New analysis of data and photo records of melting glaciers in New Zealand has found human-induced climate change increased the likelihood of mass ice melt.
The new modelling techniques were applied to 10 glaciers in New Zealand to reveal a more detailed picture of what is driving the accelerating rates of ice melt in the region.
Lead researcher Lauren Vargo said the analysis compared ice melt under pre-industrial greenhouse gas emissions and current-day emissions to uncover "a quite extreme result".
A snowline survey of New Zealand's glaciers taken in 2018. Source: Supplied/Dave Allen/National Institute of Water and Atmospheric Research New Zealand
"Some of the glaciers were at least six times more likely to have experienced that high mass loss because of humans," said Dr Vargo, who is based at the Victoria University of Wellington.
"We’re really confident with that number because that is the low end. The high end is 350 times more likely.
"But with the high ice mass loss event we saw [in 2018], it would not have happened at all without humans."
The to make a direct link between human-induced climate change and glacier melt focused on glaciers in the Northern Hemisphere where more robust data records exist. The study - published in the journal Science in 2014 - looked at records between 1851 and 2010 using a running mean over 20-year periods.
Dr Vargo said the rate of ice melt has resulted in the loss of a quarter of New Zealand's glaciers since the 1970s.
Rapid melt events in 2011 and 2018 prompted the study.
"We’re going to see more extreme temperature events. And it is going to happen with more intensity and more frequently. I think the extreme glacial melt that results from that - being able to say that is due to humans, it adds value compared to looking at long-term trends," Dr Vargo said.
"We are looking at these extreme events, compared to the [in 2014 to link glacial melt and climate change] looking at a longer-term overall global warming."
The study accounted for years like 2013 when some glaciers in New Zealand actually gained mass, but not at a rate to combat the overall decline.
Monash University Professor Andrew Mackintosh, one of the study's co-authors, said the findings were remarkable.
"If you look at the paper and look at the graphs within it, it is almost impossible to see the changes of the [2018] big melt event under natural climate change. But under our human-forced climate change, it is also pretty unusual. It is even an unusual event in a system that is modified by climate. The melt is so extreme.
"In a way, our modelling is a bit conservative, and this melt event is almost off the scale, even for a system that is being influenced by humans."
Dr Vargo said the next stage of research will be to apply the modelling framework to the world's 200 glaciers to get a more fine-tuned understanding of the impacts - particularly of big melt events - over single years, instead of decades.
About a quarter of New Zealand's glaciers have been lost since the 1970s. Source: Supplied/Dave Allen/National Institute of Water and Atmospheric Research New Zealand
The research did not include the 2020 Australian bushfire event, which turned the due to the drift of bushfire smoke.
Dr Vargo said adding the same research method and modelling to the 2020 data should reveal a similar trend.
"If we applied this framework we developed in our paper to the melt in 2020, we would probably see similar results. The bushfires do enhance the glacier melt."
