New Zealand's biggest glacier is melting at its fastest pace in recent history, a scientist said Thursday.
The Tasman Glacier on South Island was 18 miles long in 1990, with virtually no lake at its front edge, Massey University glacier expert Martin Brook said.
New measurements last week showed the glacier was 14 miles long, he said.
Meanwhile, a lake that has formed next to the glacier is now 4.4 miles long, 1.2 miles wide and 800 feet deep, he said.
Despite global warming since the 1850s, the glacier had been protected "and kind of insulated" from the sun's heat by its cover of rock debris, but eventually a lake started to form, Brook said.
The lake is now eating away at the glacier, he said.
The ice cliff at the glacier's front edge is receding at a rate of 590 feet a year, its melt rate greatly aided by the lake water, Brook said.
As the glacial lake enlarges it will speed up the glacier's melt to between 1,650 feet to 2,640 feet a year, he said.
"The deeper the lake, the faster the retreat of the glacier," he said.
But the glacier will not disappear completely because it can only shrink for another 5.6 miles before the lake hits the side of the mountains and cannot expand farther, the scientist said separately on National Radio.
"There will always be a few kilometers of it, but it will be a hell of a lot smaller," he said.
But Canterbury University glacier expert Wendy Lawson, who did not take part in Brook's survey, said she thinks the rate of ice loss will slow down, halted by the height of the glacier's bedrock.
"But there's certainly a lot of things that go on at the interface (of glacier ice and water) we don't understand," she added.
Other glaciers on South Island's central alps also are retreating rapidly, melted by warmer temperatures and lower snowfall levels.