Why ice ages lost their cool

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By Science Advisor. 

Excerpt: About 2.7 million years ago, Earth’s climate had a personality crisis. Before then, ice ages waxed and waned in long, predictable cycles tied to Earth’s orbit, tens of thousands of years at a time. But new research in Science suggests that as Northern Hemisphere ice sheets grew larger, the planet’s climate system began behaving very differently. And ice ages started “flickering,” swinging abruptly every couple thousand years. To understand when and why this shift occurred, researchers analyzed sediment cores drilled from the seafloor off the Iberian margin, near Portugal. ...For most of the Pliocene, from about 5.3 to 2.7 million years ago, the record shows only slow orbital cycles, with little to no sign of any rapid swings. But after 2.7million years ago, during the intensification of Northern Hemisphere glaciation, the first isolated cold events begin to appear. Within 200,000 years, rapid oscillations became frequent and persistent. The timing coincides with the expansion of glaciers large enough to reach the ocean in places like Greenland. As these ice masses grew, more icebergs broke off and melted into the North Atlantic—activity that may have disrupted ocean circulation, making the climate system more prone to abrupt shifts. With ice sheets growing larger, millennial-scale variability became an enduring feature of ice ages and a new mode of climate behavior that would define the Quaternary, the ice-age period that continues today. The shift overlaps with the emergence of the genus Homo, indicating that our earliest ancestors evolved in an increasingly variable world that could have influenced hominin evolution... 

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