Inland Migration of Crevasses on the Greenland Ice Sheet

格陵兰冰盖的表面在2012年夏天经历了前所未有的融化，为十年来的夏天画上了句号，融化的程度和持续时间都高于平均水平。在2012年的峰值期间，融化覆盖了约97%的冰盖，持续时间比1979-2011年的平均水平长了近两个月。这种持续加剧的融化导致了冰面的向下消融，并通过提供径流来加强冰层下的滑动，在加速冰流方面发挥了作用。在过去十年中，冰盖对海平面上升的贡献增加，总体而言，冰盖融化占了大约一半。有报道称，在冰盖上迄今没有裂缝的位置出现了表面裂缝，这与冰盖上加剧的融化相吻合。这些裂缝可以作为新生的路径，将表面融化的水输送到冰盖床，这可能会促进加速的冰流向内陆传播，并可能导致额外的表面破裂。它们的发生还对安全后勤业务(飞机着陆、地面穿越)构成潜在危险。2012年，格陵兰岛的乌鸦营地出现了裂缝，导致美国军用飞机训练活动暂停。格陵兰岛西北部的格陵兰内陆导线(GRIT)使用的路线也出现了新的裂缝，可能会严重影响未来的导线活动。已经提出了两个假设来解释裂缝的发生：一个是乌鸦下面的积雪含水层突然排水，导致上面的积雪下沉形成裂缝；另一个是冰流向下游加速，导致应变率增加，形成真正的裂缝。如果第二个假设是正确的，未来裂纹的大小和出现的频率可能会增加。这项提议将通过对冰的运动和应变率进行全球定位系统(GPS)调查，并使用非常高分辨率的卫星图像详细评估裂缝随时间的分布来检验这些假设。改进对冰盖对未来气候强迫的反应的预测，以及就格陵兰的后勤活动作出明智的长期规划决定的能力，需要了解导致新的表面裂缝出现的过程，并评估其分布随时间增加的可能性。

The surface of the Greenland Ice Sheet experienced unprecedented melting during the 2012 summer, capping a decade of summers with above average melt extent and duration. Melting covered ~97% of the ice sheet during its 2012 peak and lasted almost two months longer than the 1979-2011 average. This sustained intensified melt has led to down-wasting of the surface, and has also played a role in accelerated ice flow by supplying runoff to enhance subglacial sliding. Altogether, melting has accounted for about half of the ice sheet's increased contribution to sea level rise in the last decade. Reports of surface cracks appearing at hitherto crevasse-free locations on the ice sheet have coincided with the intensified melt. These cracks can act as nascent pathways for the delivery of surface meltwater to the ice sheet bed, which might promote the inland propagation of accelerated ice flow and may lead to additional surface cracking. Their occurrence also constitutes a potential hazard to safe logistic operations (aircraft landing, surface traversing). Cracks appeared at Raven Camp, Greenland, during the peak melt event in 2012 and led to a temporary halt in U.S. military aircraft training activities. New cracks also appeared on the route used by the Greenland Inland Traverse (GrIT) in northwest Greenland and may severely impact future traversing activities. Two hypotheses have been proposed to explain the occurrence of the cracks: one invokes the sudden drainage of a firn aquifer beneath Raven and the resulting subsidence of the overlying firn to form the cracks; the other hypothesis involves the downstream acceleration of ice flow leading to an increase in strain rates and the formation of true crevasses. If the second hypothesis is correct, crack sizes and their frequency of occurrence are likely to increase in the future. This proposal will test those hypotheses by carrying out global positioning system (GPS) surveys of ice motion and strain rate, and a detailed assessment of crevasse distributions through time using very high-resolution satellite imagery. Improved predictions of the ice sheet's response to future climate forcings as well as the ability to make informed long-term planning decisions about logistic activities in Greenland require an understanding of the processes responsible for the appearance of new surface cracks and an assessment of the likelihood that their distribution will increase with time.