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Alpine Recreation has taken visitors along the 2000m-high Ball Pass in the Aoraki/Mt Cook National Park for more than two decades, but company operations manager Gavin Lang said the pass, which gave a view of Mt Cook's Caroline face, and provided access between the Tasman and Hooker Glacier valleys, was now under threat because the Hooker Glacier was retreating at a ''noticeably'' accelerated rate.
Mr Lang said the glacier's retreat had undermined the moraine walls, which were now unsupported.
''The process is sped up by heavy rain, and in two locations the erosion has been so extreme that we've been forced to re-route the track, adding an hour to the average trip.''
He said when guides crossed over the pass from Caroline Hut, it had become necessary to ensure clients were fit enough to make what was now a 10-hour crossing.
His own data, collected by a GPS reference waypoint, indicated that the glacier's terminal face had retreated 99m between March 2009 and December 2012.
''That is an average downsizing of 2.2m every month.''
However, Niwa scientist Dr Tim Kerr said although it was clear the glacier was retreating, Mr Lang's measurements were in line with the glacier's movement since the 1990s. Previous studies showed the glacier had retreated at anything between 30m and 70m a year since the early 1980s and 1996.
''This retreat has been going on for quite a while, and some years it's more and some years it's less.
''What's going on at the moment, where the main erosion is occurring now is where there used to be a glacier, but now there is a lake.
''Just the fact that there is a lake there has an effect on the glacier. The lake eats away at the ice, so you get a rapid calving of the glacier, until the glacier actually works its way out of the lake again.
''The resultant very rapid glacier retreat is more to do with glacier/lake interactions than climate change.''
A study in the 1990s had predicted the glacier would continue to retreat until it was 5km from its original terminus, he said.
He said, according to that theory, the existing lake would have to become ''twice as long'' as it was at present before the retreat stopped.
The Caroline face of Mt Cook is featured on the $5 note as a backdrop to the image of Sir Edmund Hillary, but although Alpine Recreation already offered easier ways to complete the whole Ball Pass crossing, company director Anne Braun-Elwert said if the glacier walls collapsed completely, trampers and mountaineers would not be able to experience the Ball Pass crossing as the original pioneers did.
''Each time there is a heavy-rain warning, we wonder of this could be the time the moraine wall gives way.''