NZ Emerged From Last Ice Age While Europe Shivered

NZ Emerged From Last Ice Age While Europe Shivered, Study Finds

A geological study of a South Island glacier shows that New Zealand began warming up 13,000 years ago at the same time as Europe sank back into an ice-age freeze.

This 1000-year cold period in Europe is known as the ‘Younger Dryas’, after a cold-tolerant species of rose that became widespread at that time.

Previous studies of core samples from Arctic and Antarctica ice sheets revealed a climate see-saw at the end of the last ice age, with warming in the north matching cooling in the south, and vice versa.

But scientific debate has raged over whether the Arctic or the Antarctic conditions were the more important influence worldwide.

The study involved 10 scientists led by Michael Kaplan of Lamont-Doherty Earth Observatory in the United States. The group, including two New Zealanders, studied glacier moraines at the head of the Irishman Stream in the Ben Ohau Range on the eastern side of the Southern Alps.

The findings are published this week in the international science journal Nature.By using improved methods for dating glacier moraines, the Southern Alps study has shown that the Antarctic climate signal matched the behaviour of glaciers in New Zealand.

Flowing glaciers carry rocks and dirt which build up mounds and ridges, called moraines, at the downhill end of the glacier. When glaciers retreat, cosmic rays bombard these moraines, producing concentrations of distinctive isotopes in the glacial rocks.

Scientists can work out when a glacier retreated by measuring the amount of the cosmogenic isotope beryllium-10 in moraines.

The beryllium-10 dating method enabled the scientists to track the glacier’s retreat through time and indirectly work out how much New Zealand’s climate had warmed.

The scientists showed that the Irishman Stream glacier more than halved in size over 1000 years and that this shrinkage was due to local climate warming of as much as 1 degree Celsius.

David Barrell of GNS Science, one of the authors of the paper, said Irishman Stream was the perfect target for this study.

“Although no glacier survives there today, the ice-age glacier lay in a semi-circular basin with simple topography and the moraines are perfectly preserved fossil landforms,” Mr Barrell said.

“We could reconstruct the shape and size of the glacier to a high degree of certainty and estimate the rise in the height of its snowline over time. Thanks to the precise dating, we knew exactly what time interval was represented.

“Glaciers are remarkable thermometers for average summer temperatures, because the summer melt determines the average position of the snowline. It is exceedingly rare to find such a perfectly preserved set of landforms for this type of study. There is probably nowhere else in Australasia where we could have obtained such precise results for this critical time period at the end of the last ice age.”

The scientists offer two alternative explanations for temperatures in the northern and southern hemispheres being out of sync at the end of the last ice age.

In one, the weakening of the Gulf Stream reconfigured the planet’s wind belts, pushing warm air and seawater south, and pulling carbon dioxide out of the ocean and into the air, causing further warming.

The other explanation has the weakened Gulf Stream triggering a worldwide change in ocean currents, allowing warm air to pool in the south, heating up the climate there.

According to study lead author, Dr Michael Kaplan, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, “Knowing that the Younger Dryas cooling in the northern hemisphere was not a global event brings us closer to understanding how Earth finally came out of the ice age.”

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