In recent years, the thinning and retreat of the glaciers making up the West Antarctic ice sheet has become increasingly apparent. But the trend isn't new. Research now suggests the ice sheet's ongoing retreat began in the 1940s.

Analysis of sediment cores collected from beneath the Pine Island Glacier ice shelf suggests the glacier began retreating from a prominent seafloor ridge no later than 1945. The analysis also suggests the ice shelf's grounding line fully separated from the ridge in 1970.

"Our results suggest that, even when climate forcing -- such as El Ninos, which create warmer water -- weakened, ice-sheet retreat continued," lead researcher James Smith of the British Antarctic Survey said in a news release.

As satellite monitoring has revealed, glaciers flowing into the West Antarctic ice sheet have grown increasingly thin over the last three decades. But satellite data prior to 1990 is incomplete.

The latest study combines satellite data with sediment analysis to show the seemingly recent thinning phenomenon is in fact several decades old.

"This finding provided the first hint that the recent retreat could be part of a longer-term process that started decades or even centuries before satellite observations became available," Smith said.

The melting West Antarctic ice sheet has significant implications for future sea level rise. By better understanding the history of glacial retreat in Western Antartica, scientists can improve long-term predictions for ice-sheet melting and rising seas.

Researchers published their latest work in the journal Nature.

"Despite a return to pre-1940s climatic conditions in the ensuing decades, thinning and glacier retreat has not stopped and is unlikely to be reversible without a major change in marine or glaciological conditions," Smith concluded. "A period of warming in the Antarctic shelf waters triggered a substantial change in the ice sheet, via the mechanism that we see today -- that is, ocean-driven thinning and retreat of ice shelves leads to inland glacier acceleration and ice-sheet thinning."