That Comet Theory

Did a comet impact 12,900 years ago cause the Younger Dryas cold event and otherwise have big impacts on the planet and the course of human civilization? A science writer named Zach St. George, whom I never heard of until today, has an excellent article in the NY Times on the subject and the very weird debate that surrounds it.

The Younger Dryas is a rather mysterious climate event that took place at the end of the last Ice Age. Consider the graph above, which weirdly runs from right to left. You can see that the Northern Hemisphere was getting warmer, up to nearly modern levels, and then, boom, the temperature fell off a cliff, back to Ice Age conditions for a thousand years. This is very clearly visible in the archaeological record from my part of the world. We have a decent record of human presence in that warm period – called the Bølling–Allerød Interstadial – in the form of Clovis people. But we have next to nothing from the Younger Dryas, none of the distinctive stone tools known from farther south during that period.

Nobody knows what caused the Younger Dryas. But that isn't especially surprising, since nobody knows what causes Ice Ages or Interglacial Periods, either. Geologists used to say that it was caused by a huge flow of fresh water from melting glaciers into the Atlantic, disrupting the currents that bring warm water northward. But nobody has been able to make that work in detail, and lately even the theory's originator has backed away from it; a flood of meltwater that large ought to have left lots of evidence, and there just isn't any along the St. Lawrence River or any other plausible route to the Atlantic.

Enter a band of semi-plausible scientific types and their weird theory. Back in 2006-2007, archaeologist William Topping, physicist Richard Firestone, and geophysicist Allen West published a popular book arguing that the Younger Dryas was caused by a comet that struck the North American ice sheet, and they followed this up with a major scientific publication that had 22 co-authors. The theory intrigued scientists and archaeologists from the start because of the remarkable variety of evidence introduced: carbon nanospheres, nanodiamonds said to have been created by impact shocks, metal spherules, fullerenes with extraterresterial Helium, a weird soil layer they called the "black mat," and more. The scientific publication was fairly restrained, but the popular books was wild. St. George:

In The Cycle of Cosmic Catastrophes, a book published around the same time, two of the researchers described the scene more vividly. The impact caused the ground to shake and the sky to glow, they wrote. A hail of tiny molten particles sank into flesh and set forests ablaze. Soot blotted out the sun. Earth’s magnetic field wavered, and living things were bombarded by cosmic rays, confounding the navigational senses of turtles and porpoises, which beached themselves en masse. Addled birds plummeted from the sky.

Most disastrous of all, the impact shattered the ice dam holding back Lake Agassiz, a vast expanse of glacial meltwater that stretched across Manitoba, Ontario, Saskatchewan, Wisconsin and Minnesota. The lake cascaded into the Atlantic Ocean, where the freshwater pooled over the denser seawater, disrupting the convection current carrying warm water north from the tropics. The Northern Hemisphere plunged back into full-glacial cold.

The initial scientific response to these claims was muted partly because nobody had the expertise to evaluate more than a small part of the evidence they presented. But people's skeptical radars were very much on, partly just because these guys made the mistake of publishing the popular book before the scientific paper. Scientists hate that. The book also (see the title) tied their theory to the ancient global folklore of lost civilizations and repeated catastrophes, which is something else that scientists hate.

So people began to look into the evidence for a cometary impact 12,900 years ago. A raft of skeptical publications followed, arguing that 1) nanodiamonds are not a real thing; 2) nanodiamonds are ubiquitous and have nothing to do with impacts; 3) carbon spheres are ubiquitous and have nothing to do with fires; 4) carbon spheres are related to fires but are not especially common around 12,900 years ago; 5) carbon spheres are actually made by fungi; 6) there isn't any evidence that their Helium was extraterrestrial; 7) their Helium might be extraterrestrial but it can't be dated, so we have no idea that it was related to any particular impact; "black mats" are common archaeological sediments in no way related to comet impacts or climate change (there's one in post-Roman London); and their theory about fresh water flow into the Atlantic has the same problems as all other such theories. We also know, thanks to the wonderful record of the Greenland ice cores, that sudden cooling episodes like the Younger Dryas are not rare.

You can see that this was a little confused. Which makes me think that the initial publication was a stroke of genius: to get your whacky theory into print, just introduce so many kinds of new evidence that nobody can evaluate them all. This will intrigue people enough to get them talking about your ideas, and by the time the dust settles you are famous and have a book contract.

If you are curious about the current state of the scientific debate, the place to look is a long article from December, 2023, by Vance Holliday and many others, titled, "Comprehensive Refutation of the Younger Dryas Impact Hypothesis." I think this article is excellent and makes the whole Topping-Firestone-West theory look very bad. On the other hand you can learn from this paper that research on many of these topics – nanodiamonds, carbon spherules, etc. – is still in a primitive state, with very little agreement about what these objects represent.

