Climate scientist Dr Andrew Lorrey said the annual end-of-summer snowline survey showed it was equal to the basic drinking water requirements for all New Zealanders over the same timeframe.
However, the volume lost from Brewster Glacier is only a small part of the Southern Alps’ ice storage, which has been in decline since annual monitoring of the glaciers began in the late 1970s.
Dr Lorrey, who co-ordinates the aerial survey each March, said about 30% of the Southern Alps’ ice volume had been lost over the past four decades, which was equivalent to 15.9trillion litres of water.
This year, scientists from Niwa and Victoria University (Wellington) recorded more snow on the South Island glaciers than the previous two years, but warned it could not be considered any kind of recovery.
Dr Lorrey said this year’s snowline was part of standard climate variability, in which some good snow years were to be expected among the trend of increasing frequency for warmer temperatures.
However, he said it would take between 20 and 30 years like this one, to "even start to consider whether the recent damage can be reversed to any degree".
That was because the previous two years had produced particularly harsh conditions, as marine heatwaves and record temperatures took their toll on the glaciers and pushed snowlines off the top of many mountains.
In order to grow glaciers, the long-term permanent snowline must be below the mountain top, so winter snow can be retained to form ice.
He said the damage sustained by some small glaciers between 2018 and 2019 had placed them on a path to extinction.
In addition, ash from the Australian bushfires blanketed the Southern Alps ice, absorbing more solar radiation and increasing the potential for more melting.
Dr Lorrey said the photos taken of key index glaciers during each annual survey, were increasing in importance because they provided scientists with a better idea of this part of New Zealand’s water reserves.
"Over the past few years, our observations of extreme and variable conditions highlight strong impacts on water, which is arguably our most precious natural resource."
He said the long-term glacier photo archive was a valuable scientific resource for future generations and also helped establish how the climate was changing in New Zealand.
