Extending the timescales of glacier volume change in regions of GRACE-gravity anomalies in Northern Greenland

格陵兰冰盖及其目前和未来对海平面上升的影响吸引了科学家、媒体、公众和政策制定者的极大关注。这是因为最近的结果显示，冰盖发生了意想不到的变化，这意味着它正在以比科学家预期的更快的速度导致海平面上升。一些最重要的科学成果来自于使用卫星测高测量冰盖体积，以及使用GRACE（重力恢复和气候实验）使命测量冰盖重量。GRACE卫星测量了整个格陵兰冰盖的重力变化，并显示了东南，东北和西北低海岸地区的同步和多区域质量损失，而北部高海拔内陆地区的质量正在增加。在局部范围内，许多格陵兰冰川，如东南部的两个主要出口Helheim和Kangerdlugssuaq，都报告了冰流速度的同步退缩，显着变薄和急剧加速。冰川流量的增加也增加了冰盖对海洋的贡献。然而，最新的结果表明，这两个冰川再次放缓，突出了我们在预测格陵兰冰盖未来时面临的最大问题之一，即：由于几乎所有可用的测量都只有10到15年的长度，而且人们对驱动这些变化的物理过程知之甚少，观察到的变化可能反映了对最近北极空气和海洋温度上升的深刻调整，因此表明了一种重大趋势，或者只是预期的波动，这种波动遵循一个尚不清楚的自然周期。迫切需要扩大时间范围，以了解格陵兰迅速变化的出口冰川的自然变异性（趋势与周期）。科学界广泛认为这是未来研究的优先目标。在我们的项目中，我们将针对这一关键问题，在GRACE数据显示质量损失率高的地区，对70 oN以北的15-20个主要出口进行多年体积变化估计，并将其与冰量，冰川流速和冰范围的近期变化进行比较。激光雷达和摄影测量的结合将被用来揭开档案立体照片的秘密，这些照片可用于20世纪80年代的格陵兰岛沿海地区和20世纪40年代的几个冰川。这些技术涉及从激光雷达数据中识别和提取稳定的特征，然后将其用作摄影测量处理中的地面控制，以从档案摄影中产生数字高程模型（DEM）。还将创建来自ASTER、Landsat和激光雷达数据的当代DEM和速度估计值，以比较同一冰川。激光雷达数据将在与美国航天局戈达德空间飞行中心（Krabill）冰冻圈科学分支合作的飞行活动中收集。飞越每一冰川两侧的山脉和冰原岛峰的飞行线路将产生必要的地形控制，重复的冰川流线剖面将用于快速速度测量和高分辨率高程参考，以计算体积差异。在2008年的一次类似活动中，也是由斯旺西冰川学小组和美国宇航局进行的，收集了格陵兰东南部16个出口冰川的激光雷达和雷达数据，目标是该地区的主要GRACE异常。该使命的成功直接导致了本项目提案，其中我们将瞄准格陵兰岛北方GRACE数据中相对未知的剧烈变化区域。根据我们在2008年夏季与美国航天局进行飞行的经验，将获得格陵兰岛北方至少15个、最有可能20-25个快速流动的潮汐出水口的数据，为评价高度敏感地区的长期行为提供一个独特的数据库。

The Greenland ice sheet, and its current and future contribution to sea level rise, has attracted a great deal of attention from scientists, media, public and policy makers. This has arisen because recent results show unexpected changes to the ice sheet that mean it is contributing to sea level rise at faster rates than scientists expected. Some of the most important scientific results come from measurements of the ice sheet volume using satellite altimetry, and its weight using the GRACE (Gravity Recovery and Climate Experiment) mission. The GRACE satellites measure gravity changes over the entire Greenland ice sheet, and have shown synchronous and multiregional mass loss in the low coastal regions in the southeast, northeast, and northwest, whereas the high-elevation interior in the north is gaining mass. On a local scale, synchronous retreat, significant thinning and dramatic acceleration of ice flow velocities has been reported for many Greenland glaciers like Helheim and Kangerdlugssuaq, two major outlets in the southeast. These increases in glacier flow also increase the contribution from the ice sheet to the ocean. However, the very latest results show that these two glaciers have slowed down again, highlighting one of the biggest problems facing us in predicting the future of the Greenland ice sheet, namely: As almost all available measurements are only 10 to 15 years in length and the physical processes driving these variations are poorly understood, the observed changes could reflect profound adjustments to recent increases in Arctic air and ocean temperatures, and therefore indicate a significant trend, or simply expected fluctuations that follow a natural cycle which is not yet known. There is an urgent need to extent the temporal time-frame to understand the natural variability of the rapidly changing outlet glaciers in Greenland (trend vs. cycle). This is widely agreed in the scientific community as a priority target for future research. In our project we will target this crucial issue by deriving multi-decadal volume change estimates for 15-20 major outlets north of 70oN in regions where the GRACE data shows high rates of mass loss, and compare them to recent changes in ice volume, glacier flow rates and ice extent. A combination of lidar and photogrammetry will be used to unravel the secrets of archival stereo photographs, which are available for the coastal regions of Greenland from the 1980s and for several glaciers back to the 1940s. The techniques involve identifying and extracting stable features from lidar data, which are then used as ground-control in photogrammetric processing, to produce digital elevation models (DEMs) from the archival photography. Contemporary DEMs and velocity estimates from ASTER, Landsat and lidar data will also be created for comparison for the same glaciers. The lidar data will be collected during a flight campaign in cooperation with the Cryospheric Sciences Branch of NASA Goddard Space Flight Centre (Krabill). Flight lines over mountains and nunataks on both sides of each glacier will produce the necessary topographic control, and repeated glacier flowline profiles will be used for snap-shot velocity measurements and high resolution elevation reference for calculating volume differences. In a similar campaign in 2008, also run by the Swansea Glaciology Group and NASA, lidar and radar data for 16 outlet glaciers in SE Greenland were collected, targeting the major GRACE anomaly in that area. The success of that mission has led directly to this project proposal, in which we will target the relatively unknown area of dramatic change in the GRACE data in northern Greenland. Based on our experience of flights with NASA during summer 2008, data over at least 15, and most probably 20-25 fast-flowing tidewater outlets in the northern part of Greenland will be acquired, providing a unique database for evaluating the long-term behaviour of a highly sensitive area.