Photo Credit: Mike Kaplan

Scientist Alice Doughty from the University of Maine takes notes during fieldwork in New Zealand. The boulder she is sitting on is glacial debris from when a glacier advanced and then retreated, allowing researchers to reconstruct past climate by dating the rock's exposure to cosmic rays.

A sudden cold snap in the northern hemisphere about 13,000 years ago, shortly after the end of the last ice age, apparently never touched Antarctica.

In fact, much of the southern half of the planet appeared to be warming up with the atmospheric increase of the heat-trapping gas carbon dioxide while the other side of the equator cooled off for about a thousand years.

That’s the finding from a new study in the prestigious journal Nature that offers evidence that New Zealand was also warming in step with Antarctica. The deep-freeze that affected the northern hemisphere is known as the Younger Dryas, named after the white flower that grows near glaciers.

“Glaciers in New Zealand receded dramatically at this time, suggesting that much of the southern hemisphere was warming with Antarctica,” said lead author Michael Kaplan , a geochemist at Columbia University’s Lamont-Doherty Earth Observatory , in a press release.

“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,” Kaplan added.

Funding for the study came from the National Science Foundation’s Division of Earth Sciences .

The scientists estimate that New Zealand glaciers lost more than half of their extent over a thousand years as the local climate warmed by as much as 1 degree centigrade. Meanwhile, temperatures in Greenland dropped by 15 degrees centigrade, based on data from ice cores.

To reconstruct New Zealand’s past climate, the study’s authors tracked one glacier’s retreat on the South Island’s Irishman Basin. When glaciers advance, they drag mounds of rock and dirt with them. When they retreat, cosmic rays bombard these newly exposed ridges of rock and dirt, called moraines.

By crushing this material and measuring the build-up of the cosmogenic isotope beryllium-10, scientists can pinpoint when the glacier receded. The beryllium-10 method allowed the researchers to track the glacier’s retreat upslope through time and indirectly calculate how much the climate warmed.

The overall trigger for the end of the last ice age, called the Last Glacial Maximum, came as Earth’s orientation toward the sun shifted about 20,000 years ago, melting the northern hemisphere’s large ice sheets. As fresh melt water flooded the northern Atlantic Ocean, the Gulf Stream weakened, driving the north back into a deep freeze.

For years, scientists have tried to explain how the so-called Younger Dryas cooling fit with the simultaneous warming of Antarctica — based on data from ice cores — that eventually spread across the globe.

The Nature paper discusses the two dominant explanations without taking sides, according to the press release from Columbia University’s The Earth Institute . In one scenario, the weakening of the Gulf Stream reconfigured the planet’s wind belts, pushing warm air and seawater south, and pulling carbon dioxide from the deep ocean into the air, causing further warming.

In the other, the weakened Gulf Stream triggered a global change in ocean currents, allowing warm water to pool in the south, heating up the climate.

Bob Anderson , a geochemist at Lamont-Doherty who argues the winds played the dominant role, said the Nature paper adds another piece to the Younger Dryas puzzle.

“This is one of the most pressing problems in paleoclimatology, because it tells us about the fundamental processes linking climate changes in the northern and southern hemispheres,” he said. “Understanding how regional changes influence global climate will allow scientists to more accurately predict regional variations in rain and snowfall.”

NSF-funded research in this story: Michael Kaplan, George Denton and Joerg Schaefer, Columbia University, Award No. 0745781 ; and Robert Finkel, University of California, Berkeley, Award No. 0746190 . Other researchers involved in the Nature study include  Roseanne Schwartz, Columbia University; Aaron Putnam, University of Maine; David Barrell, GNS Science, New Zealand; Trevor Chinn, Alpine and Polar Processes Consultancy, New Zealand; Bjørn Andersen, University of Oslo; and Alice Doughty, Victoria University of Wellington. Adapted from a press release from Columbia University’s The Earth Institute.