A part of the Antarctic normally icebound in February is now virtually clear water, a sailor has reported.
He said he had also found an ice shelf in "dramatic" retreat.
The reports will fuel concern that the Antarctic is being seriously affected by climate change.
But they will be dismissed by those who question the evidence that global temperatures are rising.
Blake's progress
The sailor, Sir Peter Blake, was talking via satellite telephone with the conference here of the United Nations Environment Programme (Unep), which ended on 9 February.
In his yacht Seamaster, he is currently off the Antarctic Peninsula, at 69 degrees 15' South.
"We are in an area that normally is solid ice at this time of year," he told Unep's director, Dr Klaus Toepfer, and several ministers attending the conference, "now it has many bergs in it, but is essentially a free waterway, an almost unheard-of occurrence".
"The captain of a cruise ship that has been coming to the Antarctic Peninsula every year since the mid-1970s told us he has never seen the area so free of ice, and that the average temperature in that time has increased by about 1.4 degrees C."
Sir Peter reported that he had sailed to the King George VI ice shelf that normally fills the channel between Alexander Island and the mainland at the base of the peninsula. The indications are that it has retreated dramatically, especially over the past eight to ten years.
"We weren't able to make it to the face of the ice shelf, because it is dropping so much old ice into the sea as it recedes," he said, "the channel is full of it".
Sir Peter, a winner of the Whitbread Round the World yacht race and a former setter of the record time for
circumnavigating the world non-stop under sail, said
he and his crew had sailed through seas that would not
have been navigable in the era of early Antarctic
explorers such as Sir Ernest Shackleton.
Eroding ice shelf
In reply, Dr Toepfer said analysis of whaling records and modelling studies showed that Antarctic sea ice had retreated south by 2.8 degrees of latitude between the mid 1950s and the early 1970s.
He said recent reports by the Intergovernmental Panel on Climate Change (IPCC), which has estimated the possible global temperature rise by 2100 as 5.8 degrees C, had confirmed a "spectacular" retreat and collapse of ice shelves on the Antarctic peninsula.
"Climate change in polar regions is expected to be among the greatest of any region on the Earth, and it will cause major physical, ecological, sociological and economic impacts," Dr Toepfer said.
The Canadian Environment Minister, David Anderson, who was also taking part in the link-up, said: "In Canada's north, we are seeing dramatic changes that affect permafrost and sea ice, which has major implications for species on which traditional Inuit life depends, such as polar bears and seals."
Sir Peter's report is consistent with IPCC predictions of what will happen as climate change progresses.
But a report last year that an expedition had found open water at the North Pole was dismissed as largely meaningless by critics who said it happened every year.
While the IPCC recently said it was more certain that climate change was happening, and that human activities were at least partly to blame, the sceptics say it has failed to explain several anomalies.
In particular, they say measurements taken near the Earth's surface, which show a steady rise in global temperatures, are not supported by measurements taken at higher altitudes.