As the last ice age was ending, about 13,000 years ago, a final blast of cold hit Europe, and for a thousand years or more, it felt like the ice age had returned.
But oddly, despite bitter cold winters in the north, Antarctica was heating up.
For the two decades since ice core records revealed simultaneous warming and cooling at opposite ends of the planet during this time period, scientists have looked for an explanation.
“New advances in the use of cosmogenic isotopes [used in this research] allow dating with hundreds of years’ resolution, and correlation of key deposits such as the moraines in New Zealand,” said Enriqueta Barrera, program director in the National Science Foundation’s (NSF) Division of Earth Sciences, which funded the research.
About the image: Geologists have found clues to past climate changes in New Zealand’s melting glaciers. Image credit: Government of New Zealand
“Further application of this technique will reveal the details of climate change in different regions since the last glaciations.”
The finding indicates that the deep freeze up north, called the Younger Dryas for the white flower that grows near glaciers, bypassed much of the southern hemisphere.
“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.”
Ice core records show that warming of the southern hemisphere, starting 13,000 years ago, coincided with rising levels of the heat-trapping gas, carbon dioxide (CO2).
About the image: Thick ice once filled New Zealand’s Irishman Basin, from which geologists took samples. Image credit: Aaron Putnam
The study is the first to link this spike in CO2 to the impressive shrinking of glaciers in New Zealand.
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.
About the image: Glacial debris samples allow scientists to retrace ancient glaciers’ paths. Image credit: Mike Kaplan
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 came as Earth’s orientation toward the sun shifted, about 20,000 years ago, melting the northern hemisphere’s large ice sheets.
As fresh meltwater flooded the North Atlantic Ocean, the Gulf Stream weakened, driving the north back into the ice age.
During this time, temperatures in Greenland dropped by about 15 degrees C.
The Nature paper discusses the two dominant explanations without taking sides.
Bob Anderson, a geochemist at Lamont-Doherty who argues that winds played the dominant role, says the Nature paper adds another piece to the puzzle.
Other researchers involved in the study were: Joerg Schaefer and Roseanne Schwartz, also of Lamont-Doherty; George Denton and Aaron Putnam, University of Maine; David Barrell, GNS Science, New Zealand; Trevor Chinn, Alpine and Polar Processes Consultancy, New Zealand; Bjørn Anderson, University of Oslo; Robert Finkel, University of California, Berkeley; and Alice Doughty, University of Wellington.
*Source: National Science Foundation (NSF)