Poles apart: why has Antarctic sea ice increased, and why can't coupled climate models reproduce observations?

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Due to its pale colour, sea ice reflects much of the incoming solar radiation back into space, keeping local temperatures relatively cold. However, if warming occurs and sea ice melts, it is replaced by darker ocean. This absorbs more solar energy, causing warming, and so the cycle, the so-called 'ice-albedo feedback' loop, continues. Sea ice also modifies the regional surface energy balance by capping the upper layer of the ocean, reducing its loss of heat to the atmosphere. In addition, sea ice is important because it plays a role in the exchange of carbon dioxide between the atmosphere and ocean, thereby affecting how much of this greenhouse gas is in the atmosphere and contributing to global warming. Moreover, sea ice formation is an important element in driving the global thermohaline circulation of heat and salt through the world's oceans. One component of this circulation is the North Atlantic Drift current that carries warm tropical water across the Atlantic and keeps the UK's winter temperatures much warmer than they would be otherwise.The Intergovernmental Panel on Climate Change (IPCC) assessment reports are an important tool in drivng government policy around the world. However, the present generation of climate models, which are used to predict the future climate scenarios described in these reports, are unable to consistently reproduce the recent increase in Antarctic sea ice. As a result considerable uncertainty must be attached to their predictions of future climate.This proposal aims to both advance our understanding of the Earth's climate and facilitate improved predictions of its future change to aid policy makers. This will be achieved through the following objectives: 1. To explain the key climate processes involved in the recent Antarctic sea ice increase. We know from observations that changes in the near-surface wind around Antarctica are predominantly responsible for the observed increase in sea ice but we don't know exactly how the wind and the ice interact. Using a state-of-the-art computer model of sea ice and the ocean forced by the latest atmospheric data we will establish the key processes through which changes in the wind are causing the ice to increase.2. To establish the ultimate driver of the sea ice increase. Policymakers need to know whether we can attribute the observed changes in Antarctic sea ice to human activity. This might happen through changes in the near-surface winds around Antarctica caused by the 'ozone hole' or greenhouse gas increases for example. Alternatively, it may be simply due to natural variations in the Antarctic climate system. If the former is true, we must determine which human activities are responsible. If the latter is correct, we must try to understand connections between the key processes and wider aspects of the climate system.3. To understand why current climate models fail to simulate the growth in Antarctic sea ice. We will examine the current UK climate model in detail to diagnose which components are to blame and, with our Met Office partner, we will design a development programme to ensure that our findings are transferred into future model improvements in time for the next IPCC report. To help other climate model developers around the world, we will also analyse whether the failings are common to the other models used in the IPCC reports.