The glacial lake below the Hooker Glacier (centre left).
The glacier's retreat means the moraine walls on the
right-hand side are eroding, affecting access by visitors.
A Lake Tekapo guided tour company fears that an alpine
vista shown on the New Zealand $5 note could become lost to
view at any moment, as the Hooker Glacier retreats at a rate of
more than 2m every month.
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
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.''