KEY POINTS:
Climate change will see most of the Tasman Glacier in the Southern Alps melt away over the next 20 years, scientists say.
"In the past 10 years the glacier has receded a hell of a lot," said glaciologist Dr Martin Brook.
"It's just too warm for a glacier to be sustained at such a low altitude - 730 metres above sea level - so it melts rapidly and it is going to disappear altogether".
The Tasman Glacier is the largest in the Southern Alps and at 29km was noted as one of the longest in the world's temperate zones.
In 1973 there was no lake in front of the Tasman Glacier, while new measurements taken last week indicate the lake at its foot is now 7km long, 2km wide and 245m deep.
The lake has attracted regular excursions by boatloads of tourists, but Dr Brook today warned they may be at risk from massive chunks of ice unexpectedly breaking loose underwater and surfacing as far as 60m from the glacier face.
"There's actually a sub-surface apron of ice that slopes away under the water for at least 50m or 60m from the front of the glacier," Dr Brook said. As this ice-apron melts, blocks of ice break off and float to the surface.
"This happens pretty quickly and is potentially a hazard for the tour boats that cruise up to the cliff: the blocks just pop out on the surface and some are between 5m and 10m in size."
The lake has been formed as the ice which makes up the glacier melts, and is a key factor in its destruction: the deeper the lake, the faster the retreat of the glacier.
According to another glaciologist, Trevor Chinn, the development of the lake was a "tipping point": no amount of snow at the head of the glacier, the neve, can compensate melting triggered by the lake.
Dr Brook, a lecturer in physical geography at Massey University, said the lake could only grow to a length of about 16km - allowing room for another 9km glacier retreat.
"We could expect further retreat of between 477m and 822m each year," he said. "At these rates it would take between 10 and 19 years for the lake to expand to its maximum."
His work has vindicated predictions made in 1990 by Dr Martin Kirkbride.
The last major survey of the glacier was in the 1990s and since then the glacier has retreated an average of 180m a year, exposing a basin carved out of rock more than 20,000 years ago when the glacier was a lot larger and more powerful.
Dr Brook and his research students are using a sonar and echo sounding equipment to measure the depth of the lake and analyse and analyse sediments under the lake.
Over the past couple of decades, a notch would develop at the waterline in the cliff of ice which is the face of the glacier, then melt back into the glacier to undercut the ice above, causing it to collapse into the lake.
At one time large blocks of blue ice, some about the size of the Dunedin Railway Station, were deposited in the lake.
But because a much larger part of the glacier is submerged, the 2degC water is causing a faster retreat of the ice face, leaving a "foot" of ice extending out into the lake.
"The result is large pieces of ice fracturing off the ice foot and floating on the surface," Dr Brook said.
Dr Brook said the team was also investigating a the way the glacier's melt differs to the "clean-ice" glaciers on the West Coast. These smaller, steeper glaciers, such as the Fox and Franz Josef, retreat and advance more erratically.
Tasman is covered in rock and debris, and has a different relationship with the climate, and different patterns of retreat.
- NZPA
