The fastest draining glacier in Antarctica began to melt during an unusually strong El Niño in the 1940s. New research shows that once set in motion, the process has turned out to be irreversible.
Researchers led by David Vaughan at the British Antarctic Survey in Cambridge, UK, set out to discover what caused the Pine Island Glacier in West Antarctica to retreat so far from a prominent anchoring ridge that marine water was able to flood underneath and detach it from the seafloor.
They found that the start of the glacier’s melt coincided with an extra-warm Pacific Ocean in about 1945, resulting from an El Niño pulse. This caused sea water to rush beneath the ice, forming an ocean cavity. The cavity then continued to expand, finally “ungrounding” the glacier from its anchoring ridge around 1970.
The team’s findings are published in Nature.
The study is significant because until the advent of satellite imaging in the 1990s, it was impossible to track precisely any changes to the glacier’s “grounding line” – the point at which its floating component began.
The team drilled three 20-centimetre-wide holes through the glacier’s ice shelf, opening lines to the ocean cavity, allowing access to sediment built up below.
Once analysed, the cores revealed two different types of rock, each with its own distinct set of characteristics, which essentially documented the transition from a grounded glacier to a free-floating ice shelf.
Using techniques based on lead isotope levels, the researchers dated the two sediment sets to the 1940s and 1970s respectively. The latter is in robust agreement with satellite data that suggests the glacier experienced its final ungrounding around 1973.
“We propose that the grounded ice bulldozed sediment off the ridge crest, generating down slope mass flows,” the researchers explain. “This process can be seen from acoustic imagery of the sea floor under [the glacier].”
The sediment record correlates to climate pattern data, that shows warming sea temperatures hit a peak mid-way through the 20th century.
“The El Niño record includes a major central Pacific warm event in the early 1940s, and analysis of ice cores shows that this was a time of exceptional warmth in West Antarctica,” the researchers write.
“If conditions in the Amundsen Sea were anomalously warm in the past, this could have been the trigger for the present ice thinning.”
Crucially, the timeline of glacial retreat suggests that even after the extreme weather pattern had subsided, the ice continued to shift. The movement appears irreversible and is still continuing today.
“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,” the researchers write.
“Our findings suggest that ice-sheet retreat can continue even when the forcing reverts to its earlier state.”
Amy Middleton is a Melbourne-based journalist.
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