DATING frost-shattered rocks in Australia has pinpointed the peak of the last ice age at 22,000 years ago. The finding confirms that the ice age started in the north and spread south鈥� some earlier ice cores had suggested otherwise.
Ice ages are driven by wobbles in the Earth鈥檚 orbit that change the distribution of solar radiation the planet receives. According to the most popular theory, variations in the amount of solar radiation falling on the northern hemisphere drive climate change across the globe. But recent comparisons of ice cores from the Arctic and Antarctic suggested the opposite, that the last ice age might have started in the southern hemisphere and spread north.
Now a study by Timothy Barrows of the Australian National University in Canberra and his colleagues suggests that the north led the way after all. To work out when rocks in eastern Australia were shattered by subzero temperatures, the team looked at isotopes in the rock that act as a marker of exposure to cosmic radiation.
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Previous estimates put the peak of the ice age in Australia somewhere between 15,000 and 30,000 years ago. The new results, which will appear in Quaternary Science Reviews, narrow it down to 22,000 years ago. 鈥淲e are confident we鈥檙e within 1000 years,鈥� says Barrows.
That fits what would be expected if the ice age was triggered when solar radiation in the northern hemisphere hit a low. The next step to understanding how the cold climate spread around the globe is to date similar rock deposits in the tropics and elsewhere in the southern hemisphere, says Barrows.
Knowing the temperature at which the eastern Australian rocks shattered also led the team to conclude that the climate then was far colder than previously supposed, around 9 掳C colder than today, and much drier.