Airborne geophysical investigations of basal conditions at flow transitions of outlet glaciers on the Greenland Ice Sheet

170万平方公里的格陵兰冰盖被划分为一系列主要的流域，每个流域的面积通常约为50-10万平方公里。这些盆地中的大多数通过相对狭窄和严重裂隙的出口冰川排入峡湾系统的海水中，这些冰川解剖了该岛周围的山脉。在过去的几年里，很明显，冰盖正在失去质量，并已成为全球海平面上升的重要贡献者。这与以下因素有关：第一，几个出口冰川的速度加倍，增加了流入海洋的冰通量；第二，受夏季融化和冰盖表面径流影响的面积大幅增加。这两个变化都发生在过去十年，并与格陵兰及其周围地区气温和水温变暖有关。科学家和政策制定者面临的一个主要问题是，格陵兰冰盖将如何继续对一套气候模型预测的下一个世纪气温上升做出反应，特别是在北极变暖的速度可能高于全球平均水平的背景下，这是由于周围海冰覆盖的持续丧失以及海洋反照率的变化，从而导致的能量平衡。我们将使用机载穿冰雷达、激光高度计、重力仪、磁强计和GPS仪器获取格陵兰冰盖10个出口冰川的地球物理数据。之所以选择这些冰川系统，是因为：(A)它们是冰盖内的主要流域，向海洋提供高冰通量；(B)它们代表格陵兰冰盖及其相关气候和海洋环境内的不同亚环境。我们将重点调查每条出口冰川的三个关键区域：第一，有大量裂缝的快速出口冰川本身，在狭窄的水道中流经格陵兰的边缘山脉；第二，在冰盖内部和这些狭窄的出口冰川之间的上部过渡区；以及第三，接地带，标志着快速流动的出口冰川向位于格陵兰许多峡湾头部的漂浮冰舌过渡。我们的科学目标是：1.确定冰面高度和冰床下高度，包括在快速流动的出口冰川的严重裂隙区域下进行测量。2.确定冰下基质的特征，特别是它是结晶基岩还是可变形沉积物。3.确定冰下融化的分布，描述有水的冰下水文系统。4.识别内陆冰川、出口冰川和基底带之间的过渡带，并揭示与之相关的基本特征变化。5.描述从内陆冰川到出口冰川过渡带内冰分层的三维性质，以测量堆积、应变和底部融化的分布。这些信息将对冰盖的计算机模拟，特别是格陵兰未来如何应对未来几十年空气和海洋温带的变化做出根本贡献。这是因为这些模型需要关于床的形状和正在进行的过程的信息，即所谓的边界条件，以便做出有用的预测。例如，到目前为止，我们对这些出口冰川下的水分布知之甚少。冰盖因表面融化和冰山产生而失去的冰量的变化，反过来对于预测格陵兰未来对北极变暖的海平面上升的贡献是重要的。这一点具有超出学术界的重要意义。在英国和其他地方，国家和地区政府正在要求提供有关海平面上升速度和所需补救措施(如海防)的信息。

The 1.7 million km2 Greenland Ice Sheet is divided into a series of major drainage basins, each typically about 50-100,000 km2 in area. Most of these basins drain into the marine waters of fjord systems via relatively narrow and heavily crevassed outlet glaciers that dissect the mountains fringing the island. Over the past few years it has become clear that the Ice Sheet is losing mass and has become a significant contributor to global sea-level rise. This is related to, first, the doubling in speed of several outlet glaciers, increasing ice flux to the sea and, secondly, a major increase in the area affected by summer melting and runoff from the ice-sheet surface. Both of these changes have taken place in the past decade and have been linked with warmer air and water temperatures over and around Greenland. A major question for both scientists and policymakers is how the Greenland Ice Sheet will continue to react to the temperature rises that are predicted for the coming century by a suite of climate models, particularly in the context that the Arctic is likely to warm at a greater rate than the global average due to the continuing loss of its surrounding sea-ice cover and the changes in ocean albedo and, therefore, energy balance that will result. We will acquire geophysical data from a series of ten outlet glaciers of the Greenland Ice Sheet using airborne ice-penetrating radar, laser altimeter, gravimeter, and magnetometer and GPS instruments. These glacier systems have been selected because: (a) they are major drainage basins within the ice sheet which provide a high ice flux to the sea; and (b) they represent different sub-environments within the Greenland Ice Sheet and its related climate and ocean setting. We will focus our investigations on three key areas of each outlet glacier: first, the heavily crevassed fast-flowing outlet glaciers themselves, that flow in narrow channels through Greenland's fringing mountains; secondly, an upper transition zone between the ice-sheet interior and these narrow outlet glaciers; and thirdly, the grounding zone marking the transition of fast-flowing outlet glaciers to floating ice tongues that are present at the head of many Greenland fjords. Our scientific objectives are: 1. To determine ice surface elevation and subglacial bed elevation, including measurement beneath areas of heavy crevassing in fast-flowing outlet glaciers. 2. To characterize the substrate beneath the ice, in particular whether it is crystalline bedrock or deformable sediments. 3. To establish the distribution of subglacial melting and characterize the subglacial hydrological system where water is present. 4. To identify the transition zones between inland ice, outlet glaciers and the grounding zone and reveal basal character changes associated with them. 5. To describe the three-dimensional nature of internal ice layering within transition zone from inland ice to outlet glacier to measure the distribution of accumulation, strain, and basal melting. This information will make a fundamental contribution to the computer modelling of ice sheets, and how Greenland in particular may respond in future to changes in air and ocean temperate over the coming decades. This because these models require information, known as boundary conditions, on the shape of the bed and also the processes that are going on there in order to make useful predictions. To date, we know little about, for example, the distribution of water beneath these outlet glaciers. The changing amount of ice lost from the ice sheet by surface melting and iceberg production is important, in turn, for predictions on the future contributions of Greenland to sea-level rise in a warming Arctic. This is of significance beyond the academic community. In the UK and elsewhere, governments at national and regional level are requiring information about rates of sea-level rise and the remediation measures, such as sea defences, that are needed.