A third of the permanent snow and ice on New Zealand’s Southern Alps has now disappeared, according to research based on aerial surveys by the National Institute of Water and Atmospheric Research.
The researchers say that that since 1977, the volume of ice on the nation’s Southern Alps has shrunk by more than 18 cubic kilometres or 34%, and that the ice losses have been accelerating rapidly over the past 15 years.
The disappearance of this vast quantity of ice has been very dramatic. And, when lined up with rapid glacier retreats in many parts of the world, it raises serious questions about future sea level rise and coastal climate impacts.
In 1977 aerial photography began to be used 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 institute and now the results can be used to calculate the annual glacier mass balance and hence volume changes of small to medium sized glaciers in the alps.
The researchers say that small to medium glaciers respond quickly to annual variability of weather and climate, and are in balance with the current climate. But not so the 12 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 takes many decades to respond.
Glaciers are large-scale, highly sensitive climate instruments, which in an ideal world, the researchers say they 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. It flows downhill and crosses the permanent snowline from the area of snow gain to the zone of ice loss,” according to the researchers.
“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.”
Up until the 1970s, the surfaces lowered like sinking lids maintaining their original areas. Thereafter, glacial lakes have formed and they have undergone rapid retreat and ice loss.
To produce their calculations, the researchers used the snowline survey data plus earlier topographic maps and a GPS survey of the ice levels of the largest glaciers to calculate total ice-volume changes for the Southern Alps up until 2014.
Over that time, ice volume had decreased 34%, from 54.5 km3 to 36.1 km3 in water equivalents. Of that reduction, 40% was from the 12 largest glaciers, and 60% from the small- to medium-sized glaciers.
In an article in The Conversation, Jim Salinger from the University of Auckland, Blair Fitzharris from the University of Otago and Trevor Chinn, a glaciologist with the institute, say the New Zealand results mirror trends from mountain glaciers globally.
From 1961 to 2005, the thickness of ‘small’ glaciers worldwide decreased approximately 12 metres, the equivalent of more than 9,000 km3 of water.
Martin Hoelzle and associates at the World Glacier Monitoring Service have estimated the 1890s extent of ice volume in New Zealand’s Southern Alps was 170 km3, compared to 36 km3 now.
Salinger and colleagues say that disappearance of 75% to 80% of the ice in the alps is graphic evidence of the local effects of global warming.
Further large losses of ice in the alps have been projected by glaciologists Valentina Radic and Regine Hock, suggesting that only 7-12 km3 will remain by the end of the 21st century. This is based on regional warming projections of 1.5°C to 2.5°C and represents a likely decimation of ice cover of the alps over two centuries because of global warming.
Where does all this melted glacier ice go? Into the oceans, thus making an important contribution to sea level rise, which poses a serious risk to low-lying islands in the Pacific, and low-lying coastal cities from Miami in the United States to Christchurch in New Zealand.
In 2013, the Intergovernmental Panel on Climate Change estimated mountain glacier melt has contributed six to seven centimetres of sea level rise since 1900 and project a further 10 to 20 centimetres from this source by 2100.
The New Zealand team says this disappearing ice story demands a robust and effective climate policy to moderate the effects on landscape and coasts: “Otherwise, future generations will have little of New Zealand’s ‘long white cloud’ of alpine ice left to enjoy.”
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