Thresholds for the future of the Greenland ice-sheet

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Sea-level change is one of the mostly widely known and potentially serious consequences of anthropogenic climate change due to emissions of greenhouse gases, because of its adverse impact on the populations and ecosystems of coastal and low-lying areas. This impact is expected to increase for centuries to come. One of the contributors to global-mean sea-level rise is the Greenland ice-sheet, which is presently shrinking, with the ice which it is losing being added as water to the ocean. In a warmer climate, increased melting of the ice-sheet is projected, which will exceed the expected increase in snowfall on the ice-sheet, and hence the ice-sheet will lose mass more quickly in future.Existing scientific information indicates that global warming exceeding a certain threshold would lead to the near-complete loss of the Greenland ice-sheet over a millennium or more, causing a global-mean sea-level rise of about 7 metres. The threshold is very uncertain, but it could be as low as 1-2degC of global warming above pre-industrial. If warming passes above the threshold, and later falls back below it, the ice-sheet might regrow, but this depends on how long and how far the warming was above the threshold. If the ice-sheet has lost too much mass, it might continue to contract and could be eliminated even if global climate returned to a state like that which existed before the industrial revolution. In that case, the sea-level rise due to the Greenland ice-sheet would be irreversible.Irreversible global-mean sea-level rise of several metres over many centuries is a scenario which would present an extreme challenge to adaptation in the coastal zone, and avoiding it is crucial for mitigation. Thus, the long-term future of the Greenland ice-sheet is a critical uncertainty, and our project aims to provide clearer information about it. We will do this by predicting the changes in the ice-sheet in this century and for many millennia into the future using a computer model which we have developed for studying changes that occurred during the ice-ages of the last 100,000 years. There is a close relationship between these scientific interests, because what happened in the past can inform us about what could happen in the future. The model represents both the climate, on a grid covering the world, and the Greenland ice-sheet, in much greater detail. Both components are necessary because as the ice-sheet changes in shape and size it modifies the climate it experiences, and this affects the rates of melting and snowfall. We will use the model to study the consequences for the ice-sheet of various levels of global warming, maintained for various lengths of time. We will make our results available to the public, the scientific community, and policy-makers in the UK and abroad. They are relevant to international climate policy because of the global warming target of 1.5degC, which is the aspiration expressed in the Paris climate agreement signed in 2016.