Ice loss in the Canadian Arctic Archipelago

Though much attention has been focused in recent years on the melting of ice from Greenland and Antarctica, nearly half of the ice volume currently being lost to the ocean is actually coming from other mountain glaciers and ice caps. Ice loss from a group of islands in northern Canada accounts for much of that volume. In a study published in April 2011 in the journal Nature, a team of researchers led by Alex Gardner of the University of Michigan found that land ice in both the northern and southern Canadian Arctic Archipelago has declined sharply. The maps above show ice loss from surface melting for the northern portion of the archipelago from 2004-2006 (left) and 2007-2009 (right). Blue indicates ice gain, and red indicates ice loss. In the six years studied, the Canadian Arctic Archipelago lost an average of approximately 61 gigatons of ice per year. (A gigaton is a billion tons of ice.) The research team also found the rate of ice loss was accelerating. From 2004 to 2006, the average mass loss was roughly 31 gigatons per year; from 2007 to 2009, the loss increased to 92 gigatons per year. Gardner and colleagues used three independent methods to assess ice mass, all of which showed the same trends. The team used a model to estimate the surface mass balance of ice and the amount of ice discharged. They also compiled and analyzed measurements from NASA's Ice, Cloud and Land Elevation Satellite (ICESat) to assess changes in the surface height of ice. Finally, they gathered observations from NASA's Gravity Recovery and Climate Experiment (GRACE) to determine changes in the gravity field in the region, an indicator of the amount of ice gained or lost. The Canadian Arctic Archipelago generally receives little precipitation, and the amount of snowfall changes little from year to year. But the rate of snow and ice melting varies considerably, so changes in ice mass come largely from changes in summertime melt. During the 2004 to 2009 study period, the Canadian Arctic Archipelago experienced four of its five warmest years since 1960, likely fueling the melting. Gardner notes that from 2001 to 2004, the sum of melting from all mountain glaciers and ice caps around the world (but not the Greenland and Antarctic ice sheets) contributed an estimated 1 millimeter per year to global sea level rise. Recent estimates suggest the Greenland and Antarctic ice sheets add another 1.3 millimeters per year to sea level. "This means 1 percent of the land ice volume--mountain glaciers and ice caps--account for about half of all ice loss to the world's oceans," Gardner said. "Most of the ice loss is coming from the Canadian Arctic Archipelago, Alaska, Patagonia, the Himalayas, and the smaller ice masses surrounding the main Greenland and Antarctic ice sheets."近年来，尽管学界与公众的注意力多集中于格陵兰岛与南极洲的冰体消融，但当前全球海洋新增的冰体流失量中，近半数实际来自其他山地冰川（mountain glaciers）与冰盖（ice caps）。加拿大北部群岛群的冰体流失量，便占据了这部分流失总量的绝大多数。 2011年4月发表于《自然》（Nature）期刊的一项研究中，由密歇根大学亚历克斯·加德纳（Alex Gardner）领衔的研究团队发现，加拿大北极群岛（Canadian Arctic Archipelago）南北两部分的陆地冰体均出现了显著衰减。上图展示了该群岛北部区域2004-2006年（左图）与2007-2009年（右图）因表面消融导致的冰体流失情况：蓝色代表冰体增益，红色代表冰体流失。 在本次研究覆盖的6年时间里，加拿大北极群岛年均冰体流失量约为61吉吨（1吉吨即十亿吨冰体）。研究团队同时发现，冰体流失速率正持续加快：2004-2006年间，年均冰体质量流失约31吉吨；到2007-2009年间，这一数值攀升至92吉吨/年。 加德纳及其团队采用了三种独立方法评估冰体质量，所有方法均得出了一致的变化趋势。其一，他们借助模型估算冰体的表面质量平衡与冰体输出量；其二，整合并分析了美国国家航空航天局（NASA）冰、云和陆地高程卫星（Ice, Cloud and Land Elevation Satellite, ICESat）的观测数据，以评估冰体表面高度的变化；其三，利用NASA重力恢复与气候实验卫星（Gravity Recovery and Climate Experiment, GRACE）的观测结果，推断该区域的重力场变化——而重力场变化正是冰体增减量的重要指示指标。 加拿大北极群岛整体降水量稀少，年降雪量波动幅度极小。但冰雪消融速率存在显著差异，因此冰体质量变化主要受夏季消融量的影响。在2004-2009年的研究周期内，该群岛经历了自1960年以来5个最暖年份中的4个，这大概率加剧了冰体消融进程。 加德纳指出，2001-2004年间，全球除格陵兰岛与南极洲冰盖之外的所有山地冰川与冰盖的消融总量，预估每年会使全球海平面上升约1毫米。近期的估算显示，格陵兰岛与南极洲冰盖每年会额外使海平面上升1.3毫米。加德纳表示："这意味着，仅占陆地冰体总量1%的山地冰川与冰盖，贡献了全球海洋冰体总流失量的约一半。其中绝大多数冰体流失来自加拿大北极群岛、阿拉斯加、巴塔哥尼亚、喜马拉雅山脉，以及格陵兰岛与南极洲主冰盖周边的小型冰体群。"