Glaciers are large-scale, highly sensitive climate instruments which ideally should be picked up and weighed once a year.
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 always 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).
One of us - the late Trevor Chinn - began aerial photography to measure the end-of-summer snowline for 50 index glaciers throughout the Southern Alps of New Zealand since 1977.
The annual surveys have been continued by the National Institute of Water and Atmospheric Research (Niwa).
These are used to calculate annual glacier mass balance and hence volume change of small to medium glaciers in the Southern Alps and are in balance with the current climate.
Not so the 12 larger debris-mantled glaciers. These have long response times maintaining their areas while their surfaces have lowered like sinking lids and glacial lakes have formed since the 1970s.
An example of the two types of glacier response is seen in the 20th century response, as temperatures have warmed by 1C.
During the 65 years up to the mid-1970s, glacial retreat was massive.
Most of the numerous fast-response, small and moderate-sized glaciers had shrunk rapidly enough to virtually keep pace with climatic warming.
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These were free to respond to current climate variability of the positive snow gains that came with the increased and decreased precipitation that occurred in the Southern Alps after 1976.
Not so the larger debris-mantled glaciers, however; these have continued to adjust to the post-1910 warming phase through to the present. Rapid glacial lake expansion and glacier calving is now changing this situation.
The story of disappearing ice in New Zealand's Southern Alps since 1977 has been very dramatic. In early 2007 we used the snowline survey data plus earlier topographic maps and a GPS survey of the ice levels of the larger glaciers to calculate total ice-volume changes for the Southern Alps, and this data has been updated to 2018.
Between the start of official monitoring in 1977 and last year, the volume had decreased 31 per cent, from 54 km3 to 37.3 km3 in water equivalents.
Forty-five per cent of this was from the 12 largest glaciers, and 55 per cent from the small- to medium-sized glaciers.
This mirrors global trends from mountain glaciers globally.
From 1961 to 2005, the thickness of "small" glaciers decreased about 25 metres, the equivalent of more than 20,000 km3 of water.
Various researchers at the World Glacier Monitoring Service have estimated the 1890s extent of ice volume in the Southern Alps of between 140 and 170 km3.
Further large losses of ice in the Southern Alps have been projected, by glaciologists Valentina Radic and Regine Hock, to between 7 and 12 km3 by the end of the 21st century.
This is based on regional warming projections of 1.5C to 2.5C. This represents a 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, contributing to sea-level rise.
Michael Zemp and colleagues at the University of Zurich estimated recently that mountain glacier melt has contributed 2.7cm of sea level rise (SLR) between 2001 and 2016, and they project at least a further 20cm of SLR from this source by 2100.
The disappearance of 75 per cent of Southern Alps ice in New Zealand from its former volume is the most graphic evidence of the local effects of global warming.
From the coldest part of the 20th century (around 1900) to the 2010s New Zealand regional temperatures have increased by 1.3C, from 13C to 14.3C.
The disappearing ice in turn has global influences as it contributes to SLR, already starting to inundate low-lying islands in the Pacific, and low-lying coastal cities such as Christchurch and Miami.
The disappearing ice story calls for robust, effective climate policy to moderate effects on our landscape and coasts, otherwise future generations will witness the demise of New Zealand into the "land of the short white cloud", with inundation of the sea into Aotearoa's coastal towns and cities.
Professor Jim Salinger is a visiting professor at the University of Florence, the late Dr Trevor Chinn pioneered the first entire mountain range glacier monitoring in the world, and Professor Blair Fitzharris is from the University of Otago.
