A third of the permanent snow and ice of New Zealand’s Southern Alps has now disappeared, according to our new research based on National Institute of Water and Atmospheric Research aerial surveys.
Since 1977, the Southern Alps’ ice volume has shrunk by 18.4 km3 or 34 per cent, and those ice losses have been accelerating rapidly in the past 15 years.
The story of the Southern Alps’s disappearing ice has been very dramatic – and when lined up with rapid glacier retreats in many parts of the world, raises serious questions about future sea level rise and coastal climate impacts.
Glaciers are large-scale, highly sensitive climate instruments, which in an ideal world we would pick up and weigh once a year, because their fluctuations provide one of the clearest signals of climate change.
A glacier is simply the surplus ice that collects above the permanent snowline where the losses to summer melting are less than the gains from winter accumulation. A glacier flows downhill and crosses the permanent snowline from the area of snow gain to the zone of ice loss. The altitude of this permanent snowline is the equilibrium line: it marks the altitude at which snow gain (accumulation) is exactly balanced by melt (ablation) and is represented by the end-of-summer snowline.
In 1977, one of us (Trevor Chinn) commenced aerial photography to measure the annual end-of-summer snowline for 50 index glaciers throughout the Southern Alps.
These annual end-of-summer surveys have been continued by the National Institute of Water and Atmospheric Research (NIWA). We then use the NIWA results to calculate the annual glacier mass balance and hence volume changes of small to medium sized glaciers in the Southern Alps. Small to medium glaciers respond quickly to annual variability of weather and climate, and are in balance with the current climate.
Not so the twelve largest glaciers: the Tasman, Godley, Murchison, Classen, Mueller, Hooker, Ramsay, Volta/Therma, La Perouse, Balfour, Grey, and Maud glaciers. These have a thick layer of insulating rocks on top of the ice lower down the glaciers trunk. Their response to new snow at the top is subdued, and take many decades to respond.
Up until the 1970s, their surfaces lowered like sinking lids maintaining their original areas. Thereafter, glacial lakes have formed and they have undergone rapid retreat and ice loss.
Continue reading the rest of the story here.
