Losing their Cool: are high-elevation heat exchanges warming Himalayan glaciers?

我们最近发现，在世界最高的冰川之一——尼泊尔的昆布冰川（Khumbu Glacier），一半以上的消融面积都是由处于熔点的冰构成的。此外，在过去的40年里，上层消融区内的冰已经升温了2-3摄氏度，并且与当地气候不平衡。综上所述，这些观测结果表明，高海拔喜马拉雅冰川意外地容易受到21世纪气候变暖的影响，并且正在接近一个临界点，超过这个临界点，质量损失将大大加速。然而，决定该地区冰温的过程仍然知之甚少，这使得对未来冰川变化的预测不确定。失冷（LtC）的总体目标是研究大气与高海拔冰川表面之间的物理相互作用（平均海拔6000米），首次提供对雪和冰过程的洞察，这些过程规定了喜马拉雅冰的温度。工作的假设是，在冰形成之前，在积累区域内融化和再冻结足以有效地将坚硬层的温度提高几度。为了验证这一点，LtC将从珠穆朗玛峰西侧昆布冰川的积累区收集第一个可靠和持续的天气条件测量数据。我们将在海拔6000 - 6800米的地方钻探20-25米长的钻孔并进行仪器测量，以在两年的时间内测量冰川的温度和冰的温度。我们还将使用360度摄像机对钻孔内部进行成像，以表征岩石密度，并量化以前重新冻结事件的强度和频率。我们将在海拔不存在自动气象站的地方安装自动气象站，并从岩心中采集样本，供相关领域（如生物地球化学）的合作者使用。我们将利用这些经验数据来校准并验证一个数值模型，该模型既可以模拟驱动地表变暖的能量通量，也可以模拟随之而来的地下融水流动和再冻结过程。这将使我们能够分离出融水重新冻结对冰温的影响，并确定这种影响在气候变暖中的变化程度。最后，我们将模拟整个冰川系统，并跟踪冰温随冰川下游距离的演变，以评估冰川消融过程在多大程度上可以解释我们之前在冰川消融区观察到的意外高温，以及对2100年冰损失的改进预测。这项工作将提供对全球类似环境下所有冰川相关的冰川消融过程的新认识。特别是，它将改进我们在动态冰川模型中表示冰刚度和冰流过程的方式。它将解决文献中关于世界最高海拔地区净质量损失可能性的悬而未决的争论，并指出喜马拉雅地区的其他冰川可能也包括意想不到的暖冰的程度。我们的工作将提供对很少观测到的冰冻圈区的见解，为政府间气候变化专门委员会（ipcc）编制的议程设定报告提供信息，并直接和间接地解决几个关键的可持续发展目标。随着气候变化对该地区的持续影响，我们将进一步为联合国开发计划署（UNDP）和尼泊尔政府等支持机构提供证据，帮助他们准备和缓解融水供应现在不可避免的变化。

We recently discovered that over half of the ablation area of one of the world's highest glaciers, Khumbu Glacier, Nepal, comprises ice that is at the melting point. Moreover, ice within the upper ablation area has warmed by 2-3 degrees Celsius over the last 40 years and is out of equilibrium with local climate. Combined, these observations indicate that high-elevation Himalayan glaciers are unexpectedly vulnerable to 21st Century climatic warming, and approaching a tipping point beyond which greatly accelerated mass loss will occur. However, the processes that determine ice temperatures within this region remain poorly understood, making projections of future glacier change uncertain.The overarching aim of Losing their Cool (LtC) is to investigate the physical interactions between the atmosphere and the glacier surface at high-elevation (>6,000 m a.s.l.), providing insight into the snow and firn processes that prescribe Himalayan ice temperatures for the first time. The working hypothesis is that melting and refreezing within the accumulation area is sufficiently effective to raise firn-layer temperatures by several degrees prior to ice formation. To test this, LtC will collect the first robust and sustained measurements of firn conditions from Khumbu Glacier's accumulation area in the Western Cwm of Mount Everest. We will drill and instrument 20-25 m-long boreholes at elevations of 6,000-6,800 m a.s.l. to measure englacial firn and ice temperatures over a two-year period. We will also use a 360 degree camera to image the interior of the boreholes to characterise firn density and quantify the magnitude and frequency of previous re-freezing events. We will install automatic weather stations at elevations where they do not already exist, and take samples from the cores for collaborators working in relevant fields (e.g. biogeochemistry). We will use these empirical data to calibrate, and then validate, a numerical model that can simulate both the energy fluxes driving warming at the surface, and the consequent subsurface meltwater flow and refreezing processes. This will enable us to isolate the impact of meltwater re-freezing on ice temperature, and determine the extent to which this changes in a warming climate. Finally, we will simulate the whole glacier system, and track the evolution of ice temperatures with distance downglacier, to assess the extent to which firn processes can account for the unexpectedly high temperatures we previously observed in the glacier ablation area, as well as yield improved forecasts of ice loss up to 2100.This work will provide new understanding of firn processes that are relevant for all glaciers within similar settings world-wide. In particular it will improve the way we represent ice stiffness and processes of ice flow in dynamic glacier models. It will resolve outstanding debates in the literature about the possibility of net mass loss at the world's highest elevations, and indicate the extent to which other glaciers within the Himalaya may also comprise unexpectedly warm ice. Our work will provide insights into a rarely observed cryospheric zone that can inform agenda-setting reports such as those produced by the Intergovernmental Panel on Climate Change, as well as addressing, directly and indirectly, several key Sustainable Development Goals. We will further provide evidence for supporting agencies such as UNDP, and the Nepalese government, to help prepare for, and mitigate against, a now inevitable change in meltwater supply as climatic changes continue to impact this region.