You may have heard about a new study identifying a “young” impact crater under the Greenland ice sheet. This article does a pretty good job of providing some background and context: science.sciencemag.org/content/362/64…
In 2007, a team of scientists suggested that an impact event in North America caused an abrupt cold period known as the Younger Dryas 12,900 years ago, causing widespread fires, megafaunal extinctions, and devastating human populations (the Clovis culture).
This was understandably met with skepticism. Impact experts found the evidence shaky. There were problems with other teams replicating the results. Paleoecology and archaeology data didn’t line up. Here’s a paper summarizing those concerns: researchgate.net/profile/Jacque…
The proposed impactor has been a moving target. First it was to have hit the North American ice sheet and destabilized it, leaving no crater. Then it was a series of bolides, leaving no crater. The Greenland crater may be 3 million years old, or as young as 12,000. Is this it?
Firstly, I would be extremely skeptical if anyone saying this is a smoking gun before the crater is dated. Remember the method of multiple working hypotheses (auburn.edu/~tds0009/Artic…). We can’t decide something is what we want it to be just because it’s what we’re looking for.
I’ve had people point at a charcoal peak in one of my sediment cores and say “that’s the impact right there!” even though it was thousands of years too early or late. “Well, your radiocarbon dates must be bad, then.” Be careful to not force-fit data to fit your interpretations.
We really have two questions here: 1) Was there an extraterrestrial impact 12,900 years ago? 2) If there was, what impact (ha) did it have? I’m not an impact expert, though I’ve worked with some, like @MarkBoslough. So I’ll focus on the ice age environment, which is my jam.
The Younger Dryas is an abrupt climate event we’ve known about for a few decades. It’s named for Dryas octopetala, this cute little arctic plant. Why? In cores, its pollen signals an abrupt return to glacial conditions, a brief reversal as we were coming out of the last ice age. 
There’s a signal of the Younger Dryas in ice cores, but it took a while for folks to agree it was ecologically important, in part because it doesn’t show up everywhere in the fossil record. It mostly seems to be a North Atlantic event, seen in New England and northwestern Europe.
The YD wasn’t the only abrupt event as we came out of the last ice age; there was an abrupt warm period several centuries earlier. Pollen records that do record the Younger Dryas show a sudden return colder, drier conditions for a few centuries, then we abruptly warmed again.
In some places, the Younger Dryas brought boreal forest expansion; in others, the tundra moved southward again, even as it had been pushed north by the warming climate and melting ice sheets. Overall, though, the vegetation trends match what we’d predict from a cold event.
Work by @mudfire on fire records that span the last 21,000 years (reconstructed from charcoal in sediment cores) don’t show an increase in fire at the start of the Younger Dryas, though they show increases in fire at other abrupt climate events: pnas.org/content/pnas/1…
Megafauna populations were collapsing well before the Younger Dryas (as my work supports), and @ChrisWidga and others have shown that mammoths and other extinct species persisted in North America for millennia after the YD.
Archaeologists also find no evidence that paleoundian populations were decimated (or driven “extinct”) by the Younger Dryas climate changes. The end of the Clovis culture is a cultural shift in tool use, not a die-off. pdfs.semanticscholar.org/14ef/a36931a3d…
(As a side note, I don’t know if anyone has asked Native people about their stories or traditional knowledge about the time of the Younger Dryas. I suspect not.)
To summarize, the ecological and cultural responses we see 12,900 years ago are in line with a cooler, drier climate, but these effects aren’t seen everywhere. There’s no evidence of widespread upheaval or landscape devastation, even in the northeast (closest to Greenland).
“But what caused the Younger Dryas,” you might be asking. Good question! It’s been proposed that increased melting during the rapid warming at the end of the last ice age added freshwater to the North Atlantic, disrupting the Gulf Stream’s transport of warm water northward.
(This is basically the idea behind The Day After Tomorrow, and one possible mechanism where global warming can screw with climate in counterintuitive ways). The melting hypothesis is highly debated, partly because there hasn’t been evidence for a meltwater channel until recently.
Figuring out the cause of the Younger Dryas is one of the holy grails for folks researching ice age climate change. It’s been a tantalizing puzzle. Could it have been something big smashing into Greenland? I defer to the impact experts to tell us how an ice sheet would react.
The proposed age of the crater is 3 million to 12,000 BP. A LOT happened in Earth’s climate system in that window. 1) the onset of ice age cycles started 2.8 million years ago. 2) ~900,000 years ago, we saw an abrupt shift from 40,000-year glacial cycles to 100,000-year cycles.
When the age of the impact crater is independently dated, and it’s chemistry is analyzed, we’ll be able to better place it in context. Until then, any definitive conclusions are impossible, and calling this crater a smoking gun for the Younger Dryas would be irresponsible.
Finally, it’s entirely possible there was an impact event ~12,900 years ago. But aside from a short-term shift in response to cooler and drier conditions (in some places), the, er, impacts to people and ecosystems seem to have been relatively minor.
More on a possible meltwater channel from earlier this year: nature.com/articles/s4156… (HT @paleoMunoz, another person to follow).