So the debate goes on, even among people with no connection to the original team. Questions about impacts are very hard, as the still-not-completely-settled wrangling over the Chicxulub Crater and the end of the dinosaurs shows, and, as I want to keep emphasizing, nobody knows what caused the Younger Dryas.

But there is one thing that I feel quiet confident in asserting, which is that whatever caused the Younger Dryas it had no particular impact on the course of human evolution.

One of the claims made over and over by comet impact theorists is that the Clovis culture of North America "disappeared," and that the impact provides an explanation for this disappearance. The Clovis culture did not disappear; it evolved into other, obviously related cultures. At the Clovis type site in New Mexico it evolved into the Folsom culture, and despite what Graham Hancock says, there is no thousand-year gap in the sequence. (The obvious, well-documented evolution of Clovis into a dozen other cultures in different parts of the Americas is the best piece of evidence for the "Clovis First" theory about when humans first came to the Americas.) Impact theorists want the comet to be the cause of the mass extinction of North American mammals, but a model based on human predation and introduced dieases fits the evidence much better. The origins of agricultural in the Middle East are indeed rather puzzling, but the process was well under way before 12,900 years ago and so far as we can tell it continued along a pretty smooth curve right through this period.

And once we get into Graham Hancock "the comet impact destroyed the great white civilization of Atlantis" level crap, we're outside of science altogether.

Zach St. George is particularly good on the weird internet "debate" that has grown up around this stuff:

There are now many dozens of videos about the Younger Dryas impact hypothesis on YouTube. Some YouTubers doubt the hypothesis or even try to debunk it, but many more treat it as true. In their retellings, the hypothesis takes on the sheen of legend, with new embellishments, new twists, new conclusions. Some YouTubers use the impact and its supposed connection to rapid climatic cooling to challenge the importance of modern anthropogenic contributions to climate change. Others tie the impact to biblical events. Skeptics of the hypothesis, meanwhile, swell into villains — members of the “scientific cabal,” as one YouTuber describes them, or victims of groupthink.

“What’s crazy is that this evidence has already existed for years but has been shunned by the mainstream scientific and academic communities,” says Jimmy Corsetti, who runs the YouTube channel “Bright Insight,” in one video. “They don’t want to talk about it, and the reason is, is because this is people’s livelihoods. A lot of people in the scientific community have become very wealthy.” Reporters, too, are complicit. “The failure to properly report evidence for the Younger Dryas Impact will one day be understood as the worst intellectual crime in the history of science journalism,” writes the Comet Research Group member George Howard, who describes himself as an “avocational expert” and “noncredentialed scientist,” on his blog, “The Cosmic Tusk.”

This kind of nonsense fascinates me. Once you decide that your view is absolutely true, you have to imagine why some people nonetheless oppose it. The claim that they do so for money shows up over and over again in this kind of theorizing, which is completely crazy. Sure, tenured scientists have pretty good lives, but they don't make any extra money for opposing crackpot theories. The only people who have made real money out of this affair are the proponents, notably Graham Hancock, who got a best-selling book and a Netflix TV series out of it. 

But there is a deeper misunderstanding of science here. These guys assume that scientists derive their status from defending a status quo to which they are attached, so new theories are a threat to them. But nobody ever got to be a famous scientists just by defending the status quo. The way to become a famous scientist is to either come up with a bold new theory or latch onto one early on and become one of its leading proponents. Some people oppose new theories because they are conservative curmudgeons, but in science that is never enough to stop a new theory. In our world, the incentives are all on the side of the bold and the new. 

The same is true for journalists. And everybody else, for that matter; we live in an age of rebellion so constant that it is sometimes hard for would-be rebels to find any establishment to rebel against.

Besides, some of my best friends are climate scientists, and I can tell you that they responded to the initial claim of cometary impact with great curiosity. The thing seemed kind of mad, but, again, the variety of new kinds of evidence got everyone's attention and led to all kinds of new work and new partnerships between people in different disciplines. (Who do we know who can help us with nanodiamonds?) The biggest hurdle for any radical theory is to get established scientists to pay attention to it at all, and the amount of attention that has been showered on the cometary impact theory shows how seriously it has been taken.

Now I have reached the point where I have to offer my own explanation of why my opponents cling to their false beliefs. I suggest two: first, we love anti-establishment rebels, and here we have some bona fide anti-establishment rebels with a bold new theory that is really quite cool, and might potentially explain a lot of things. Second, many humans for thousands of years have been fascinated by catastrophes that destroyed ancient civilizations. It's just an idea that many people find cool and meaningful. Graham Hancock has written a bunch of books that mention archaeological evidence, but he cares not a fig for evidence. He believes in lost ancient civilizations as a spiritual pursuit, and his ideas are no more subject to refutation than faith in the resurrection to come.

Research on possible cometary impacts continues, in North America and other places. Comets and asteroids are certainly real, and their impacts might have numerous and varied effects. Right now, though, the Younger Dryas Impact Hypothesis looks bad, and the work of its initial advocates looks very sloppy.