Sea ice around Antarctica may have decreased by as much as 20% since 1950, new findings suggest.
Until now there has been no clear evidence of Antarctic sea ice melting due to global warming.
Satellite measurements only span the last 22 years, making comparisons difficult.
But the new study relied on an algae chemical, drilled out of the ice, which provided a longer record of change.
Levels of the chemical, methanesulphonic acid (MSA), are influenced by the presence of sea ice in the Southern Ocean.
A team of Australian scientists compared MSA measurements from an ice core taken from the coastal Law Dome region of Antarctica with satellite data over the past 22 years.
They were found to match up well, so the team extended the MSA record from the core back to 1840.
The findings showed that sea ice in that part of the ocean was stable from 1840 to 1950, but had declined sharply since then.
Writing in the journal Science, the researchers said there had been a 20% reduction in sea ice since 1950.
They suspected that the results reflected an overall decline of sea ice throughout the whole region.
The emerging picture was consistent with other evidence from whaling records, penguin observations, reports from early voyages, and climate models.