Assessing the role of oceanic forcing in West Antarctic Ice Sheet retreat since the Last Glacial Maximum

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Accurate predictions of sea-level rise are critical if governments are to plan for the future in a warming world. For London and other low-lying parts of the UK, knowing when and by how much sea level will rise will determine when costly infrastructure like the Thames Barrier is upgraded. The Intergovernmental Panel on Climate Change has identified rapidly melting ice sheets as the main source of accelerating sea level rise and stated that collapse of the West Antarctic Ice Sheet will cause sea-level to rise at rates much higher than currently predicted.Understanding the behaviour of glaciers flowing into the Amundsen Sea sector of the West Antarctic Ice Sheet is key to the accuracy of such predictions. They represent one-third of the total discharge of the West Antarctic Ice Sheet and are currently contributing to sea level rise at a significant and accelerating rate. It is widely understood that increased glacier melting in this region is driven by incursions of warm ocean water, called Circumpolar Deep Water (CDW). This warm water flows onto the continental shelf and beneath the floating parts of the glaciers where it melts the glacier ice. Measurements have shown that the temperature and volume of CDW in the Amundsen Sea has increased during the past decade, which has coincided with increased glacier melting and sea level rise. We also know that the arrival of CDW to the area is affected by the weather systems over the ocean which means that CDW is sensitive to changes in atmospheric conditions. Although the idea that warm water is driving glacier retreat is now firmly established, it is unclear (and a factor limiting our ability to predict future changes) how the volume and temperature of CDW has varied over longer timescales. The current generation of predictive ice sheet models assume that melting of the glaciers in the Amundsen Sea will be maintained or increase in future. However with only two decades of ocean temperature data from the Amundsen Sea it is difficult to confirm whether the models are accurate. Given the rate of ice loss in this area and the implications for sea defence planning worldwide, there is a fundamental need to understand the long-term history of CDW incursion and whether the ocean temperatures we see today are unique or have varied substantially in the past. This research will directly address this lack of knowledge by reconstructing ocean temperature in the Amundsen Sea over the past 25,000 years and its relationship to past ice sheet retreat. To achieve this we will apply two independent methods to reconstruct past ocean temperatures from well-dated marine sediment cores from the Amundsen Sea. The first method uses specific organic remains (from marine microbes that live in the surface waters) whilst the second method uses the chemical composition of calcareous shells found in the sediments. Using these different techniques we will be able to reconstruct surface, sub-surface and deep water temperatures and compare them to well-dated records of ice sheet retreat over the past 25,000 years. If our results show that past ice sheet retreats coincided with warm ocean temperatures, then we can quantify the relationship between incursions of CDW and ice sheet retreat. One implication of this could be that modern changes are part of a long term 'trajectory' that needs to be incorporated into predictive models. On the other hand, if the timing of ice sheet retreat did not coincide with the presence of warm water, or that incursions of CDW has varied substantially in the past then this would also have significant implications for future predictions. Ultimately our data will help underpin the next generation of ice sheet models and in turn, well-validated ice sheet models will be able to better predict future sea-level rise. Overall this project will deliver significant improvements in our understanding of the sensitivity of ice sheets to incursions of warm water.