What Really Causes Global Ice Ages?

Researchers at Dartmouth College are casting doubt on the leading theory of what causes ice ages around the world, which could possibly help shed light on what we can expect in the future as our global climate continues to change.

According to the popular Milankovitch theory of climate, the advance and retreat of ice sheets in the Northern and Southern Hemisphere is caused by changes in the Earth's orbit. That is, when our planet's orbit wobbles, it results in cyclic fluctuations in solar radiation intensity that influences glaciers on Earth.

However, a new study published in the journal Geology indicates that sea surface temperature and levels of atmospheric carbon dioxide, not Earth's orbit, primarily influence glacier movement in the Southern Hemisphere.

This raises questions about the Milankovitch theory, which says that orbital fluctuations should have an opposite effect on Southern Hemisphere glaciers compared to what's seen in the Northern Hemisphere.

To get to the bottom of this conundrum, researchers mapped out New Zealand's Southern Alps, where the glaciers were much bigger in the past. They used a dating technique that involves beryllium-10, a nuclide produced in rocks when they are struck by cosmic rays.

It turns out 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. And while it only makes sense that during an ice age the whole world would be cold at the same time, the results do not add up according to the Milankovitch theory. If this concept were correct, the researchers should have seen opposite effects for the Northern and Southern Hemispheres.

"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. 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," lead author Alice Doughty, a glacial geologist at Dartmouth College, said in a statement.

"Our results point to the importance of feedbacks - a reaction within the climate system that can amplify the initial climate change, such as cool temperatures leading to larger ice sheets, which reflect more sunlight, which cools the planet further," she added. "The more we know about the magnitude and rates of these changes and the better we can explain these connections, the more robust climate models can be in predicting future change."

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