However, the climate simulations used by the IPCC often fail to realistically capture large scale properties of the Arctic sea ice, such as the extent, variability and recent trends. Therefore, there is a need to improve simulations of Arctic sea ice to provide better understanding of the recent observed changes and credible projections of the future to help assess risks and opportunities and inform important policy decisions about adaptation and mitigation. Observations of the Arctic have improved in recent years: new satellites are measuring sea ice thickness and topography from space, and substantial recent and planned field programmes are contributing to our knowledge of the in situ processes controlling the evolution of the ice cover. These observations reveal not only that the extent and thickness of the Arctic ice cover is reducing in all seasons, but also show us how the ice is decreasing. In particular, recent observations demonstrate how surface melt ponds weaken the ice, fragmenting it into many floes. No operational environmental or climate model of sea ice evolution takes into account the processes of ice fragmentation or the impact of having a range of different sea ice floe sizes. Sea ice floe size affects lateral melting, air to ocean momentum transfer, rheology, and wave-ice interaction. Idealised model studies demonstrate that accounting for floe fragmentation has a leading order impact on the spatial distribution of sea ice and decreases its overall extent and volume. Because existing climate models do not contain this physics, their utility for understanding and predicting variability and change in the Arctic is compromised. This leads to impairment of climate model accuracy at lower latitudes also, due to errors in meridional atmospheric and oceanic circulations, as well as ice export from the Arctic. This proposal will determine the role of sea ice fragmentation in the accelerated retreat of Arctic sea ice. Using a combination of new and emerging observations, new theory and process modelling, we will build fundamental understanding of sea ice fragmentation. We will a) develop a high fidelity, high resolution model of sea ice thermodynamic and mechanical evolution and use this, and observations, to explore sea ice fragmentation, b) build parameterisations of fragmentation processes, including the first model of fragmentation by melt, and incorporate these into the sea ice component of climate and weather models, and c) use ice-ocean and fully coupled climate models to identify the contribution of fragmentation to: rapid ice loss over past decades, projections of ice loss in the coming decades, and creating skill for operational ice forecasts now. In addition to the scientific outcomes, the proposed work will result in a new sea ice fragmentation module delivered to climate and weather modelling groups: within the UK, the Met Office, the National Oceanography Centre, the British Antarctic Survey, and the European Centre for Medium-Range Weather Forecasts.