal timescale. A modelled period of warming following low CDW influx in the late 1980's and early 1990's coincides with a period of observed thinning and acceleration of Pine Island Glacier.

Oceanographic data are presented from the eastern Bellingshausen Sea, representing the first near-contemporaneous sampling of conditions near both the northern and southern ice fronts of George VI Ice Shelf. Circumpolar Deep Water (CDW) with a temperature in excess of 1 degrees C floods the entire continental shelf and forms the main inflow to the cavity beneath the ice shelf. We use measurements of salinity, potential temperature, stable isotope ratios and dissolved oxygen, helium, and neon to show that the outflows contain meltwater in concentrations that rise to a maximum of around 3%. Assuming that the currents are in geostrophic balance, we calculate relative velocities along the ice front sections, then estimate the absolute velocity by inversion of the tracer conservation equations. We obtain an overall mean melt rate of 3-5 m a(-1) and a net south-to-north throughflow beneath the ice shelf of 0.17-0.27 Sv. The mean melt rate exceeds that required for equilibrium, consistent with recent observations of ice shelf thinning and retreat. Melting beneath the ice shelf drives upwelling of about 0.1 Sv in total of CDW into the surface mixed layer at the two ice fronts. The effective vertical heat flux per unit area of ice shelf cover is 8 W m(-2), more than 4 times that estimated for vertical diffusion through the main pycnocline of the neighboring open water region. The south-to-north throughflow carries a particularly strong signature of upwelled CDW, including low dissolved oxygen and high nutrient concentrations, north into Marguerite Bay.

Introduction George VI Ice Shelf, sandwiched between the western coast of Palmer Land and the eastern coast of Alexander Island, is the largest and most studied of the west Antarctic Peninsula ice shelves. It covers an area of approximately 25,000 km2 and is underlain by Circumpolar Deep Water (CDW), with temperatures in excess of 1ºC, giving rise to rapid basal melting (Bishop and Walton, 1981; Lennon et al., 1982). The maximum ice thickness of about 500 m occurs about 70 km from the southern ice front, where a ridge of thick ice extends across George VI Sound (near 70ºW, see Figure 1) effectively dividing the upper water column into northern and southern regions. The northern ice front, which faces Marguerite Bay, appears to be near the geographical limit of ice shelf viability and has undergone a gradual retreat in recent decades (Lucchitta and Rosanova, 1998), a timeframe over which much of the nearby Wordie Ice Shelf disintegrated (Doake and Vaughan, 1991). There is extensive surface melting over the northern parts of the ice shelf and much of the ice column near the northern ice front appears to be temperate (Paren and Cooper, 1986). Conditions in the south, where the ice front faces into Ronne Entrance, are colder and the ice front position appears to be steady. The vast majority of the flow into the ice shelf comes from Palmer Land, but basal melting is sufficient to remove most of this, so that the ice is derived almost exclusively from local accumulation by the time it reaches the ice fronts (Potter et al., 1984). There is some evidence at the margins of the ice shelf to suggest that it may have disappeared completely during the early Holocene before reforming (Sugden and Clapperton, 1981; Bentley et al., 2005).