New Zealand's longest glacier has lost 5km to global warming and is expected to lose at least as much again if the climate keeps heating up.
The Tasman Glacier, the massive ice river that sweeps past Aoraki-Mt Cook, has already shrunk to 23km, from the formation of a 5km lake at its snout in the past 30 years.
In that time, New Zealand's glaciers have lost almost 11 per cent - 5.8 cubic kilometres - of their ice, new research released yesterday has found.
Twelve of the largest in the Southern Alps are unlikely to return to their earlier lengths without "extraordinary cooling of the climate", says the National Institute of Water & Atmospheric Research (Niwa).
The warming climate is responsible for more than 90 per cent of the ice loss.
The report comes a day after the starkest warning yet from the Intergovernmental Panel on Climate Change, which declared the impact of global warming could be "abrupt or irreversible" and no country would be spared.
Niwa said the shrinking of New Zealand's glaciers had continued despite there being virtually no change in the amount of snow feeding them last year.
The shrinkage of the big glaciers, mostly in the Mt Cook region, is driven mainly by the formation of glacier-snout lakes - which encourage big lumps of ice to break off and accelerate the shrinkage - and surface melting.
However, the Franz Josef Glacier on the West Coast grew 170m from 2005 to this year from increased snowfall, following earlier shrinkage.
Niwa principal scientist Dr Jim Salinger said the 12 big glaciers all had lakes and had now "passed a tipping point", although it was unclear if they would disappear completely with future warming.
Niwa contractor Dr Trevor Chinn said that in the present climate, the Tasman Glacier would eventually reach equilibrium when its lake had doubled to nearly 10km long, but he was unsure about the more-distant future. "If the climate keeps warming, the Tasman will be at least 10km shorter [than before its lake formed]."
Glacial recession causes major problems for today's mountaineers - unlike in 1882, when the Rev. William Spotswood Green nearly conquered the mountain. His party had to climb up onto Ball Glacier, a tributary of the Tasman. But even then, shrinkage was visible, from the slumping at the middle of the glacier.
Now many climbers face scrambling more than 100m down "Garbage Gully" to the glacier - with the constant threat of rockfall and a slip - or must pay for a skiplane ride. Similar problems exist elsewhere in the national park,.
Alpine guide Gottlieb Braun-Elwert said that when he first visited the Tasman Glacier in 1976 it had no lake, but two years later "big ice puddles" had formed. He said plans to build a new hut below Mt Malte Brun had been abandoned because the Tasman's recession had made access too dangerous.