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缝的出水口冰川流入峡湾系统的海水，这些冰川将岛屿周围的山脉切开。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一个重要因素。这与以下因素有关：第一，几个出海口冰川的速度增加了一倍，流入海洋的冰量增加；第二，受夏季融化和冰盖表面径流影响的面积大大增加。这两种变化都发生在过去十年中，并与格陵兰岛及其周围的空气和水温升高有关。科学家和决策者面临的一个主要问题是，格陵兰冰盖将如何继续对一系列气候模型预测的下个世纪的温度上升做出反应，特别是在北极可能以高于全球平均水平的速度变暖的背景下，这是由于其周围海冰覆盖的持续消失和海洋反照率的变化，因此会导致能量平衡的变化。我们将利用机载破冰雷达、激光高度计、重力仪、磁力计和GPS等仪器获取格陵兰冰盖10个出口冰川的地球物理数据。选择这些冰川系统的原因是：(a)它们是冰原内的主要流域，向海洋提供大量冰通量；(b)它们代表了格陵兰冰盖内不同的亚环境及其相关的气候和海洋环境。我们将把调查重点放在每个出口冰川的三个关键区域：首先，严重裂缝的快速流动的出口冰川本身，它们在格陵兰岛的边缘山脉中以狭窄的渠道流动；其次，在冰盖内部和这些狭窄的出口冰川之间有一个上部过渡带；第三，接地带标志着快速流动的出口冰川向漂浮的冰舌过渡，这些冰舌存在于许多格陵兰峡湾的头部。我们的科学目标是：1。确定冰面高程和冰下床高程，包括测量快速流动的出水口冰川中严重裂缝区域下的高程。2. 表征冰下的基底，特别是它是结晶基岩还是可变形的沉积物。3. 建立冰下融化的分布和表征存在水的冰下水文系统。4. 识别内陆冰、出口冰川和接地带之间的过渡带，揭示与之相关的基底特征变化。5. 描述内陆冰到出入口冰川过渡带内部冰层的三维性质，测量堆积、应变和基底融化的分布。这些信息将对冰原的计算机模拟，特别是格陵兰岛未来如何对未来几十年空气和海洋温度的变化作出反应，作出根本性的贡献。这是因为这些模型需要信息，即所谓的边界条件，关于床的形状以及那里正在发生的过程，以便做出有用的预测。例如，到目前为止，我们对这些出水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损失的冰量的变化，反过来对于预测格陵兰岛未来对变暖的北极海平面上升的贡献很重要。这是超越学术界的意义。在英国和其他地方，国家和地区一级的政府正在要求提供有关海平面上升速度和所需的补救措施（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.