Investigating the Dynamic Response of the Greenland Ice Sheet to Climate Forcing using a Geophysical, Remote-Sensing and Numerical Modelling Framework

摘要越来越多的科学研究表明，人类活动，例如燃烧化石燃料，增加了大气中吸热气体的浓度。据估计，如果我们继续向大气中添加二氧化碳和其他温室气体，本世纪全球气温将上升2-5摄氏度。在北极，变暖的速度预计会更快，年平均气温可能会上升4-7摄氏度。全球变暖的影响是巨大的，因为冰川和冰盖融化的速度会更快，变得越来越容易崩溃。在格陵兰岛，从出口冰川排放的冰块约占每年冰块损失的一半。另一半损失是由于地表融化水的径流。目前，冰山排放和地表融化的综合影响大于格陵兰冰盖上的降雪量。因此，在向大西洋释放淡水的同时，这块冰盖正在缩小。不平衡在1996年达到每年-90立方公里，到2000年增加到每年-140立方公里。2005年，这种不平衡可能会增加到每年-220立方公里。因此，格陵兰冰盖的大小似乎正在以更快的速度缩小。这一趋势引起了全世界的关注，因为冰盖退化导致全球海平面上升，甚至可能阻碍海洋循环，在北大西洋等关键地区，海洋循环对融化的冰块释放的淡水很敏感。格陵兰冰盖位于海平面以上或接近海平面的基岩上。冰川学家多年来一直认为，这样的位置将是稳定的，即使在极端全球变暖的情况下，冰盖的消亡也需要数千年的时间。这一假设可能需要修正。最近有研究表明，地表融化的水可以穿透超过1公里的冰层到达格陵兰冰盖底部，并由于基底部滑动速度更快而导致冰流加速。这种机制是潜在的危险，因为加速的冰流导致变薄，进而导致表面融化的增加，因为冰盖的大部分移动到更低和更温暖的海拔。因此，格陵兰冰盖可能比早先对全球变暖的预测更容易衰退。然而，到目前为止，表面融化和冰流之间的这种动态反应的机制只被普遍理解，而用于预测未来海平面变化的当代气候-冰盖模型没有以任何严格的方式包括这些机制。在以下方面尤其如此：1)地面、内部和基础水管和冰流系统可以在多大程度上缓和、放大并将动态响应传递到冰盖内部，从而将内陆冰水库拉低；2)未来格陵兰温度的变化可能会增加冰盖直接经历这些影响的面积和时间长度。该项目直接解决了当前模型中的这两个缺点，并将实施一套实地考察、卫星遥感和全面的格陵兰冰盖模型模拟，以全面评估和实施当前模型中未包括的与冰流相关的动态过程……这可能会增加冰盖对气候变暖的脆弱性，从而增加未来海平面上升的可能性。(气专委，WG1-2007)。

Summary An increasing number of scientific studies show that human activities, e.g. burning of fossil fuels, have increased the concentration of heat-trapping gasses in the atmosphere. It is estimated that global temperatures will increase by 2-5 degrees C during this century if we continue to add carbon dioxide and other 'greenhouse' gasses to the atmosphere. In the Arctic, warming is expected to be even faster and mean annual temperature may increase by 4-7 degrees C. The implications of global warming are of immense proportions because glaciers and ice sheets will melt faster and become increasingly prone to collapse. In Greenland, discharge from outlet glaciers is responsible for about half the annual loss of ice. The other lost half is due to runoff of surface meltwater. The combined effect of iceberg discharge and surface melt are currently greater than the total amount of snowfall falling onto the Greenland Ice Sheet. This ice sheet is therefore shrinking while releasing freshwater into the Atlantic Ocean. The imbalance amounted to -90 cubic km per year for 1996 and increased to -140 cubic km per year by 2000. In 2005, this imbalance may have increased to as much as -220 cubic km per year. The size of the Greenland Ice Sheet is thus diminishing at what appears to be a growing rate. Worldwide concern is associated with this trend because ice-sheet decay results in global sea-level rise and possibly even an obstruction of oceanic circulation, which in key places - such as the North Atlantic - is sensitive to freshwater released from melting ice masses. The Greenland Ice Sheet rests on bedrock above or close to sea level. Glaciologists have for years assumed that such position would be stable and that demise of the ice sheet would require thousands of years even under extreme global warming scenarios. This assumption may need revision. It was shown recently that surface meltwater could penetrate through over 1km of ice to the base of the Greenland Ice Sheet and cause ice-flow speed-up due to faster basal sliding. This mechanism is potentially dangerous because accelerated ice flow leads to thinning, which in turn leads to an increase in surface melt since a larger part of the ice sheet moves into lower and warmer elevations. The Greenland Ice Sheet may therefore be far more prone to decay than it was assumed in earlier projections of global warming. However, up until now the mechanisms by which this dynamic response between surface melt and ice flow have only been generally understood and the present generation of climate-ice sheet models which are used to forecast future sea-level change do not include them in any rigorous manner. This is particularly true in respect of: 1) the extent to which the surface, interior and basal water-plumbing and ice flow systems can moderate, amplify and transmit the dynamic response away into the interior of the ice sheet thereby drawing the inland ice reservoir down and, 2) the extent to which future changes in Greenland temperatures may increase both the area and length of time of which the ice sheet directly experiences these effects. This project directly addresses both of these shortcomings in current models and will implement a set of fieldwork, satellite remote-sensing and comprehensive Greenland Ice Sheet modelling simulations that will fully assess and implement those 'dynamical processes related to ice flow not included in current models... (which) could increase the vulnerability of the ice sheets to warming, increasing future sea-level rise.' (IPCC, WG1 - 2007).