Louise C. Sime

Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history

Rapid warming over the past 50 years on the Antarctic Peninsula is associated with the collapse of a number of ice shelves and accelerating glacier mass loss. In contrast, warming has been comparatively modest over West Antarctica and significant changes have not been observed over most of East Antarctica, suggesting that the ice-core palaeoclimate records available from these areas may not be representative of the climate history of the Antarctic Peninsula. Here we show that the Antarctic Peninsula experienced an early-Holocene warm period followed by stable temperatures, from about 9,200 to 2,500 years ago, that were similar to modern-day levels. Our temperature estimates are based on an ice-core record of deuterium variations from James Ross Island, off the northeastern tip of the Antarctic Peninsula. We find that the late-Holocene development of ice shelves near James Ross Island was coincident with pronounced cooling from 2,500 to 600 years ago. This cooling was part of a millennial-scale climate excursion with opposing anomalies on the eastern and western sides of the Antarctic Peninsula. Although warming of the northeastern Antarctic Peninsula began around 600 years ago, the high rate of warming over the past century is unusual (but not unprecedented) in the context of natural climate variability over the past two millennia. The connection shown here between past temperature and ice-shelf stability suggests that warming for several centuries rendered ice shelves on the northeastern Antarctic Peninsula vulnerable to collapse. Continued warming to temperatures that now exceed the stable conditions of most of the Holocene epoch is likely to cause ice-shelf instability to encroach farther southward along the Antarctic Peninsula.

Evidence for warmer interglacials in East Antarctic ice cores.

Stable isotope ratios of oxygen and hydrogen in the Antarctic ice core record have revolutionized our understanding of Pleistocene climate variations and have allowed reconstructions of Antarctic temperature over the past 800,000 years (800 kyr; refs 1, 2). The relationship between the D/H ratio of mean annual precipitation and mean annual surface air temperature is said to be uniform ±10% over East Antarctica and constant with time ±20% (refs 3–5). In the absence of strong independent temperature proxy evidence allowing us to calibrate individual ice cores, prior general circulation model (GCM) studies have supported the assumption of constant uniform conversion for climates cooler than that of the present day. Here we analyse the three available 340 kyr East Antarctic ice core records alongside input from GCM modelling. We show that for warmer interglacial periods the relationship between temperature and the isotopic signature varies among ice core sites, and that therefore the conversions must be nonlinear for at least some sites. Model results indicate that the isotopic composition of East Antarctic ice is less sensitive to temperature changes during warmer climates. We conclude that previous temperature estimates from interglacial climates are likely to be too low. The available evidence is consistent with a peak Antarctic interglacial temperature that was at least 6 K higher than that of the present day —approximately double the widely quoted 3 ± 1.5 K (refs 5, 6).

Ice core evidence for a 20th century decline of sea ice in the Bellingshausen Sea, Antarctica

[1] This study uses ice core methanesulphonic acid (MSA) records from the Antarctic Peninsula, where temperatures have been warming faster than anywhere else in the Southern Hemisphere, to reconstruct the 20th century history of sea ice change in the adjacent Bellingshausen Sea. Using satellite‐derived sea ice and meteorological data, we show that ice core MSA records from this region are a reliable proxy for regional sea ice change, with years of increased winter sea ice extent recorded by increased ice core MSA concentrations. Our reconstruction suggests that the satellite‐observed sea ice decline in the Bellingshausen Sea during recent decades is part of a long‐term regional trend that has occurred throughout the 20th century. The long‐term perspective on sea ice in the Bellingshausen Sea is consistent with evidence of 20th century warming on the Antarctic Peninsula and may reflect a progressive deepening of the Amundsen Sea Low due to increasing greenhouse gas concentrations and, more recently, stratospheric ozone depletion. As a first‐order estimate, our MSA‐based reconstruction suggests that sea ice in the Bellingshausen Sea has retreated southward by ∼0.7° during the 20th century. Comparison with other 20th century sea ice observations, reconstructions, and model simulations provides a coherent picture of Antarctic sea ice decline during the 20th century, although with regional‐scale differences evident in the timing and magnitude of this sea ice decline. This longer‐term perspective contrasts with the small overall increase in Antarctic sea ice that is observed in post‐1979 satellite data.

