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How NZ’s largest, longest glacier is melting from within

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Haupapa Tasman Glacier – the longest in the Southern Hemisphere – has been retreating by more than 100m each year for the past few decades. Photo / Dave Allen, Niwa

Scientists have discovered fresh evidence showing how New Zealand’s largest, longest glacier is melting from the inside as well as out, posing wide implications for these icy jewels of our alpine environment.

Flowing some 23km through the heart of Aoraki-Mt Cook National Park, Haupapa Tasman Glacier has shrunken dramatically over recent decades – all while a milky blue proglacial lake that formed at its tongue has been expanding at a rate of more than 100m each year.

Eventually, the glacier as we know it today could become unrecognisable: perhaps just languishing as a remnant wedge of ice, disconnected from its feeder sources higher in the mountains.

While Tasman Lake itself has contributed to much of the glacier’s recent decline through gradually carving its exposed terminus, researchers have been working to understand how its many deep crevasses may be trapping heat.

This effect was something that struck University of Canterbury glaciologist Dr Heather Purdie when she and colleague Tim Kerr were exploring how glacier retreat was impacting mountaineering on Aoraki Mt Cook around the commonly-climbed Linda Glacier route.

Crevasses along Haupapa Tasman Glacier that pose a hazard for mountaineers have also been shown to trap a surprising amount of heat. Photo / Heather Purdie
Crevasses along Haupapa Tasman Glacier that pose a hazard for mountaineers have also been shown to trap a surprising amount of heat. Photo / Heather Purdie

“Our conversations with climbers really highlighted a perception that the crevasses were becoming exposed earlier presenting challenges to the route, this got me thinking about the wider implications to glacier health.”

Purdie explained that, when glaciers had rougher surfaces, this generated turbulence that could help drive melting – and previous theoretical studies had also explored how crevasses could trap incoming radiation from the sun.

Given numerical models typically treated glacier accumulation areas as relatively smooth, snow-covered surfaces, Purdie suspected we may be underestimating the amount of melting going on within the region, which means they would be overestimating mass balance.

Haupapa Tasman Glacier, where she’s been closely studying mass balance processes since 2007, proved the ideal place to test her hypothesis.

Over three field campaigns, she and colleagues installed weather stations at the glacier’s surface, along with sensors measuring temperature and wind speed within its crevasses.

The team also used a drone to map the surface and an infrared camera to measure its surface “skin” temperature.

Their study, supported by a Marsden Fund grant and published in the Journal of Glaciology, showed day-time temperatures inside the crevasses were higher than her team had expected.

“The crevasses essentially trap heat and can at times air inside the crevasse can even be warmer that the air over the glacier surface,” Purdie said.

“I suspected that the air inside the crevasses would warm up, but I was quite surprised how warm they can get.”

Crevasses in Haupapa Tasman Glacier have been trapping a surprising amount of heat - creating a feedback loop in which the crevasses widen and melt the glacier further. Photo / Julian Thomson
Crevasses in Haupapa Tasman Glacier have been trapping a surprising amount of heat – creating a feedback loop in which the crevasses widen and melt the glacier further. Photo / Julian Thomson

Even five metres down inside a crevasse, they found air temperatures could be higher than 8C on clear, sunny days.

“We also discovered that this heat can get transported even deeper into the crevasse with the help of wind turbulence.”

This heating could create a feedback loop, where the crevasses melt more quickly and become larger, enabling greater heat storage and further melting – but Purdie said there was much more to understand.

“There are a number of characteristics that appear to influence how warm the in-crevasse air gets, such as how wide the crevasse is, as well as orientation to the sun and prevailing wind directions.”

Still, the findings raised new questions for similar glaciers, here and overseas.

“The implications of these findings are that currently we will be underestimating melt rates in crevassed regions of alpine glaciers,” she said.

“The next objective of my research is to determine the magnitude of this effect, so that we can develop a simple parameterisation scheme and make recommendations for how this enhancement of melt can be captured in existing models.”

The new insights come as New Zealand heads into its third consecutive La Niña summer – something that doesn’t bode well for glaciers.

“In general, La Niña conditions during summer can be associated with more anticyclones sitting over the central South Island, which tend to be accompanied by sunny settled weather,” Purdie said.

Aoraki Mt Cook National Park's Tasman Lake - which lies at the foot of Haupapa Tasman Glacier - didn't exist until a few decades ago. Photo / Sarah Ivey
Aoraki Mt Cook National Park’s Tasman Lake – which lies at the foot of Haupapa Tasman Glacier – didn’t exist until a few decades ago. Photo / Sarah Ivey

“While that is nice for people out enjoying the mountains, it can also result in more melting.”

All the while, background global warming – which contributed to our glaciers losing a third of their volume within the last few decades – was wreaking a worsening toll.

In one disastrous melt year – New Zealand’s record-warm summer of 2017-18 – scientists estimated global warming made the extreme ice loss at least 10 times more likely.

A new study, also supported by the Marsden Fund, was now underway to tease out the likely impact of human-driven climate change on 230 glaciers around the country.

• People can experience a new online audio-visual exhibition on the glacier’s decline – titled Haupapa: The Chilled Breath of Rakamaomao, and combining western and mātauranga Māori – here.



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