When the world's glaciers shrunk, New Zealand's grew bigger

Wednesday, 15 February 2017

Julian Thompson from GNS Science at Franz Josef Glacier in 2005.

The question has puzzled scientists: when the world's glaciers were retreating, why were New Zealand's glaciers getting bigger?

New research from a group of New Zealand scientists may have solved the mystery, but it's not good news; New Zealand's glaciers are now likely to continue melting at a dramatic rate.

Between 1983 and 2008, when the vast majority of the world's glaciers were shrinking in a warming world, at least 58 New Zealand glaciers grew bigger.

The Southern Alps was one of a handful of areas internationally where glaciers were growing – In 2005, 15 of the 26 advancing glaciers worldwide were in New Zealand.

* Our frozen assets slowly melting away

* Plight of New Zealand glaciers is global news**

Some glaciers advanced between 1980 and 2011, but the long-term trend is of dramatic decline.

In that time, Franz Josef Glacier regained nearly half the mass it had lost during the 20th century.

The unusual period of advance came after several decades or rapid decline, which for many glaciers has resumed since 2008.

Many of the country's largest glaciers have retreated substantially since 2011 with several on track to disappear entirely.

The reason for that 25 year period of growth had been unknown, but newly published research points at a run of cooler temperatures caused by the South Pacific's unusual climate variability.

Scientists from Victoria University of Wellington and Niwa used computer modelling to analyse more than a decade of field observations and 30 years of glacier photographs.

Their hypothesis had been that higher rainfall levels were the primary cause of the advancement.

While conducting their research, they found a stronger link to cooler air temperatures, caused by southerly flows and a cooler Tasman Sea.

It explained why New Zealand's glaciers were almost uniquely affected – the cooler temperatures were specific to the climate variability in this part of the world.

'It may seem unusual, this regional cooling during a period of overall global warming, but it's still consistent with human-induced climate change,' lead author Andrew Mackintosh of Victoria University said.

'New Zealand sits in a region where there's significant variability in the oceans and the atmosphere – much more than many parts of the world. The climate variability that we identified was also responsible for changes in the Antarctic ice sheet and sea ice during this period.'

Since 2011, most New Zealand glaciers had gone into rapid decline.

Both Franz Josef Glacier and Fox Glacier are no longer accessible by foot due to their retreat.

Franz Josef has one of the fastest melt rates observed of any glacier in the world, and Fox Glacier is predicted to lose five kilometres of mass by the end of the century.

In the 1970s, Tasman Lake at the foot of the Tasman Glacier didn't exist. It is now seven kilometres long and deeper than Lake Pukaki due to the glacier's ice loss, and is expected to grow much larger.

In total, the Southern Alps had lost about a third of its ice mass over the last four decades.

Mackintosh said that although the period of advance may seem promising, the future 'doesn't look good' for New Zealand's glaciers.

'New Zealand's glaciers are very sensitive to temperature change. If we get the two to four degrees of warming expected by the end of the century, our glaciers are going to mostly disappear.

'Some may experience small-scale advance over that time due to the regional climate variability, but overall they will retreat.'

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