Thresholds for the future of the Greenland ice-sheet

由于海平面变化对沿海和低洼地区的人口和生态系统产生不利影响，海平面变化是由温室气体排放引起的人为气候变化最广为人知和潜在的严重后果之一。预计这种影响在未来几个世纪还会加剧。导致全球平均海平面上升的因素之一是格陵兰冰盖，目前它正在萎缩，失去的冰正在以水的形式加入海洋。在气候变暖的情况下，预计冰盖融化的速度会加快，这将超过预计冰盖上降雪量的增加，因此未来冰盖的质量损失将更快。现有的科学资料表明，全球变暖超过某一阈值将导致格陵兰冰盖在一千年或更长时间内几乎完全消失，导致全球平均海平面上升约7米。这个阈值非常不确定，但可能比工业化前低1-2摄氏度。如果变暖超过阈值，然后回落到阈值以下，冰盖可能会重新生长，但这取决于变暖超过阈值的时间和程度。如果冰盖失去了太多的质量，它可能会继续收缩，即使全球气候恢复到工业革命前的状态，它也可能会消失。在这种情况下，格陵兰冰盖导致的海平面上升将是不可逆转的。在许多世纪里，全球平均海平面将不可逆转地上升几米，这将对沿海地区的适应构成极端挑战，避免这种情况对减缓至关重要。因此，格陵兰冰盖的长期未来是一个关键的不确定性，我们的项目旨在提供更清晰的信息。我们将利用一个计算机模型来预测本世纪和未来几千年冰盖的变化，这个模型是我们为研究过去10万年冰河期发生的变化而开发的。这些科学兴趣之间有着密切的关系，因为过去发生的事情可以告诉我们未来可能发生的事情。该模型在一个覆盖全球的网格上同时表示气候和格陵兰冰盖，其细节要详细得多。这两个组成部分都是必要的，因为随着冰盖形状和大小的变化，它会改变它所经历的气候，从而影响融化和降雪的速度。我们将使用这个模型来研究不同程度的全球变暖对冰盖的影响，持续不同的时间长度。我们将向公众、科学界以及英国和国外的政策制定者提供我们的研究结果。它们与国际气候政策相关，因为全球变暖目标是1.5摄氏度，这是2016年签署的巴黎气候协议所表达的愿望。

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.