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A new study confirms controversial findings that ice ages may have ended due to warmer summers in both northern and southern hemispheres, not just the northern hemisphere as was previously believed.

The study was led by researchers at the University of Newcastle working with the University of Melbourne and international colleagues in Italy, France, Germany and the UK.

“It was known that wobbles in the earth’s orbit drive the ice age cycle, but there are several theories as to how they do this,” says study co author Dr John Hellstrom of the School of Earth Sciences at the University of Melbourne. 

“We have disproved the most accepted theory and found that variation of the earth’s axial tilt alone has ended recent ice ages, by making summers warmer in both hemispheres at once.

“Previously it was believed that the northern hemisphere alone was key to the ice ages,” he says.

The research team used three stalagmites collected from an Italian cave to support their new findings. Records of past rainfall found in the stalagmites matched previously reported ocean temperature changes recorded in sediment cores from the nearby sea floor.

Lead researcher, Dr Russell Drysdale, of the School of Environmental and Life Sciences at the University of Newcastle, says ocean sediment cores contain a wealth of information about past global climate. But beyond about 50,000 years it is very difficult to determine the exact age of these sediments.

“In contrast, stalagmites from limestone caves can be very precisely dated using trace amounts of uranium incorporated within their structure, “Dr Drysdale says.

“By combining the accurate timescales of the stalagmites with past climatic data recorded in the ocean floor core samples, we have shown that the ice age before last began to end 141,000 years ago.

“This is as much as 8,000 years earlier than previously thought, too early to be caused by stronger northern hemisphere summers alone which is the prevailing theory of the ice ages.”

”At the time the glacial period ended, the Earth’s tilt angle was increasing. Higher tilt angles change the solar energy near the poles of both hemispheres, where the glacial ice sheets are positioned. This makes summers warmer in both hemispheres at once.”

“If anything, there are indications that the southern hemisphere may have a more important role than that of the northern hemisphere”, he says.

Dr Hellstrom says that improving our understanding of earth’s climate prior to any possible human impacts is important for understanding future climatic changes.

The study was funded by the Australian Research Council and was published in the prestigious scientific journal Science.