For the first time, oceanographers have documented in detail what climate scientists have long suspected: a warm body of water known as Circumpolar Deep Water has expanded southward and onto the Antarctic continental shelf over the past twenty years, undermining the floating ice shelves that buttress the continent's glaciers. The findings, published this week in Communications Earth & Environment, draw on two decades of moored sensors and Argo float profiles.
The paper's authors describe a sustained migration of waters in the 1.5 to 3.5°C range from the open Southern Ocean towards the grounding lines of West Antarctic glaciers, in particular Thwaites and Pine Island. Both glaciers are anchors of the West Antarctic Ice Sheet and, between them, hold enough ice to raise global sea levels by more than three metres if their floating extensions disintegrate.
Lead author Dr Jia Xu of the British Antarctic Survey said the data showed the warmer state was no longer episodic. "What we are seeing is not a passing pulse linked to El Niño. It is a regime shift. The shelf has spent more than half of the last decade in conditions our measurements would have classified as anomalous in 2005," she said.
The paper builds on a related study in Nature in March that documented "abrupt changes in the Antarctic environment" across atmospheric, sea-ice and biological measurements. Together, the two papers suggest that the southern polar region, long considered slower-moving than the Arctic, is now experiencing change at comparable speed.
Direct measurements of melt at the Thwaites grounding line by autonomous submersibles last year found warm water cutting upward channels into the underside of the ice, hollowing the buttress from below. The new study links those local observations to a basin-wide shift in the Antarctic Circumpolar Current.
The authors stop short of estimating a revised sea-level contribution. They note, however, that current Intergovernmental Panel on Climate Change projections, which envisage Antarctic loss as a tail risk, may understate the central estimate. The paper calls for accelerated deployment of long-duration moorings under the Ross and Ronne ice shelves to extend the observational record.