Rivers benefit as global warming pushes large glaciers into retreat

South Island rivers are benefiting from melting glaciers caused by global warming, but the extra flows will not last forever, warns a glacier expert.

National Institute for Water and Atmospheric Research consultant glaciologist Trevor Chinn, who watches the country's 3140 glaciers, said many of the South Island's largest were retreating rapidly.

As they melted, the glaciers added millions of litres of water to the alpine rivers.

The glaciers in the Southern Alps cover 1158sq km and hold 53cu km of snow and ice.

"Current flows are higher than what you would expect from rainfall," said Mr Chinn. "What we are seeing is borrowing water from glacier storage for the Waitaki and Clutha Rivers."

The rapidly melting Tasman Glacier, for example, is boosting flows in the Waitaki River system by more than four cumecs (cubic metres a second).

A 0.5C to 3C rise in average global temperature would see the glaciers reduce in volume 25 to 50 per cent.

That could add up to 8 cumecs of water to the Waitaki system.

"The prediction is that flows in hydro rivers will have a component of water from ice storage. They will be doing better than if the glaciers were advancing."

For the past 20 years, New Zealand glaciers as a whole had been gaining slightly rather than retreating.

But Mr Chinn estimates that in the past century the country has lost 23 to 32 per cent of its glacial area, "making today's extent of glacier ice probably less than at any other time during the past 5000 years".

"Glaciers worldwide are retreating with global warming," he said. "We predict there will be future glacier retreat in New Zealand.

"In the immediate future, a lot of rivers will be running water from ice storage, which is going to take a long time to replenish."

Mr Chinn said the glaciers could not retreat forever.

"Even with a lot of climate warming, it is not going to make them disappear, but they will retreat until they get into equilibrium with the present climate."

How long the retreat lasts depends on the size and shape of the individual glaciers.

The ice-melt flows are expected to taper off in the next 50 years.

"If there was a reversal of the climate and glaciers advanced, you would get water going back into ice storage, but that is highly unlikely," said Mr Chinn.

The flows due to ice melt would be significant in a desert country but were only a small proportion of the water in rivers because of the large amount of rain that fell in New Zealand, he said.

His calculations show that before the arrival of European settlers, there were about 100cu km of glacial ice in New Zealand. That has reduced to about 53cu km, and predictions are that by 2070 it will fall to just 25cu km.

Worldwide, glaciers have generally been in recession since the end of the so-called "Little Ice Age" - a period of cooler temperatures that lasted for 500 years and ended in the 19th century.

During this period, glaciologists believe New Zealand's glaciers reached a maximum probably around the 1600s, after which they began a gradual recession.

Large-scale wasting only started last century, notably after 1950.

An example of extreme retreat is New Zealand's eighth-largest glacier, the Godley, in Mt Cook National Park. In 1862 explorer Julius Haast saw the Godley Glacier bulldozing vegetation as it advanced, but since then it has been in serious retreat, and its terminus is now about 6km farther up the valley.

- NZPA