The current, rapid retreat of Pine Island Glacier, West Antarctica, began around 1945, following a period of strong oceanic warming associated with El Nino activity, finds a paper published online in Nature this week. The study provides the first quantitative evidence that the present thinning and retreat of Pine Island Glacier is part of a climatically forced trend triggered in the 1940s.

Pine Island Glacier, which drains into the Amundsen Sea in West Antarctica, is retreating and thinning rapidly, but the initial triggering mechanism has long been unclear. James Smith, Martin Truffer, David Vaughan and colleagues study three sediment cores recovered from beneath the floating Pine Island Glacier ice shelf. Detailed analysis of the sediments in the cores documents the transition, near a prominent seafloor ridge, from a grounded glacier to a free-floating ice shelf. The authors use dating techniques applied to the sediments to show that an ocean cavity under the ice shelf, behind the seafloor ridge, began to form around 1945, following a pulse of warmth associated with El Nino events in the tropical Pacific Ocean. They find that final detachment of the ice shelf at the studied location occurred around 1970.

The authors note that, despite a return to pre-1940s climatic conditions in the ensuing decades, thinning and retreat of the Pine Island Glacier have not stopped. These results help to clarify the mechanisms that underlie ice sheet retreat and indicate that retreat can continue even when climate forcing weakens.