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barystatic海平面上升的单一来源（总增长占20％），每年从冰盖中损失的质量中有一半以上来自地面熔融水径流。这一比例及其规模越来越多，随着气候变暖的持续增长，但未来的预测以及对海平面上升影响的社会计划受到了不确定性的根本来源的破坏。在格陵兰冰盖的绝大多数积累区域中，我们不知道在下面的FIRN（即多年降雪）中从表面熔化的重新冻结中产生的水中有多少水或变成径流。当冰片的表面融化时，下雪的密度和温度，FIRN和不可渗透的冰结合在一起，以确定融化的降雪中的融化和FIRN中的重新冻结，还是成为海洋的径流。如果熔融水可以渗透到深度（例如最高C.10 m）并访问冷密度的冷FIRN，则可以在气候变化和海平面上升之间进行重新冻结。另外，如果在相对温暖的FIRN内融化浅冰层（通过以前的重新冻结创建），熔体将无法到达冷雾，并且更多的熔体将变成径流。仅这两种情况之间的差异就可以在21世纪中叶加倍冰盖径流。我们依靠表面熔体，重新冻结和径流的模型模拟来准确投影格陵兰冰盖对海平面上升的未来贡献。但是，基于模型的估算值对过去六十年的冰盖年度重新冻结能力差异很大，并破坏了它们融合到可靠的未来预测范围的能力。不确定性的主要原因是模型对冰层渗透性的截然不同的假设，从而极大地改变了冰盖重新冻结能力。如果假定FIRN中的冰层是不可渗透的（可渗透的），则它们将抑制（允许）融合水的深度，降低（维持）重新冻结能力，增加（减小）径流（减少）径流，从而增加（减少）预计全球海平面上升。如果没有改进的冰层渗透性处理，现有的表面质量平衡模型将无法提供Greenland Ice Plath的未来重新冻结能力和融化水径流的可靠预测，从而使冰盖对海平面的未来贡献高度不确定。首先，我们需要了解控制相对较薄的冰层（<0.5m厚）的积雪和FIRN的物理和热条件，因为在我们的变暖气候中，这些材料越来越多地确定了融合水可以渗透的深度，从而控制了下层FIRN的重新冻结能力。为此，我们将进行温度控制的实验室实验，系统地模拟和监视雪/firn/冰融化/重新冻结/径流。其次，我们需要建模雪和FIRN中冰层的有效渗透性及其对变化外部和内部条件的敏感性，因为它们共同控制了多少融化或径流。为此，我们的实验室工作将为建模提供新的发展，以模拟测得的北极冰盖积雪演变。最后，我们将在格陵兰冰盖的冰盖量表模型中纳入改进的冰层渗透率标准，以在熔体极端期间对径流和重新冻结进行更准确的模拟，并改善长期质量平衡模型预测的统一，从而改善下一个世纪的全球海平面上升预测。最近有多个“异常”熔体季节导致近地面冰层通过以前的低密度FIRN增殖。这些极端将是将来的新规范，因此迫切需要新的模型参数化，以有效地表征冰层控制质量平衡。