Ocean and Atmosphere Forcing of Larsen Ice Shelf Thinning

The ice shelves of the Antarctic Peninsula (AP) have shown a progressive decline over the last five decades, including the spectacular collapses of Larsen A Ice Shelf in 1995 and Larsen B in 2002. These collapses have accelerated the flow of ice inland, contributing significantly to sea-level rise, and have also freshened the Antarctic Bottom Water formed nearby. Larsen C Ice Shelf (LCIS), the largest on the peninsula, has progressively lowered since 1992, but the origin of this lowering remains controversial; it has been attributed to ocean melting, but most evidence has favoured enhanced firn (snowpack) compaction. By applying a novel method to the data from eight separate radar surveys of LCIS spanning a 15-year period, we show that the lowering is caused by both ice loss and firn air loss. The ice loss may be caused by unbalanced ocean melting, so oceanic changes have contributed to the wastage of LCIS in addition to the well-documented atmospheric warming in the region. If naively extrapolated in space and time, the air loss rate would deplete LCIS firn within 2-3 centuries, while the ice loss could cause LCIS to unground from Bawden Ice Rise within 2-4 centuries. Until we better understand the atmosphere and ocean forcing of the AP Ice Sheet, it will not be possible to project its future stability or determine the anthropogenic contribution to its decline.