Scientists suspect they’ll find “devastation” when they assess the toll of what’s likely to have been the most damaging summer yet for New Zealand’s melting glaciers.

It’s been a particularly harsh decade for our postcard mountain glaciers, which have lost about a third of their ice volume since scientists began surveying them annually in the late 1970s.

But this dramatic La Niña summer – which brought record-warm sea surface temperatures to the West Coast and historic deluges to the north – may have proven the most damaging yet for our icy wonders.

“I’m expecting devastation,” said Dr Drew Lorrey, who’s preparing for what will be the 45th year of aerial survey of 50 “index glaciers” dotted across the Southern Alps.

Melting glaciers are seen as global warming’s coalmine canaries, as they form the sum of a range of climate components, from temperature to precipitation and sunshine.

The glacier atop Mt Butler pictured last year. The snowline seen across the glacier marks the altitude where a balance between retention or melt of snow and ice are equal. Photo / Drew Lorrey, Niwa

That signal is becoming ever clearer: one recent study estimated how, over the record-warm summer of 2017-18, climate change made the extreme ice loss at least 10 times more likely.

Lorrey said it was possible that this season, the third hottest on Niwa’s books, could have brought similar - or worse - rates of melt.

In January, swathes of water off the West Coast were shown to be running even warmer than balmy eastern shores of the Far North, in an extreme marine heatwave that broke local records.

Typically, sea surface temperatures (SSTs) in the region would be sitting somewhere in the mid-teens at this time of year – but observations over summer showed some coastal waters were as warm as 21C to 22C.

The wider southwest coast region, stretching down to Fiordland, had shown an average anomaly of 3.7C over the week to January 17, with some pockets registering whopping anomalies of more than 5C.

This map shows sea surface temperatures around New Zealand by anomaly (left) and temperature (right) in mid-January. Amid a widespread marine heatwave, sea temperatures off the South Island's West Coast were as warm as - and in some places hotter - than those observed in the upper north. Image / NOAA

For glacier health, warm waters in the Tasman Sea mattered much.

That warmth translated to warmer local air temperatures, which, when carried up to altitude, meant more heat – and more melt – for alpine glaciers.

The melt not only diminished the amount of material built up from the last season’s snow, but also ate into what had fallen over the years prior.

“We’ve had a string of very warm years, many of them associated with protracted marine heatwaves in the Central Tasman Sea,” Lorrey said.

“But the last time we had one this strong was that 2018 event.”

In the latest case, marine temperatures had peaked over a much longer period – something that correlated to a significant impact on snowlines.

“I suspect that, with the longer peak in the marine heatwave - along with the fact the snowline was already riding pretty high – we could be looking at something on par, or maybe even beyond, what we got in 2018,” Lorrey said.

“And if that’s the case, then we’re going to see snowlines at the highest they’ve been since we started measuring in 1977.”

Just as with 2017-18, warmer seas weren’t the only contributing factors to melt.

This summer had also coincided with a third consecutive La Niña - a climate pattern that came with regular hot spells along the West Coast, including a record-high reading of 30.9C in Greymouth on January 8.

Another driver called the Southern Annular Mode had also been predominantly positive – a phase that’s ripened conditions for glacier loss over time – while background climate change continued to steer a warming trend.

One major study published last year found that if contemporary decadal trends continued, the Southern Alps’ average snowline elevation would be displaced at least 200m higher than normal, as early as 2025 to 2034.

Westland's Ivory Glacier, as pictured in March 2022. Photo / George Macara

“If we continue the trend of where our snowlines are heading, then really, by next decade, a significant proportion of the glaciers that we survey are going to be sitting permanently below the summer snowline,” Lorrey said.

For glaciers to survive summer, they had to be sitting near the snowline – but recent surveys had shown some snowlines sitting at the very top of mountains, with others having vanished altogether over the season.

“If this summer has been worse than 2018, then some of the glaciers that are small and lying low are going to have taken such a beating that they’re not really a meaningful body of ice anymore.”

Eleven of the original 50 glaciers, selected by the late glaciologist Trevor Chinn as a baseline for monitoring, were expected to disappear by the middle of next decade, with another 10 swiftly approaching a similar fate.

Park Pass Glacier in Mt Aspiring National Park, as viewed at the end of summer 2022. Photo / Warren Chinn

Last autumn’s survey showed how several glaciers – including the Brewster, a jewel of Mt Aspiring National Park – had snowlines at least as high as in 2016, one of the top five high snowline years.

The nearby Volta Glacier had also been eaten away to the point that layers of ice and snow accumulated from previous years were sitting exposed.

Victoria University of Wellington glaciologist Dr Lauren Vargo said that, over the decade she’d been tracking loss, there’d virtually been just one year where there hadn’t been large rates of melt.

“Because of this ongoing trend, it’s almost been hard to differentiate between years.”

Vargo is now leading a Marsden Fund-supported study exploring the link between climate change and ice loss across 230 glaciers worldwide – making it among the first and largest of its kind.

Glaciers remain important for a range of reasons.

They help regulate streams and rivers, support alpine ecosystems, contribute tens of millions of dollars to our tourist economy and were cherished by mountaineers and mana whenua alike.

“People in New Zealand have a connection with our glaciers, even if they only know the Fox and Franz Josef – which leaves us feeling sadder as we start to lose more and more of them,” Vargo said.

“And that will keep happening as long as temperatures keep warming, if we don’t slow down climate change.”