What Caused the Global Ice Ages?

What caused the ice ages around the world? That's a good question and now, scientists have an answer. Researchers have found that glacier movement in the southern Hemisphere is influenced primarily by sea surface temperature and atmospheric carbon dioxide rather than changes in the Earth's orbit, which are thought to drive the advance and retreat of ice sheets in the Northern Hemisphere.

The Milankovitch theory of climate states that the expansion and contraction of Northern Hemisphere continental ice sheets are influenced by cyclic fluctuations in solar radiation intensity due to wobbles in the Earth's orbit; these orbital fluctuations should have an opposite effect on Southern Hemisphere glaciers.

"Records of past climatic changes are the only reason scientists are able to predict how the world will change in the future due to warming," said Alice Doughty, lead author of the new study, in a news release. "The more we understand about the cause of large climatic changes and how the cooling or warming signals travel around the world, the better we can predict and adapt to future changes. Our results point to the importance of feedbacks-a reaction within the climate system that can amplify the initial climate change, such as cool temperature leading to large ice sheets, which reflect more sunlight, which cools the planet further."

In order to learn a bit more about climate change, the researchers used detailed mapping and beryllium-10 surface exposure dating of ice-age moraines, which are rocks deposited when glaciers move, in New Zealand's Southern Alps. The dating method measures a nuclide produced in rocks when they are struck by cosmic rays.

The researchers identified seven episodes of maximum glacier expansion during the last ice age. In addition, they dated the ages of four sequential moraine ridges. These results revealed that New Zealand glaciers were large at the same time that large ice sheets covered Scandinavia and Canada during the last ice age about 20,000 years ago. Yet the Milankovitch theory should have had the opposite effect for the Northern and Southern Hemispheres and thus cannot explain the synchronous advance of glaciers around the globe.

The New Zealand ridges align with times of cooler sea surface temperature off of the coast of New Zealand. The findings reveal that ocean currents play a significant role in climate.

The findings are published in the journal Geology.

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