Deglacial grounding-line retreat in the Ross Embayment, Antarctica, controlled by ocean and atmosphere forcing

Modern observations appear to link warming oceanic conditions with Antarctic ice sheet grounding-line retreat. Yet, interpretations of past ice sheet retreat over the last deglaciation in the Ross Embayment, Antarctica’s largest catchment, differ considerably and imply either extremely high or very low sensitivity to environmental forcing. To investigate this, we perform regional ice sheet simulations using a wide range of atmosphere and ocean forcings. Constrained by marine and terrestrial geological data, these models predict earliest retreat in the central embayment and rapid terrestrial ice sheet thinning during the Early Holocene. We find that atmospheric conditions early in the deglacial period can enhance or diminish ice sheet sensitivity to rising ocean temperatures, thereby controlling the initial timing and spatial pattern of grounding-line retreat. Through the Holocene, however, grounding-line position is much more sensitive to subshelf melt rates, implicating ocean thermal forcing as the key driver of past ice sheet retreat

Basal conditions of two Transantarctic Mountains outlet glaciers from observation-constrained diagnostic modelling

This is the published version. Copyright 2014 International Glaciological SocietyWe present a diagnostic glacier flowline model parameterized and constrained by new velocity data from ice-surface GPS installations and speckle tracking of TerraSAR-X satellite images, newly acquired airborne-radar data, and continental gridded datasets of topography and geothermal heat flux, in order to better understand two outlet glaciers of the East Antarctic ice sheet. Our observational data are employed as primary inputs to a modelling procedure that first calculates the basal thermal regime of each glacier, then iterates the basal sliding coefficient and deformation rate parameter until the fit of simulated to observed surface velocities is optimized. We find that the two glaciers have both frozen and thawed areas at their beds, facilitating partial sliding. Glacier flow arises from a balance between sliding and deformation that fluctuates along the length of each glacier, with the amount of sliding typically varying by up to two orders of magnitude but with deformation rates far more constant. Beardmore Glacier is warmer and faster-flowing than Skelton Glacier, but an up-glacier deepening bed at the grounding line, coupled with ice thicknesses close to flotation, lead us to infer a greater vulnerability of Skelton Glacier to grounding-line recession if affected by ocean-forced thinning and concomitant acceleration

Modelled glacier response to centennial temperature and precipitation trends on the Antarctic Peninsula

The northern Antarctic Peninsula is currently undergoing rapid atmospheric warming1. Increased glacier-surface melt during the twentieth century2, 3 has contributed to ice-shelf collapse and the widespread acceleration4, thinning and recession5 of glaciers. Therefore, glaciers peripheral to the Antarctic Ice Sheet currently make a large contribution to eustatic sea-level rise6, 7, but future melting may be offset by increased precipitation8. Here we assess glacier–climate relationships both during the past and into the future, using ice-core and geological data and glacier and climate numerical model simulations. Focusing on Glacier IJR45 on James Ross Island, northeast Antarctic Peninsula, our modelling experiments show that this representative glacier is most sensitive to temperature change, not precipitation change. We determine that its most recent expansion occurred during the late Holocene ‘Little Ice Age’ and not during the warmer mid-Holocene, as previously proposed9. Simulations using a range of future Intergovernmental Panel on Climate Change climate scenarios indicate that future increases in precipitation are unlikely to offset atmospheric-warming-induced melt of peripheral Antarctic Peninsula glaciers

8000 years of North Atlantic storminess reconstructed from a Scottish peat record: implications for Holocene atmospheric circulation patterns in Western Europe

North Atlantic storminess can affect human settlements, infrastructure and transport links, all of which strongly impact local, national and global economies. An increase in storm frequency and intensity is predicted over the Northeast Atlantic in the 21st century because of a northward shift in storm tracks and a persistently positive North Atlantic Oscillation (NAO), driven by recent atmospheric warming. Although documentary records of North Atlantic storminess exist, these are generally limited to the last c. 1000‐2000 years. This paper presents a continuous high‐resolution proxy record of storminess spanning the last 8000 years from a 6 m long core taken from a peat bog in Northern Scotland. Bromine concentrations in the peat, derived from sea spray, are used to reconstruct storm frequency and storm intensity, and mire surface wetness is used as an indicator of longer‐term climate shifts. The results suggest a relationship between positive phases of the NAO and increased North Atlantic storminess. However, subtle differences between bromine concentrations and mire surface wetness suggest that high intensity but perhaps less frequent periods of storminess are not necessarily associated with a wetter climate

Global environmental consequences of 21st century ice sheet melt

Government policies currently commit us to surface warming of three to four degrees Celsius above pre-industrial levels by 2100, which will lead to enhanced ice-sheet melt. Ice-sheet discharge was not explicitly included in Coupled Model Intercomparison Project phase 5, so effects on climate from this melt are not currently captured in the simulations most commonly used to inform governmental policy. Here we show, using simulations of the Greenland and Antarctic ice sheets constrained by satellite-based measurements of recent changes in ice mass, that increasing meltwater from Greenland will lead to substantial slowing of the Atlantic overturning circulation, and that meltwater from Antarctica will trap warm water below the sea surface, creating a positive feedback that increases Antarctic ice loss. In our simulations, future ice-sheet melt enhances global temperature variability and contributes up to 25 centimetres to sea level by 2100. However, uncertainties in the way in which future changes in ice dynamics are modelled remain, underlining the need for continued observations and comprehensive multi-model assessments

Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial

This is the final version of the article. Available from Springer Nature via the DOI in this record.Contrasting Greenland and Antarctic temperatures during the last glacial period (115,000 to 11,650 years ago) are thought to have been driven by imbalances in the rates of formation of North Atlantic and Antarctic Deep Water (the 'bipolar seesaw'). Here we exploit a bidecadally resolved (14)C data set obtained from New Zealand kauri (Agathis australis) to undertake high-precision alignment of key climate data sets spanning iceberg-rafted debris event Heinrich 3 and Greenland Interstadial (GI) 5.1 in the North Atlantic (~30,400 to 28,400 years ago). We observe no divergence between the kauri and Atlantic marine sediment (14)C data sets, implying limited changes in deep water formation. However, a Southern Ocean (Atlantic-sector) iceberg rafted debris event appears to have occurred synchronously with GI-5.1 warming and decreased precipitation over the western equatorial Pacific and Atlantic. An ensemble of transient meltwater simulations shows that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train.A challenge for testing mechanisms of past climate change is the precise correlation of palaeoclimate records. Here, through climate modelling and the alignment of terrestrial, ice and marine (14)C and (10)Be records, the authors show that Southern Ocean freshwater hosing can trigger global change.This work was funded by the Australian Research Council (FL100100195, DP170104665 and SR140300001) and the Natural Environment Research Council (NE/H009922/1 and NE/H007865/1)

Antarctic ice-sheet sensitivity to obliquity forcing enhanced through ocean connections

Deep sea geological records indicate that Antarctic ice-sheet growth and decay is strongly influenced by the Earth's astronomical variations (known as Milankovitch cycles), and that the frequency of the glacial-interglacial cycles changes through time. Here we examine the emergence of a strong obliquity (axial tilt) control on Antarctic ice-sheet evolution during the Miocene by correlating the Antarctic margin geological records from 34 to 5 million years ago with a measure of obliquity sensitivity that compares the variance in deep sea sediment core oxygen-isotope data at obliquity timescales with variance of the calculated obliquity forcing. Our analysis reveals distinct phases of ice-sheet evolution and suggests the sensitivity to obliquity forcing increases when ice-sheet margins extend into marine environments. We propose that this occurs because obliquity-driven changes in the meridional temperature gradient affect the position and strength of the circum-Antarctic easterly flow and enhance (or reduce) ocean heat transport across the Antarctic continental margin. The influence of obliquity-driven changes in ocean dynamics is amplified when marine ice sheets are extensive, and sea ice is limited. Our reconstruction of the Antarctic ice-sheet history suggests that if sea-ice cover decreases in the coming decades, ocean-driven melting at the ice-sheet margin will be amplified.Published132-1371A. Geomagnetismo e Paleomagnetismo4A. Oceanografia e clima5A. Ricerche polari e paleoclimaJCR Journa

Reassessing the post-Last Glacial Maximum retreat history from the Southwest Ross Sea

Constraining the timing of the retreat of the Last Glacial Maximum (LGM) Antarctic Ice Sheet in the Ross Sea provides insights into the processes controlling marine-based ice sheet retreat. The over-deepened Ross Sea continental shelf is an ideal configuration for marine ice-sheet instability, and this region was thought to be one of the largest Antarctic contributors to post-LGM sea level rise. However, the chronology and pattern of retreat of the LGM ice sheet in the Ross Sea is largely constrained by coastal records along the Transantarctic Mountain front in the Western Ross Sea. Although these offer more reliable dating techniques than marine sediment cores, they may be influenced by local glaciers derived from East Antarctic outlet glaciers. Consequently, these coastal records may be ambiguous in the broader context of retreat in the central regions of the Ross Sea. However, previous studies have inferred that records in this region retreated in a north to south pattern, and was fed by ice sourced from the central Ross Sea – with the implication that broader ice sheet retreat in the central Ross Sea occurred as late as the mid Holocene. We present two lines of evidence that counter this established interpretation of the pattern of retreat in the Ross Sea: 1) a sedimentary facies succession and foraminifera-based radiocarbon chronology from within the Ross Sea embayment that indicates glacial retreat and open marine conditions to the east of Ross Island was already in place before 8.6 cal ka BP, at least 1 kyr earlier than indicated by terrestrial records in McMurdo Sound; and 2) a new multibeam swath bathymetry data that identifies well-preserved glacial features indicating thick (>700m) marine-based ice derived from the East Antarctic Ice Sheet (EAIS) coastal outlet glaciers dominated the ice sheet input into the southwestern Ross Sea during the last phases of glaciation – and thus may have acted independent of any ice in the central Ross Sea embayment. Comparing these data to new modelling experiments, we hypothesize that marine-based ice sheet retreat was triggered by oceanic forcings along most of the Pacific Ocean coastline of Antarctica, but continued early Holocene retreat into the inner shelf region of the Ross Sea occurred primarily as a consequence of marine ice sheet instability. Keywords: Ross Sea, deglaciation, Last Glacial Maximum, Holocen