Kiwi scientists have just drawn one of the first direct links between climate change and glacier melt, with a new study suggesting that warming made a recent hit on our icy wonders 10 times more likely.
The study, just published in major journal Nature Climate Change, further highlights how glaciers have been proving themselves coalmine canaries in flagging some of the earliest and dramatic impacts of a fast-heating planet.
It was also just the second to directly and formally link the two.
"This is important, because having multiple studies in agreement means we can be even more confident that there is a link between human activity and glacier melt," said lead author Dr Lauren Vargo, of Te Herenga Waka—Victoria University of Wellington's Antarctic Research Centre
"This confidence is especially important for documents like the Intergovernmental Panel on Climate Change reports, which use findings like ours to aid and inform policy-makers on climate change."
The study focused on a group of 10 glaciers in the South Island.
"It's very likely that the extreme melt of these New Zealand glaciers was caused by human greenhouse gas emissions," Vargo said.
"Specifically, our results show that high levels of melt in 2011 were six times more likely to have happened due to climate change, and high levels of melt in 2018 were at least 10 times more likely to have happened due to climate change.
"These increases in likelihood are due to temperatures that are 1C above pre-industrial levels, confirming a connection between greenhouse gas emissions and high annual ice loss."
This study began in response to observations made during the 2018 End of Summer Snowline Survey – a flying stocktake scientists have been carrying out around the Southern Alps since the late 1970s.
The survey has been tracking how the snowline position on select glaciers and ice geometry – or their thickness and flow – has changed using aerial photos, which then allowed them to determine the level of growth or shrinkage.
"This research shows one application of our work to monitor glaciers and reinforces the value of long-term snow and ice observations," said Niwa scientist Dr Drew Lorrey, who co-ordinates the survey and served as a co-author on the latest paper.
"The impacts from recent extreme years we've seen is concerning—the results from this study indicate human activities contribute to those years and load the dice against our glaciers."
During 2018 – the year of New Zealand's hottest summer – the survey team observed the least amount of snow on the glaciers since the survey began.
They then wanted to determine to what extent this lack of snow was due to human influence.
"We used a method called extreme event attribution, which is used to calculate the human influence on extreme climate events like heatwaves and droughts," Vargo said.
"To get these results, we developed a framework that uses extreme event attribution together with calculating glacier mass changes with computer models.
"For example, the extreme loss of mass we saw of one glacier—the Rolleston glacier—in 2011 would be a 1 in 100 year event under natural conditions, but due to climate change this has become a one in 8 year event.
"Our results show that New Zealand glaciers are melting because of greenhouse gases emitted by humans.
"Glaciers in New Zealand are important for many reasons, including tourism and water resources, so we hope that our findings will encourage and convince people around the world, but especially Kiwis, that we need to take stronger actions to stop climate change."
Vargo and her colleagues now want to apply this method to studying more glaciers.
"There are over 100 glaciers globally that have annual measurements of mass change available, so we can use these with our new method to calculate the human fingerprint on glacier melt around the world," she said.
"We know that many glaciers globally experienced their highest levels of melt in the last decade, so we look forward to investigating the link between this melt and human climate influence.
"Ultimately we hope this research can contribute to evidence-based decision making on climate change."
The research comes after earlier calculations presented by climate scientist Professor Jim Salinger indicated that, over 2017-18, the alps experienced their biggest snow melt on record, losing about 9 per cent of their total ice volume.
Other figures, reported to the American Geophysical Union in 2017, revealed how New Zealand's total glacier area had shrunk from 1240sq km to 857sq km - a decrease of 31 per cent since the late 1970s, or just under 1 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 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 stabilising by the middle of the century.
But if emissions continued to ramp up without any efforts to curb them, glaciers could become virtually unrecognisable 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.
The new study also involved Dr Huw Horgan, Dr Ruzica Dadic, and Associate Professor Brian Anderson from the Antarctic Research Centre, Professor Andrew Mackintosh from Monash University and Dr Andrew King from the University of Melbourne.