Synthesized grounding line and ice shelf mask for Antarctica

Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 gigatonnes per year. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-sheet base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of -1,454 ± 174 gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering and enhanced discharge. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.

此前学界普遍认为，冰山崩解（Iceberg calving）是南极冰盖（Antarctic Ice Sheet）物质损失的主导成因，过往对崩解通量（calving flux）的估算值超过每年2000吉吨。而近期研究证实，在冰架接地线（grounding line）附近及崩解前沿（calving front）周边，海洋消融的重要性已得到验证。然而截至目前，尚无研究能够对整个南极区域的崩解通量及基底物质平衡（basal mass balance，即冰架底部的积增与消融平衡）进行可靠量化。因此，南大洋（Southern Ocean）淡水的分布及其在液态与固态相之间的分配情况，目前仍缺乏有效约束。本研究采用卫星观测得到的崩解通量与接地线通量（grounding-line flux）、模拟所得的冰架积雪积累速率，以及针对未勘测区域的区域尺度修正方案，对南极所有冰架的物质平衡组分进行了估算。本研究得到的总崩解通量为每年1321±144吉吨，总基底物质平衡为每年-1454±174吉吨（负号代表物质净损失）。这意味着，冰架通过表面过程获得的物质中，约有一半在抵达冰架前沿前便因海洋侵蚀发生损失；同时本次估算的崩解通量较此前基于冰山追踪得到的结果低约34%。此外，不同冰架的基底过程所致物质损失占比差异显著，区间约为10%至90%。对于出现表面降高与冰输出增强的冰架，我们发现其基底物质损失与表面高程变化之间存在显著的正相关关系。本研究认为，基底物质损失可作为预测冰架未来受海洋强迫（oceanic forcing）影响脆弱性的重要指标。