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Major climate shift a million years ago linked to ocean currents

About a million years ago, the Mid-Pleistocene Transition (MPT) occurred – a major shift in the response of Earth’s climate system to variations in our planet’s orbit around the Sun. If before MPT, cycles between warmer (interglacial) and colder (glacial) periods had occurred rarely (approximately every 41,000 years), afterwards they became more frequent and intense, giving the Earth the regular ice-age cycles that have persisted since the origins of humanity. 

Until recently, scientists believed that the main cause for this transition had been a change in the phenomena called Milankovitch cycles – cyclic changes in the Earth’s orbit and orientation towards the Sun which affect how much solar energy can the Earth absorb. However, new research has shown that these cycles did not undergo any significant change during the MPT.

By analyzing cores of deep-sea sediments from the South and North Atlantic, a research team led by Columbia University has found that about a million years ago, a large system of ocean currents that helps heat move around the globe, called the Atlantic Meridional Overturning Circulation (AMOC), experienced a severe weakening. 

“What we found is the North Atlantic, right before this crash, was acting very differently than the rest of the basin,” said study lead author Maayan Yehudai, a doctoral student at Columbia University. Immediately before AMOC’s weakening, ice sheets in the Northern Hemisphere had begun to stick more effectively to their bedrock, causing glaciers to grow thicker and thus leading to a global cooling that initiated the new climatic pattern characterized by stronger and more frequent ice ages.

“Our research addresses one of the biggest questions about the largest climate change we had since the onset of the ice ages,” explained Yehudai. “It was one of the most substantial climate transitions and we don’t fully understand it. Our discovery pins the origin of this change to the Northern Hemisphere and the ice sheets that evolved there as driving this shift towards the climate patterns we observe today. 

“This is a very important step toward understanding what caused it and where it came from. It highlights the importance of the North Atlantic region and ocean circulation for present and future climate change,” he concluded.

The study is published in the journal Proceedings of the National Academy of Sciences

By Andrei Ionescu, Staff Writer

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