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  Glaciers show north-south climate divide
Southern hemisphere glaciers evolve quite differently to those in the north, according to a new study.

Glaciologist Dr Andrew Mackintosh of Victoria University in Wellington, New Zealand and colleagues report their findings in today's issue of the journal Science.

"Don't assume that warming will be uniform over the earth," he says.

Mackintosh says the advance and retreat of glaciers are a good indication of climate change.

But, he says, previous studies of glaciers have not been truly representative of global trends.

"A lot of this work has been led by northern hemisphere researchers who come to New Zealand or Patagonia or wherever, and look for some of the features they've seen in the north," says Mackintosh.

"But the more we're learning about the southern hemisphere we understand that it has its own climate system that's somewhat different."
Reliable method

The researchers, led by Dr Joerg Schaefer of Columbia University in New York, plotted the retreat and advance of glaciers in New Zealand over the past 11,500 years and compared it to data gathered from the northern hemisphere.

They gathered their data by dating the age of the moraines, which are piles of rocks left, like tide marks, as glaciers retreat.

The method, which analyses beryllium-10 isotopes that accumulate as cosmic rays bombard quartz minerals, is the most reliable method available, says Mackintosh.

"It's certainly the most complete chronology of events that has so far been worked out," he says.

The researchers found no real correlation between data from the northern and southern hemispheres.
Bucking the trend

As reported in recent years, Mackintosh and colleagues found some New Zealand glaciers, including Fox and Franz Joseph Glaciers, are growing, despite global warming.

"Even though over the past 100 years or so there's been overall retreat … there can be changes in atmospheric circulation locally which make the glaciers in New Zealand buck that trend," says Mackintosh."

He says glaciers are growing in response to changes such as El Niño which brings cool, wet conditions to New Zealand, and changes in the position of westerly winds.

He says other New Zealand glaciers would also be advancing if it was not for other factors stopping them from responding to that local climate.

Mackintosh says such regional climate could also explain why predictions for New Zealand show less warming than the rest of the world over the next 100 years.
Further back in time

Mackintosh and colleagues found that overall, northern hemisphere glaciers grew until the end of what is known as the Little Ice Age in the 1800s, when they began to retreat.

By contrast, southern hemisphere glaciers have on the whole been shrinking throughout the Holocene.

Mackintosh says long term orbital differences, known as Milankovitch cycles, may explain this but he says more work is required to determine if this is the case.

He says the latest data will be useful in better understanding of how climate works. It will also improve climate models so they better predict how specific regions of the world will respond to global warming.
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