Reconciling the changes in atmospheric methane sources and sinks between the Last Glacial Maximum and the pre-industrial era

We know from the ice record that the concentration of atmospheric methane, [CH4], at the Last Glacial Maximum (LGM) was roughly half that in the pre-industrial era (PI), buthow much of the difference was source-driven, and how much was sink-driven, remains uncertain. Recent developments include: a higher estimate of the LGM-PI change in methane emissions from wetlands―the dominant, natural methane source; and the possible recycling of OH consumed in isoprene oxidation―the principal methane sink. Here, in view of these developments, we use an atmospheric chemistry-transport model to re-examine the main factors affecting OH during this period: changes in air temperature and emissions of non-methane volatile organic compounds from vegetation. We find that their net effect was negligible(with and without an OH recycling mechanism), implyingthe change in [CH4] was almost entirely source driven―a conclusion that, though subject to significant uncertainties,can be reconciled with recent methane source estimates.

A Decomposition of the Atlantic Meridional Overturning

A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure–dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8° and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

Automated processing to derive dip angles of englacial radar reflectors in ice sheets

We present a novel automated processing method for obtaining layer dip from radio-echo sounding (RES) data. The method is robust, easily applicable and can be used to process large (several terabytes) ground and airborne RES datasets using modest computing resources. We give test results from the application of the method to two Antarctic datasets: the Fletcher Promontory ground-based radar dataset and the Wilkes Subglacial Basin airborne radar dataset. The automated RES processing (ARESP) method comprises the basic steps: (1) RES noise reduction; (2) radar layer identification;(3) isolation of individual ‘layer objects’; (4) measurement of orientation and other object properties;(5) elimination of noise in the orientation data; and (6) collation of the valid dip information. The apparent dip datasets produced by the method will aid glaciologists seeking to understand ice-flow dynamics in Greenland and Antarctica: ARESP could enable a shift from selective regional case studies to ice-sheet-scale studies.

Interpreting temperature information from ice cores along the Antarctic Peninsula: ERA40 analysis

[1] Analysis of ERA40 temperature and accumulation data suggests that annual mean isotopic fluctuations due to temperature change will be geographically very variable across the Peninsula: isotopic variations of 0.4% at James Ross Island; 0.9% at Dyer; and 1.3% at Gomez are all likely to indicate an identical magnitude of temperature change. The reduction in the magnitude of the isotopic signal in the north and east is due to climatically dependent synoptic covariance between temperature and accumulation; whilst in the west and south seasonal covariance amplifies the isotopic temperature signal. Additionally we show that the relationship between accumulation and temperature is rather weak in the north-east regions but is stronger in the central and southerly regions. Therefore isotopes may record 11% to 30% of the variance in annual mean temperatures in the north east; 75% in central regions; and 70% in the south. This study enables physically based reconstructions of Peninsula climate based on multi-core analysis. Citation: Sime, L. C., G. J. Marshall, R. Mulvaney, and E. R. Thomas (2009), Interpreting temperature information from ice cores along the Antarctic Peninsula: ERA40 analysis, Geophys. Res. Lett., 36, L18801, doi:10.1029/2009GL038982.

Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse

Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming.

The role of atomic chlorine in glacial-interglacial changes in the carbon-13 content of atmospheric methane

The ice-core record of the carbon-13 content of atmospheric methane (δ13CH4) has largely been used to constrain past changes in methane sources. The aim of this paper is to explore, for the first time, the contribution that changes in the strength of a minor methane sink―oxidation by atomic chlorine in the marine boundary layer (ClMBL)―could make to changes in δ13CH4 on glacial-interglacial timescales. Combining wind and temperature data from a variety of general circulation models with a simple formulation for the concentration of ClMBL, we find that changes in the strength of this sink, driven solely by changes in the atmospheric circulation, could have been responsible for changes in δ13CH4 of the order of 10% of the glacial-interglacial difference observed. We thus highlight the need to quantify past changes in the strength of this sink, including those relating to changes in the sea-ice source of sea salt aerosol.

Isochronous information in a Greenland ice sheet radio echo sounding data set

The evaluation of ice sheet models is one of the pressing problems in the study of ice sheet dynamics. Here we examine the question of how much isochronous information is contained within the publicly available Center for Remote Sensing of Ice Sheets (CReSIS) Greenland airborne radio echo soundings data set. We identify regions containing isochronous reflectors using automatic radio echo sounding processing (ARESP) algorithms. We find that isochronous reflectors are present within 36% of the CReSIS radio echo sounding englacial data by location and 41% by total number of data. Between 1000 and 3000 m in depth, isochronous reflectors are present along more than 50% of the data set flight path. Lower volumes of cold glacial period ice also correspond with more isochronous reflectors. We find good agreement between ARESP and continuity index results, providing confidence in these findings. Ice structure data sets, based on data identified here, will be of use in evaluating ice sheet simulations and the assessment of past rates of snow accumulation.