What did NZ's hottest summer do to our glaciers?

View southwest toward the head of Mueller Lake and terminus of Mueller Glacier notice stream dissecting stagnant ice at head of lake.  Picture is from Noel Potter, UMaine,  2/2018 


Climate scientists expect to find some "pretty pathetic" glaciers when they make this year's aerial survey of the South Island's ice-starved, post-summer snowline.

What they find could be one of the biggest "melt years" yet seen...

Niwa this week confirmed New Zealand's summer had been the hottest on record and this would have put a major dent in the amount of snow amid the postcard Southern Alps.

The Niwa-led annual survey, which has now been running 40 years, each March recorded the snowline altitude of up to 50 glaciers across the South Island.

It revealed how much of the previous winter's snow remained to contribute to long-term glacial ice accumulation.

This time, the glaciers might likely be in a grim state...

"We have seen preliminary photos of some of them and they look sad and dirty," said Niwa climate scientist and survey leader Dr Drew Lorrey.

"At this time of year we can see the effects of the summer melt but following such an extreme summer the layers really start to peel back and you can see how harsh the effect has been on the glaciers.

"Where it becomes a concern is if there is a succession of seasons like this within a decade or two – that's when it can cause the overall volume of the glacier to decline."

Glacier fluctuations are among the clearest signals of climate change, because they are highly sensitive indicators of atmospheric temperature and precipitation levels.

Scientists believe it was a warming planet that had partly caused New Zealand's glaciers to shrink in total volume by one third in just four decades.

Victoria University glaciologist Professor Andrew Mackintosh said he was also expecting one of the largest melt years ever recorded.

"Our team has previously investigated the relationship between the South Island glaciers and sea surface temperatures," Mackintosh said.

We have seen that when the Tasman Sea is warmer than normal, you tend to lose a great deal of snow and ice in the Southern Alps.

"The marine heatwave this summer, where temperatures have been up to 6C higher in some parts of the Tasman Sea, means we are expecting to see a much higher snow line."

The survey was undertaken every March at the end of summer and carried out using specialised cameras from a light aircraft.

Lorrey said the information gathered over the past four decades has produced a unique and incredibly valuable dataset that provides an independent measure of how climate change and variability are affecting our water resources.

"We look at the surface of each glacier and the line of demarcation where there is snow from the previous winter above, and exposed bare ice below.

"That line can tell you about the amount of snow gained versus the amount lost since the start of the glacier year in April."

The five scientists on board the snowline flight – Lorrey and Trevor Chinn, together with Dr Huw Horgan, Dr Brian Anderson and PhD student Lauren Vargo from Victoria University - will take thousands of photos from different angles that will then be used to build 3D models of glaciers that can be compared year on year to give an accurate depiction of the volume of ice that has changed.

Mackintosh said powerful computing methods were used to process the photos, enabling precise measurement of snowlines, and the glacier surfaces.

Over the 40 years of the survey, the precision has evolved to the point it now "takes the guesswork out of expert judgement".

Analysis of the photographs provides absolute numbers on how the snowline had changed which act as the benchmark for evaluating computer models of the glaciers.

"It is a tremendous resource that gives us quantitative digital information on how glaciers have changed," Mackintosh said.

It also allows us to reconstruct length changes for glaciers that have never been measured on the ground."

Image / Supplied
Image / Supplied

For the first time this year a thermal imaging camera will be used that Lorrey hoped will reveal more about the debris-covered ice.

"While it is experimental, we hope it may tell us something about the thickness and extent of the debris cover and the properties of the ice underneath it."

The survey comes after scientists revealed New Zealand's total glacier area had shrunk from 1240 sq km to 857 sq km - a decrease of 31 per cent since the late 1970s, or just under one per cent of loss each year.

The number of glaciers also fell slightly from 3283 to 3180, while mean altitude climbed from 1859m to 1939m above sea level.

Those figures followed a 2014 analysis by Chinn and other scientists which showed ice volume in the Southern Alps had shrunk by 18.4cu km or 34 per cent since the 1970s, and ice losses had been accelerating rapidly since the turn of the new century.

Whether the overall trend of ongoing loss continued was dependent on how the world acted on climate change.

One scenario that assumed future warming could be limited only to another 2C - the ultimate goal of the Paris Agreement on climate change - would see glaciers keep retreating but stabilizing by the middle of the century.

But if emissions continued to ramp up without any efforts to curb them, glaciers could become virtually unrecognizable by 2100.

Around the world, glaciers were already melting at an unprecedented rate, losing on average between half a metre and metre of ice thickness every year.

An interactive juxtaposition of the Brewster Glacier, pictured in 2016 and 2017, can be viewed here.

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David Pike, Editor