A Cometary Upheaval in the History of North America

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Around 12,900 years ago, Earth's climate abruptly changed, but the upheaval was particularly felt in North America. Over the span of just a decade, temperatures fell between 3.6 and 10.8 degrees Fahrenheit on average. Glaciers which had been gradually receding through Canada reversed course and crept southward. The air dried, and droughts became frequent. Megafauna like mammoths, camels, and giant bears couldn't adapt to the sudden changes and died out in droves, their extinction abetted by hungry humans on the hunt.

For years, the accepted explanation for the Younger Dryas, as this period is called (named for the tundra-loving wildflower pictured above), is that glacial meltwater severely slowed or even halted North Atlantic "conveyor" currents that carry warm waters originating closer to the equator northward, thus depriving the air and nearby continents of heat from the ocean. But over the past decade, accumulating evidence has been gradually dislodging this narrative. Taking its place is a story that's decidedly more explosive.

Dozens, and perhaps hundreds, of scientists from all across the world now ascribe to the notion that some sort of cometary impact triggered the Younger Dryas cooling and extinctions. They cite as evidence the presence of nanodiamonds and microscopic grains associated with impact events at sites across the globe around 12,900 years old, the time that the Younger Dryas began.

But critics have countered their claims every step of the way, questioning the dating techniques used, disputing the origin of the nanodiamonds, and pointing out that no large impact craters linked to the Younger Dryas have yet been found. Their fierce skepticism is justified. Supporters of a Younger Dryas impact hypothesis are making a bold claim. It is their responsibility to back it up.

And back it up they have. In the past few years, researchers have published dozens of peer-reviewed scientific papers to support a comet impact. Just a few months ago, they released their most exhaustive research yet – two studies of 129 lake cores taken from around the world showing that 12,900 years ago there was a spike in sediment charcoal levels, a clear sign of burning on a massive scale. Moreover, ice cores clearly indicate a ten percent spike in global carbon dioxide levels at the same time! It seems something cataclysmic happened just before the Younger Dryas that sent all sorts of carbon into the atmosphere.

According to the authors, the new evidence suggests that the Earth was struck by some sort of cometary debris cloud, resulting in a multitude of impacts and airbursts on the level of the Tunguska Event of 1908, where an astronomical object exploded over Siberia, decimating some 80 million trees over 770 square miles. The impacts likely ignited wildfires across the northern hemisphere, sending clouds of smoke into the air, blocking the sun and triggering cooling.

Indeed, this latest research may serve as a scientific tipping point, where the Younger Dryas cometary impact theory supplants the glacial melting theory. This is a beautiful thing – something you don't see all too often. We may be witnessing scientific consensus change before our eyes.

"These are always the best science stories," Yale neurologist and leading scientific skeptic Steven Novella wrote. "We get to watch from the sidelines as scientists duke it out, using logic and evidence to make their case. It shows how complex and subtle science can be, and how disputes within science are resolved."

While scientists on each side of this issue have stood opposed, debating for their preferred theories, they have been counterintuitively working together. Every point and counterpoint has forced the other side to refine their ideas and search for new evidence. Indeed if the impact hypothesis does finally win out, it will take its place as the leading theory largely thanks to its most ardent critics.

This is scientific debate at its best.

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