New evidence indicates Australia's interior cooled by more than 16 degrees Fahrenheit during the last ice age, hinting that dramatic temperature drops associated with glaciations at the poles and in the Northern Hemisphere reached around the globe.
The surprising evidence comes from fossil eggshells of large, flightless birds known as emus, said University of Colorado at Boulder Professor Gifford Miller, chief author of the study. The chemical structures of amino acids in the eggshells change into mirror images of themselves at a rate that is directly dependent on air temperatures, allowing the shells to act as "paleo-thermometers," he said.
The research team first collected hundreds of fossil emu shell fragments and radiocarbon-dated them at the University of Arizona. The thumbnail-sized shells used in the study dated from about 45,000 years ago to about 16,000 years ago during the last glacial epoch. They then analyzed changes in the amino acids in Miller's CU-Boulder lab to estimate changes in air temperatures over that period.
Tests were conducted on two independent groups of shell fragments collected from two regions in Australia's sub-tropical interior, said Miller, a fellow at CU-Boulder's Institute of Arctic and Alpine Research. The researchers concluded that the mean annual air temperature in the interior was about 54 F between 45,000 and 16,000 years ago, compared to about 70 F from 16,000 years ago to the present.
"This immense temperature shift was very surprising to us," said Miller, who said the team did not expect to find significant temperature changes from Australia during the last ice age. "The fact that Australia was so much colder during that period was an indication that we were seeing some sort of a global signal. We believe Australia acts as a passive recorder of global-scale climate events."
A paper on the subject by CU-Boulder's Miller, John Magee of Australian National University and A.J.T. Jull of the University of Arizona was published in the Jan. 16 issue of Nature, one of the world's leading weekly science journals.
Although there are mounds of evidence of ice-age cooling from Earth's polar and temperate regions, cooling in tropical regions during those periods has been the subject of much debate, said Miller. Because of increased solar radiation striking the Southern Hemisphere due to a cyclic wobble in Earth's axis during the last ice age, many scientists discounted the possibility of significant cooling there, he said.
In recent years, however, researchers have found isolated evidence of tropical cooling during the last ice age in coral from Barbados, noble gases in Brazilian and African ground water and snail shells from Jamaica. "Combining these lines of evidence with our findings, which are the first quantitative temperature measurements from low elevations in a subtropical region, indicates the cooling trend during the last ice age was likely a global phenomenon," said Miller.
The study results also show that the onset of post-glacial warming in Australia and probably elsewhere around the globe began about 16,000 years ago, said Miller, who also chairs CU's geological sciences department.
The climate mechanisms that cooled Australia during the last ice age remain somewhat of a mystery, he said. Australia's sub-tropical location and relatively flat topography preclude the presence of common climate-forcing mechanisms like ice sheets, glaciers, sea-ice and permafrost believed to have acted as triggers for massive cooling episodes in the Northern Hemisphere and polar regions.
"Our feeling now is that this cooling in Australia may have been related to a reduction of water vapor in the atmosphere," he said. Less water vapor in the atmosphere would have meant the tropical oceans were significantly colder than many climate modelers have assumed.
During the ice age, ocean currents probably moved warm tropical water to the polar regions, where it cooled, he said. Cooler water returning back to the equator then helped to chill the tropical oceans.
Less water would have evaporated from the cooler tropical oceans, decreasing the amount of water vapor in the air over both hemispheres, Miller said. This would have "effectively spread the cooling throughout the planet."
"Events at the poles apparently can trigger much larger temperature changes around the globe than previously thought," he said. "If we ever are going to be able to predict future changes in Earth's climate, we need to understand how the circulation processes in the world's oceans and atmosphere are linked."