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重压海平面上升的单一来源（约占总上升的 20%），每年冰盖损失的质量一半以上来自地表融水径流。随着气候持续变暖，这一比例及其幅度正在上升，但未来的预测和海平面上升影响的社会规划却受到一个根本性不确定性来源的破坏。在格陵兰冰盖的绝大多数积聚区域，我们不知道有多少表面融化产生的水在下面的冰雪（即多年积雪）中重新冻结或变成径流。当冰盖表面融化时，下面的雪、冷杉和不渗透冰的密度和温度共同决定融化的水是否会在下面的雪和冷杉中重新冻结，或者变成流入海洋的径流。如果融水可以渗透到深度（例如深达 10 米）并进入寒冷、低密度的冰雪，它可以重新冻结，在气候变化和海平面上升之间形成重要的缓冲。或者，如果融化物在相对温暖的冰层中遇到浅的不渗透冰层（它们本身是由先前重新冻结产生的），则融化物无法到达寒冷的冰层，并且更多的融化物将变成径流。到 21 世纪中叶，仅这两种情况之间的差异就可能使冰盖径流增加一倍。我们依靠地表融化、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影响。然而，基于模型对过去六十年冰盖年度再冻结能力的估计存在巨大差异，并削弱了它们向可靠的未来预测范围收敛的能力。不确定性的一个主要原因是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。如果冰层中的冰层被假定为不可渗透（可渗透），它们将抑制（允许）融水渗入深度，减少（维持）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，现有的表面质量平衡模型就无法可靠地预测格陵兰冰盖未来的再冻结能力和融水径流，从而使冰盖未来对海平面上升的贡献高度不确定。首先，我们需要了解控制相对较薄的冰层（<0.5m 厚）的有效渗透性的雪和雪的物理和热条件，因为在我们气候变暖的情况下，这些条件越来越多地决定融水渗透的深度，从而控制底层冰的再冻结能力。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/再冻结/径流。其次，我们需要对雪和雪中​​冰层的有效渗透性及其对外部和内部条件变化的敏感性进行建模，因为它们共同控制着有多少融化物重新冻结或变成径流。为此，我们的实验室工作将为模拟测量的北极冰盖积雪演化的建模提供新的发展。最后，我们将在格陵兰冰盖的冰盖比例模型中纳入改进的冰层渗透性标准，以对极端融化期间的径流和再冻结进行更准确的模拟，并改善长期质量平衡模型预测的协调性，从而改善全球海平面上升的预测在下一个世纪。最近的多个“异常”融化季节导致近地表冰层通过先前低密度的冰层扩散。这些极端情况将成为未来的新常态，因此迫切需要新的模型参数化，以有效地表征冰层对质量平衡的控制。