Most ice loss in Greenland and Antarctica isn鈥檛 due to melting at the surface. Instead, these ice sheets are shrinking mainly because glaciers are flowing faster into the ocean, raising the risk of feedback loops that could accelerate sea level rise.
While rising air temperatures thaw ice at the surface, glaciers also lose ice at the coast. Relatively warm ocean water melts the underside of the glacier and icebergs calve off its tongue, drifting equatorward to eventually melt. The quicker a glacier flows, the more ice it will lose to the sea.
The longest satellite record ever compiled shows ice loss has sped up by a factor of four since the 1980s due to global warming. It found that 84 per cent of this acceleration was caused by glacier flow and only 16 per cent by surface melt, a much higher proportion than expected.
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鈥淣ow we can say with confidence that it has been because glaciers are flowing faster,鈥� says at Northumbria University, UK, one of the authors of the study. 聽鈥淚t was quite surprising to me and to some colleagues that actually 84 per cent of the mass loss is really driven by glaciers flowing faster to the ocean, rather than being half and half or 60-40 surface melting.鈥�
Satellites have been making consistent, regular images of the ice sheets only since 1979. But Landsat 1, the first satellite dedicated to monitoring Earth鈥檚 land surfaces, took a few measurements of glacier velocity in Greenland after NASA it in 1972. By estimating glacier flow during the gaps in this early data, Otosaka and her colleagues were able to extend the satellite ice loss record back to 1972 for Greenland.
Greenland and Antarctica have lost 11.3 trillion tonnes of ice in the past half-century, which is equivalent to an ice cube 23 kilometres high.
Scientists have known that ice loss was accelerating mainly because of an increase in the speed of glacier flow, although exactly what proportion of ice loss was due to this factor has been uncertain. The Antarctic ice sheet is cold enough that there is essentially no surface melt. But in Greenland, air temperatures do go above freezing, causing surface melt. The proportion of surface melt versus ocean loss can vary from year to year.
Glaciers flow downhill due to gravity, but water within the sediments underneath them, or water that has trickled down from surface melt, can lubricate the glacier base and speed up this flow. Ice loss at the front of the glacier can also decrease resistance. In Antarctica, the breakup of ice shelves that extend in front of the glaciers and often buttress them against outcroppings of land has caused glaciers to accelerate.
鈥淎 higher share of ocean-driven ice loss is concerning because ocean-ice feedbacks can produce faster, nonlinear responses,鈥� says at the British Antarctic Survey.
Many glaciers such as the Thwaites 鈥渄oomsday glacier鈥� in Antarctica lie over bedrock that slopes downward from the coast to the interior. As they melt, more ocean water can pour into this bowl, causing runaway melt in a feedback known as marine ice sheet instability (MISI).
Scientists also fear that as the thin snouts of glaciers retreat, the ice faces at the front of them could get too tall, collapsing and driving further retreat in what is known as marine ice cliff instability (MICI).
Ice loss in Greenland and Antarctica has raised sea levels 3 centimetres since the 1970s, putting up to 9 million more people at the risk of coastal flooding and erosion. Sea level could rise by up to 2 metres this century alone.
These estimates attempt to account for feedbacks, but if more ice loss is due to glaciers flowing into the ocean, it increases the risk that they aren鈥檛 doing so fully.
鈥淭he real concern is that poorly-understood instabilities such as MISI and MICI could push future Antarctic losses above the currently-expected range, which is why they remain a major uncertainty,鈥� says Meredith.
Nature Scientific Data
