Why Do Ice Ages Happen?

Imagine the Chicago skyline. Now imagine it under nearly 2 miles (3 kilometers) of ice. That's what the landscape looked like at the peak of the last ice age.



In the scope of Earth's recent geologic history, this wouldn't have been such an unusual sight. In the past 2.6 million years (or what's known as the Quaternary Period), the planet has undergone more than 50 ice ages, with warmer interglacial periods in between.

But what causes ice sheets and glaciers to expand periodically? Ice ages are driven by a complex, interconnected set of factors, involving Earth's position in the solar system and more local influences, like carbon dioxide levels. Scientists are still trying to understand how this system works, especially because human-caused climate change may have permanently broken the cycle. [Has the Earth Ever Been This Hot Before?]



It wasn't until a few centuries ago that scientists started recognizing hints of past deep freezes. In the mid-19th century, Swiss-American naturalist Louis Agassiz documented the marks that glaciers had left on the Earth, such as out-of-place rocks and giant piles of debris, known as moraines, that he suspected ancient glaciers had carried and pushed over long distances.



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