Fears as New Zealand's largest glacier continues to shrink

A guide on New Zealand's largest glacier says he's seen dramatic changes in the four years he's been working there.

The 23-kilometre-long Tasman Glacier holds a third of all New Zealand's glacial ice, and there are fears for its future.

Will Johnson is a Glacier Explorers Guide, located high up in the remote and beautiful Aoraki Mount Cook National Park in Canterbury.

He spends each day taking boats of tourists out onto the Tasman terminal lake at the foot of the glacier. 

After just four years, he's noticing dramatic changes in New Zealand's largest glacier.

"This is the exact spot basically where the centre of the glacier was when I first started this job back in 2015," Johnson told Newshub.

The 23-kilometre-long Tasman Glacier is retreating. As the ice melts, the terminal grows.

Scientists estimate the terminal lake is growing at a rate of about 140 metres annually. It's now over six kilometres long.

Lauren Vargo and her colleagues at Victoria University are using aerial photography to build 3D models of New Zealand glaciers, including Tasman.

"The retreat rate has increased by about three times from the early 2000s, through the later 2000s, into this decade," says Victoria University PHD student Lauren Vargo.

NIWA scientist Dr Andrew Lorrey flew over the glacier with a thermal imaging camera earlier this year. He says it's apparent the Tasman Glacier is responding to temperature changes.

"When you have warmer temperatures you have more heat in the mountains... with more heat, you have more [melting] of snow and ice, so [it] starts changing the balance of the glacier - towards where it's losing ice rather than gaining it," says Dr Lorrey.

There is no denying in Johnson's mind what is causing New Zealand's largest glacier to shrink.

"Put it this way, you don't get too many people coming out on these tours and denying climate change, especially after they see this," he said.

What the future holds for New Zealand's largest glacier - only time will tell. 

For now, Johnson will continue doing the job he loves, showing tourists the glacier he believes won't be around forever.