The annual snowline survey

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It's a bright blue morning in Queenstown. Summer has been and gone, and the first hints of autumn are starting to appear. Leaves turning, a sharpness in the mornings, the first overnight frosts.  

And as this shift begins, it's also time for another annual event - the end of summer snowline survey flight to monitor New Zealand's glaciers.  

How to build a glacier 

A glacier forms when snow builds up over time, turns to ice and then begins to flow downwards under the pressure of its own weight. For this to happen, you need snow accumulating.   

The snowline is an imaginary line that traces along mountain slopes and marks the lower limit of permanent snow cover. Below this line, snow and ice melt away, above it, snow sticks around.  

At the change of seasons, snow below this line from the previous winter will have melted, and if you time it right, and no new snow has fallen, you can fly a plane past a glacier and photograph the end-of-summer snowline. By repeating this each year, researchers can track changes happening to our glaciers over time. 

'Trev used to run on excitement and liquorice'  

The survey began in 1977. Back then, it was designed and led by a scientist called Trevor Chinn. After completing an inventory of all New Zealand's glaciers, and coming up with a total of more than 3100, Trevor realised that it just wasn't practical to monitor every individual glacier.

Instead, he developed a list of 51 'index' glaciers that would be surveyed each year. Using aerial photographs that showed the end of summer snowline height, they would be able to estimate ice volumes of the glaciers year-on-year - and keep an eye on changes.  

Though the survey baton was passed on to Dr Drew Lorrey of NIWA in 2009, Chinn continued to go on the annual flights until his passing in 2018. As a nod to him, there's still a bag of liquorice opened and passed around on each flight. 

"Trev used to run on excitement and liquorice," says Lorrey . "There's a really rich legacy that we've got to do justice to, in terms of carrying on, but also making sure that the science grows and that it actually is applied with a purpose." 

Part of 'growing the science' includes 3D modelling work, undertaken by NIWA's research collaborators since 2016. While the snowline photography does allow calculation of ice volume estimates, this new 3D method, completed by Dr Lauren Vargo at Victoria University of Wellington, uses hundreds of photos of a smaller number of glaciers to create detailed models for tracking volume changes in metres of ice.  

The glaciers are shrinking 

In her office, Vargo pulls up the data she's collected on the Brewster Glacier near Makarora in Mount Aspiring National Park. A heatmap shows the change in glacier height between 2016 and 2023. At the terminus, or end, of the glacier the map is a deep red, representing the greatest volume of ice lost from this region - about 35 metres.  

It's hard to picture this, so Lauren looks it up in terms of building size. It's shocking what 35 metres equates to: "If you were on the surface of the glacier eight years ago in 2016, you were a 10-storey building higher than if you were on the surface of the glacier today," she explains. 

In his 15 years of leading the survey, Lorrey has witnessed these changes firsthand. It's "really hard," he says, to watch it happen. But the work helps inform what our glaciers' future looks like - depending on what emissions and warming path we choose to take.   

Listen to climb aboard the survey flight with Claire Concannon and learn how and why the team go about their work. 

Learn more 

• Learn about the complete life cycle of snow in ‘The science of snow’. 

• The annual snowline survey flight photographs the glaciers of the South Island, but there are glaciers in the North Island too!