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%, 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.
To come up with our calculations, we have 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.
These 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 estimate the 1890s extent of ice volume in New Zealand’s Southern Alps was 170 km3, compared to 36.1 km3 now. That disappearance of 75-80% of Southern Alps ice is graphic evidence of the local effects of global warming.
Further large losses of ice in the Southern 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. This represents a likely decimation of ice cover of the Southern Alps over two centuries because of global warming.
And 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 US to Christchurch in NZ.
In 2013, the Intergovernmental Panel on Climate Change estimated mountain glacier melt has contributed about 6 to 7 centimetres of sea level rise since 1900, and project a further 10 to 20 cm from this source by 2100.
The disappearing ice story calls for robust effective climate policy to moderate effects on our landscape and coasts, otherwise future generations will have little of New Zealand/Aotearoa’s “long white cloud” alpine ice left to enjoy.
Read article at The Conversation.