Scientists have gained a wider view of how New Zealand's postcard glaciers are gradually being lost to climate change, with a new study pushing records back to the middle of the last century.
Since 1977, researchers have been tracking changes in the Southern Alps' snowline by photographing 50 "index" glaciers during annual fly-overs.
That record, launched by the late and influential glaciologist Dr Trevor Chinn, ultimately allowed scientists to determine that around a third of the South Island's glacier ice had been lost within just four decades.
That was the equivalent to the basic daily water use requirements of our current population for that whole period.
In a new study, scientists have used observations from New Zealand's largest glacier, the Tasman, to extend the record back as far as 1949.
When end-of-summer-snowline (EOSS) altitude measurements weren't available, researchers drew on co-relating air temperature, regional climate, and hemispheric scale climate data to piece together the picture.
"By combining data available from multiple sources, this research provides a new depth of understanding to the situation of our end-of-summer-snowlines," said study author and veteran climate scientist Professor Jim Salinger, of Victoria University's School of Geography and Earth Sciences.
"They're getting higher as the climate warms, and over the last 70 years the rise has been quickest in the last decade."
From 1949 to 1955 the altitude of snowlines varied little, but then rose until around 1980.
Thereafter, until 2010, snowline levels stayed about the same with small fluctuations – but, between 2010 and 2019, they climbed by a further 200m above normal.
The overall increase since 1949 is most striking, averaging 42 metres per decade - or a total of 300m.
Emeritus Professor Blair Fitzharris, of Otago University's School of Geography, said retreat and thinning of the world's alpine glaciers were icons of climate change and have important knock-on effects.
"Sea-level rise, and diminished water availability for purposes such as hydro-electricity production and agriculture have enormous implications for New Zealand," he said.
"Our glaciers and snowfields are like water tanks, providing river flow in times when there is little rainfall."
The study, published in the International Journal of Climatology, also credited Chinn as a co-author.
"We were writing a preliminary draft of this article when Trev died suddenly of a stroke on December 20, 2018," Fitzharris said.
"His vision and meticulous data collection over many decades, has left an important legacy to the Antarctic and New Zealand snow and ice research community."
The findings come after research published in major journal Nature found that New Zealand's glaciers had been thinning at a rate of 1.5m a year between 2015 and 2019 - a nearly sevenfold increase compared to the period from 2000 to 2004.
While Niwa's most recent aerial stocktake has found the glaciers fared better than expected last summer, the long-term prognosis for the icy wonders remains dire.