Anthropogenic Forcing of Antarctic Ice Loss (AnthroFAIL)

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Melting ice caps are widely viewed as illustrative of anthropogenic global warming. The largest ice mass on Earth - Antarctica - is losing ice, and may contribute 1 metre or more to global sea level by 2100. However, it is not clear that this ice loss has been caused by human activities. Instead, this ice sheet may be simply responding to natural variability in the climate system.Antarctic ice loss is primarily caused by increased melting of the ice sheet by the ocean. However, it is not known why this melting has changed. The deep ocean waters melting the West Antarctic Ice Sheet have been buried in the deep ocean for centuries, and have not simply warmed alongside the rest of the climate. Instead, changes in the winds are thought to have altered the transport of these warm waters towards Antarctica.This leads to two main possibilities for why Antarctica is losing ice. Firstly, anthropogenic changes in the winds, caused by greenhouse gases and/or the ozone hole, may have gradually increased the supply of warm water towards the ice sheet. Alternatively, natural variability of the winds may have caused a period of high melting that triggered a self-sustaining ice sheet retreat.It is now urgent that we understand whether human activities have increased the ocean melting of Antarctica. If the ice loss is caused by human activities, we must identify the best strategy for minimising future change. If the ice loss is natural and inevitable, we must prepare for the sea-level rise that will play out over the coming centuries. This project will carefully unpick the contributions of natural and human influences using computer simulations of the ocean surrounding West Antarctica. We will first use historical winds from climate models to drive a set of ocean simulations that comprise our 'best guess' of the real history of ocean melting of Antarctica since 1920, under all natural and human forcings. These simulations will be tested against available historical observations from ships, autonomous vehicles, and ocean sediment cores.Once we have this model history, we will work out how it has been influenced by each of the different forcings. We will perform large sets of ocean simulations with and without human greenhouse gases, the ozone hole, and natural climate variability. These simulations will tell us the contribution of each forcing to the oceanic melting of the West Antarctic Ice Sheet. When we have a good understanding of the role of historical forcings, we will be equipped to assess the impact of future forcings. The ozone hole is expected to recover before 2100, so the question is to what extent future emissions of greenhouse gases will overwhelm this recovery, and continue unchecked thereafter. This research will address the fundamental questions of whether humans have caused Antarctic ice loss, and whether there is anything we can now do about it. The answers to these questions are urgently needed to clarify our options for preventing or adapting to future sea-level rise.