Scientists are beginning to suspect that a change in global wind patterns helped end the last major ice age, and that the effects showed up first on New Zealand glaciers.
In a paper published this week in the journal Science, a team of researchers looked to a global shift in winds to explain why New Zealand and other sites in the southern hemisphere enjoyed extreme warming at a time when it was thought evidence only existed to explain a northern hemisphere warming.
Mountain glaciers in New Zealand and South America started to melt 18,000 years ago, at the peak of the last ice age. By 16,000 years ago the glaciers were in full retreat.
Research led by Columbia University's Lamont-Doherty Earth Observatory indicates that westerly winds in the southern hemisphere shifted south, which would have brought warm air and sea water to the mid latitudes and started the warming of the glaciers.
"This paper pulls together several recent studies to explain how warming triggered in the north moves to the south, ending an ice age," said study co-author Bob Anderson, a geochemist at the earth observatory.
"Finally, we have a clear picture of the global teleconnections in Earth's climate system that are active across many time scales. These same linkages that brought the Earth out of the last ice age are active today, and they will almost certainly play a role in future climate change as well."
The Earth enters an ice age about every 100,000 years as its orientation toward the sun shifts, and at the peak of the last ice age massive ice sheets stretched from the North Pole all the way down to as far as New York City and the Himalayas, and almost completely covering Canada.
The ice age was sent into retreat, at least initially, by an orbital shift of the planet which caused more sunlight to fall across the northern hemisphere, where floods of fresh water from the melting ice sheets effectively shut down the Gulf Stream carrying warm equatorial water into the North Atlantic.
Without the warm water, sea ice spread again across the North Atlantic, but the big new freeze in northern Europe triggered a profound shift in wind patterns, with tropical trade winds moving south.
Those winds also brought hot air and warm seawater south from the equator, which caused the beginning of warming in the southern hemisphere, including New Zealand.
The westerly winds in the Southern Ocean turned to southerlies which churned the ocean, releasing large amounts of carbon dioxide from the water into the air. Ice core records show that between 18,000 and 11,000 years ago atmospheric carbon dioxide levels rose from 185 parts per million to 265 parts per million, in comparison to today's levels of 393 parts per million after a sustained industrial era.
This mechanism would help scientists explain just what carbon dioxide did in the last ice age: Glaciers New Zealand and Patagonia started melting before the rise in carbon dioxide levels, but the shifting southern hemisphere westerlies could explain that rapid warming.
New Zealand researchers last year worked with a team from Columbia University to calculate in detail the retreat and advance of glaciers in New Zealand over the past 11,500 years.
They found no apparent correlation with what happened at the same time in the northern hemisphere, and said the south had its own climate drivers, which could have important implications in the context of global warming.
