August 25, 2009

Where's the CO2 in this?

FROM-Science Alert

North and south melted ice age

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

The study, led by Dr Russell Drysdale from the University of Newcastle's School of Environmental and Life Sciences, was completed by researchers from Australia, Italy, France, Germany and the UK.

"It is already known that wobbles in the Earth's orbit drive the ice-age cycle but there are several theories as to how they do this. The most popular one suggests that ice ages end during a period when the intensity of northern hemisphere summer insolation is greatest," study co-author Dr John Hellstrom, from the University of Melbourne's School of Earth Sciences, said.

The results of the new study, which focused on the end of the penultimate ice age, cast doubt on this theory and suggest that variations in the Earth's axial tilt was the key factor.

Previous research had identified precisely when these orbital "wobbles" occurred, but it had not been possible to accurately date the records of the Earth's response to them, which are found in marine sediment on the ocean floors.

Dr Russell Drysdale said ocean sediment cores contain a wealth of information about past global climate but beyond about 50,000 years ago it is difficult to determine the exact age of these sediments.

"To overcome this, we studied isotope variations in three stalagmites collected from an Italian cave, and found that these variations relate to ocean temperature changes recorded in sediment cores from the nearby sea floor," Dr Drysdale said. "Stalagmites from limestone caves can be very precisely dated using trace amounts of uranium incorporated within their structure."

"We applied the accurate timescale of the stalagmite record to the sea floor sediment data. A key property of sea-floor sediments is that they detect the growth and decay of ice sheets. So we have effectively provided an accurate timescale for the collapse of the ice sheets that ended the penultimate ice age. This collapse started at around 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."

The researchers said the findings support the theory that the changes were linked to increases in the angle of the Earth's tilt.

"At the time the glacial period ended, the Earth's tilt angle was increasing. Higher tilt angles increase the total solar energy reaching the poles of both hemispheres, where the glacial ice sheets are positioned. This makes summers warmer in both hemispheres in a given year," Dr Drysdale said.

"This mechanism has been suggested previously but up until now we have lacked a precisely dated record to test it. Our results support this mechanism and rule out the conventional theory of ice ages being driven by changes in northern hemisphere summer sunlight alone.

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

Dr Hellstrom said 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.


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