Shaun Eaves talks about glaciers in the North Island and how evidence left behind by glaciers can help reconstruct past climates.

Talk about glaciers in New Zealand and most of us think about the great glaciers of the South Island - Fox and Franz Josef glaciers on the West Coast, and Tasman Glacier near Mount Cook.

But glaciologist Shaun Eaves has been pondering the forgotten glaciers of the North Island, and their past and future.

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Making a glacier

"The recipe for a glacier? You need cold temperatures and precipitation," says Shaun. "Snow is what the glacier wants."

Over time this snow compacts to become ice and eventually becomes large enough to be counted as a glacier.

Weather is critical to a glacier's survival.

"The colder the temperature, the less snow and ice you'll lose over the course of a year, particularly during summer."

"If you manage to keep some of your winter snow through the summer then that will add mass, and either sustain or start to build a glacier," says Shaun.

Sweet spot for a glacier

Glaciers occur at higher altitudes, where it is colder.

Shaun says they exist above a hypothetical line that glaciologists call the snowline, the altitudinal point where it is cold enough for snow to last year round.

Snowline in the South Island is about 2000 metres above sea level, so there are plenty of mountains higher than that where glaciers can form.

Although glaciers are born at higher colder altitudes, their ice can flow well below the snow line, almost to sea level in the case of Fox and Franz Josef glaciers.

Shaun says there are more than 3,000 glaciers in the South Island, although some smaller ones at lower elevations have disappeared in the past few decades.

The North Island's forgotten glaciers

He says that people may be surprised to know that there are still glaciers in the North Island, although it's difficult to put an exact number on them as defining exactly what a glacier is can be difficult.

"To me, a glacier is a perennial body of snow and ice that is deforming under its own weight," says Shaun.

Shaun says that physics shows that an ice patch needs to be at least 30 metres thick for it to start deforming.

There has not been any research to measure ice thickness on Mount Ruapehu, which at 2797 metres tall is the only North Island mountain high enough to rise above the snowline. The snowline is at about 2600 metres on the central volcanic plateau…

Go to this episode on rnz.co.nz for more details