For the past million years or so, the Earth has gone through an ice age every 100,000 years. Nobody knows precisely why. Before that, colder summers occurred about every 40,000 years until the Earth entered a phase called the middle Pleistocene transition in which the intervals became over twice as long.
Researchers at Cardiff University have just put forward a new theory — published in the journal Geology— about why this cycle changed so dramatically. As it turns out, some of the answers may lie at the bottom of the ocean.
The team studied the fossils of tiny marine algae, small photosynthetic organisms that act like the trees of the ocean by sucking up carbon dioxide. Algae in the different layers of sediment were shown to have stored varying levels of carbon dioxide, and there were much higher levels in the particular layers that were formed every 100,000 years.
This means there was probably less carbon dioxide left in the atmosphere at these points, and this could have influenced lower temperatures, potentially allowing vast sheets of ice to engulf North America, Europe, and Asia.
The team looked toward the ocean for answers because of the vast amount of information that can be gained there relative to other sources.
"It's really difficult to get good records of past climate changes on land, especially from this time period, because the ice sheets themselves are really good at destroying their own geological record," lead researcher Carrie Lear, a professor of earth science at Cardiff University, told Business Insider. "But there is a continuous accumulation of sediment on the seafloor, which we can study."
As for why this happens in a 100,000-year cycle, Lear said the most likely cause is a combination of factors. For example, ocean circulation patterns — the movement of water through the seas — adjust the amount of carbon dioxide that can be trapped underwater because they affect the water's salinity and temperature.
Ocean temperatures can have a huge impact on the planet
When oceans are colder, they can absorb more carbon dioxide. As the temperature increases, more gas molecules leave the water than enter it. This theory also provides an explanation for how global warming is related to the increase of carbon in the atmosphere.
"You can start with a small change in climate, which changes the amount of carbon dioxide in the atmosphere, and then leads to a bigger change in climate," Lear said.
The Earth is currently in a warm spell, as the most recent ice age occurred about 11,000 years ago. There is no particular temperature that signifies an ice age; they are just defined as periods of time when large ice sheets cover the planet.
"Technically we are still in an ice age now, as we have a large ice sheet on Antarctica and Greenland," Lear said.
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