While climate change is the hot topic (pun intended) of the day, the truth is Earth has always gone through semi-regular cycles of warmer and colder periods. But while many of these events can be linked to specific causes, like changes in the planet's orbit (or say, human activity) the cause of the last major shift, which occurred about a million years ago, has remained a mystery. By taking samples from the ocean floor of the Bering Sea, a new study may have uncovered the mechanism behind this outlier event.

The Pleistocene period is often known casually as "The" Ice Age, and with good reason. The Earth was a much colder place than it is today, and a long-term pattern of more intense cold conditions kicked off with the Mid-Pleistocene Transition (MPT) about a million years ago.

But what exactly caused that transition has long been the subject of debate. Most of these events are triggered by what are known as Milankovitch Cycles, which include changes in the Earth's orbital path around the Sun, the tilt of its axis and the wobble of its spin. These affect how much sunlight hits the Northern Hemisphere, which can cause glaciers to either build up or melt away. Factors like CO2 levels in the atmosphere play a role, too, amplifying the effects.

The problem is that the MPT falls outside of these normal cycles, so scientists have been unsure exactly what caused this long-term global climate shift. One theory suggests that around that time, the oceans may have become more stratified, meaning the water forms distinct layers that don't mix too easily, thanks to differences in temperature and salinity. As a result, the oceans can absorb more CO2 from the atmosphere than usual and cause global temperatures to plummet.

To test the idea, the team took deep sediment cores from the bottom of the Bering Sea. By studying the chemistry of the sediment and fossil shells from marine microorganisms, the team can see a detailed history of changes in the area, including the masses of water at the surface and bottom of the ocean.

The researchers found that the Bering Sea did become more stratified around the time of the MPT. That means that CO2 locked away deep in the subarctic North Pacific Ocean stayed down there, so the sea absorbs even more of the gas, which in turn leads to global cooling. The cause of this, the team says, was that the Bering Strait – the short stretch of ocean between eastern Siberia and Alaska – iced over during a glaciation.

"Today much of the cold water produced by sea ice action flows northward into the Arctic Ocean through the Bering Strait," says Sev Kender, lead author of the study. "As glaciers grew and sea levels fell around 1 million years ago, the Bering Strait would have closed, retaining colder water within the Bering Sea. This expanded watermass appears to have stifled the upwelling of deep CO2-rich water and allowed the ocean to sequester more CO2 out of the atmosphere. The associated cooling effect would have changed the sensitivity of Earth to orbital cycles, causing colder and longer glaciations that characterize climate ever since.

"Our findings highlight the importance of understanding present and future changes to the high latitude oceans, as these regions are so important for long term sequestration or release of atmospheric CO2."

The research was published in the journal Nature Communications.