Ice-layer Permeability Controls Runoff from Ice Sheets (IPCRIS)

冰层渗透率控制冰盖径流 (IPCRIS)

• 批准号: NE/X000435/1
• 负责人: Douglas Mair
• 金额: $ 77.2万
• 依托单位: University of Liverpool
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FX000435%2F1
• 关键词: Ice; layer; Permeability; Controls; Runoff

The Greenland Ice Sheet is the world's largest single source of barystatic sea-level rise (c.20% total rise) and more than half of the mass lost annually from the ice sheet comes from surface melt-water runoff. This proportion, and its magnitude, is rising with continued climate warming but future projections, and societal planning for sea level rise impacts, are undermined by a fundamental source of uncertainty. Across the vast majority of the accumulation area of the Greenland Ice Sheet, we do not know how much of the water produced from surface melting refreezes in underlying firn (i.e. multi-year snow) or becomes runoff. When the surface of an ice sheet melts, the density and temperature of underlying snow, firn and impermeable ice combine to determine whether melt refreezes in the underlying snow and firn, or becomes runoff to the ocean. If meltwater can percolate to depth (e.g. up to c.10 m) and access cold, low density firn, it can refreeze creating a significant buffer between climate change and sea-level rise. Alternatively, if melt encounters shallow impermeable ice layers (themselves created by previous refreezing) within relatively warm firn, melt cannot reach the cold firn and more melt will become runoff. The difference between these two scenarios alone could double ice sheet runoff by the middle of the 21st century. We rely on model simulations of surface melt, refreezing and runoff to accurately project the future contribution of the Greenland Ice Sheet to sea level rise. However, model-based estimates of the annual refreezing capacity of the ice sheet over the last six decades differ dramatically and undermines their ability to converge towards a reliable range of future projections. A major cause of uncertainty follows from the quite different assumptions that models make about ice layer permeability that dramatically alters the ice sheet refreezing capacity. If ice layers in firn are assumed to be impermeable (permeable), they will inhibit (allow) meltwater percolation to depth, diminish (maintain) refreezing capacity, increase (decrease) runoff and hence increase (decrease) projected global sea level rise. Without an improved treatment of ice layer permeability, existing surface mass balance models cannot provide reliable projections of the future refreezing capacity of, and melt-water runoff from, the Greenland Ice Sheet, leaving the ice sheet's future contribution to sea level rise highly uncertain. Firstly, we need to know the physical and thermal conditions of snow and firn that control the effective permeability of relatively thin ice layers (<0.5m thick) since within our warming climate these are increasingly determining the depth to which meltwater can percolate and hence control the refreezing capacity of the underlying firn. To this end we will undertake temperature-controlled laboratory experiments, systematically simulating and monitoring snow/firn/ice melt/refreezing/runoff. Secondly, we need to model the effective permeability of ice layers in snow and firn and their sensitivity to changing external and internal conditions since these together control how much melt refreezes or becomes runoff. For this, our lab work will inform novel developments to modelling to simulate measured arctic ice cap snowpack evolution. Finally we will incorporate improved ice layer permeability criteria within ice sheet scale models of the Greenland Ice Sheet to generate more accurate simulations of runoff and refreezing during melt extremes and improve harmonisation of long-term mass balance model projections, consequently improving global sea level rise predictions over the next century. Multiple recent "exceptional" melt seasons have caused near surface ice layers to proliferate through previously low density firn. These extremes will be the new norm in the future so new model parameterisations are urgently required that can effectively characterise ice layer control on mass balance.

格陵兰冰盖是世界上最大的常压海平面上升的单一来源(总上升约20%)，每年从冰盖损失的质量有一半以上来自地表融水径流。随着气候持续变暖，这一比例及其规模正在上升，但未来的预测以及对海平面上升影响的社会规划，都受到了一个根本的不确定性来源的破坏。在格陵兰冰盖的绝大多数积累区，我们不知道表面融化产生的水中有多少在下层积雪(即多年的积雪)中重新冻结或成为径流。当冰盖表面融化时，底层积雪、积雪和不透水冰的密度和温度共同决定融化是在底层积雪和积雪中重新冻结，还是成为流入海洋的径流。如果融化的水可以渗透到深度(例如，高达约10米)并接触到寒冷、低密度的降雪，它可能会重新冻结，在气候变化和海平面上升之间形成一个重要的缓冲。或者，如果融化在相对温暖的积雪中遇到浅层不透水的冰层(它们自己是由先前重新冻结形成的)，融化不能到达寒冷的积雪，更多的融化将成为径流。到21世纪中叶，仅这两种情景之间的差异就可能使冰盖径流增加一倍。我们依靠表面融化、再冻结和径流的模型模拟来准确预测格陵兰冰盖未来对海平面上升的贡献。然而，对过去60年冰盖年度重新冻结能力的基于模型的估计有很大不同，削弱了它们收敛到可靠的未来预测范围的能力。不确定性的一个主要原因来自模型对冰层渗透性做出的截然不同的假设，这些假设极大地改变了冰盖的再冻结能力。如果降雪中的冰层被认为是不透(透)的，它们将阻止(允许)融水渗入深度，减少(保持)再冻结能力，增加(减少)径流，从而增加(减少)预计的全球海平面上升。如果不改进对冰层渗透性的处理，现有的地表质量平衡模型无法对格陵兰冰盖未来的重新冻结能力和融化水径流提供可靠的预测，这使得冰盖未来对海平面上升的贡献非常不确定。首先，我们需要知道雪和积雪的物理和热条件，这些条件控制着相对较薄的冰层(0.5米厚)的有效渗透率，因为在我们变暖的气候中，这些条件越来越多地决定了融水可以渗透到的深度，从而控制了底层积雪的再冻结能力。为此，我们将进行温度控制的实验室实验，系统地模拟和监测雪/雪/冰融化/复冻/径流。其次，我们需要对积雪和积雪中冰层的有效渗透率以及它们对外部和内部条件变化的敏感性进行建模，因为这些因素共同控制着融化重新冻结或变成径流的程度。为此，我们的实验室工作将为模拟测量到的北极冰盖积雪演变的建模提供新的发展。最后，我们将在格陵兰冰盖的冰盖尺度模式中纳入改进的冰层渗透性标准，以更准确地模拟极端融化期间的径流和重新冻结，并改善长期物质平衡模式预测的协调性，从而改善对下个世纪全球海平面上升的预测。最近的多个“异常”融化季节导致近地表冰层通过以前的低密度积雪扩散。这些极端情况将是未来的新标准，因此迫切需要新的模式参数来有效地描述质量平衡中的冰层控制。