An Antarctic ice sheet likely collapsed 120,000 years ago when global temperatures were similar to present levels, according to an analysis of octopus DNA.
Scientists say the findings are a climate change warning and mean the ice sheet could reach a melting "tipping point" even if global warming is limited to 1.5-2C under the Paris Agreement.
Concerns have increased in recent years about the melting of the West Antarctic Ice Sheet, which contains 3.2 million cubic kilometres of ice.
A study published in the journal Science compared the genetic profiles of Turquet's octopus which lives in the surrounding Weddell, Amundsen and Ross seas.
DNA analysis of almost 100 individuals found populations on opposite sides of the ice sheet intermingled about 120,000 years ago.
Researcher Nerida Wilson, a marine molecular biologist from the University of Western Australia, said the species' mixing could only have occurred if the ice sheet collapsed entirely.
She said the collapse would have opened seaways for the octopus to connect.
James Cook University molecular biologist Jan Strugnell, who also worked on the study, said it was a period when the global average temperature was 0.5-1.5C warmer than pre-industrial levels and the sea was 5-10m higher.
Professor Strugnell said the current melting of the ice sheet was Antarctica's biggest contributor to sea level rise.
"A complete collapse could raise global sea levels by somewhere between 3-5m," she said.
"Understanding how the (ice sheet) was configured in the recent past when global temperatures were similar to today, will help us improve future sea level rise projections."
The paper said marine drill core records had shown historical evidence of the ice sheet retreating but the exact timing and extent of any collapse remained ambiguous.
Sally Lau from James Cook University said the study was a piece in the puzzle in understanding the ice sheet's history.
She hoped DNA analysis of other animals could be used to paint a more detailed picture.
The study was led by researchers from the University of Western Australia, James Cook University and the Western Australian Museum.
