The problems with the Younger Dryas Impact Hypothesis

Over the past year or so, the Younger Dryas Impact Hypothesis (YDIH) has gained a great deal of public visibility, first through Graham Hancock’s Netflix series Ancient Apocalypse, and just last week, through a profile by Zach St. George in the New York Times Magazine.

The idea is that the cold snap, or rapid cooling, that ended the last ice age was caused by a cosmic impact (comet or asteroid) about 12900 years ago. The impact would have been catastrophic enough to cause widespread fires throughout the northern hemisphere, and to cause the sudden release of truly biblical quantities of water held in proglacial lakes. 

In other words, a bad day at the office for all concerned, that led not only to rapid global cooling, but to the extinction of many species of megafauna such as mammoth. Presumably, it would have had very significant consequences for our hunter-gatherer ancestors living at the time, and may even have led directly to the development of agriculture and the way of life we practice today.

The problem with the YDIH is not that cosmic impacts don’t happen, or that they are surprising, or that they don’t affect human history (or the history of life on Earth). There is plenty of evidence of cosmic impacts large and small. The Zhamanshin impact, which left a 14 km wide crater in Kazakhstan about 900000 years ago, would have significantly transformed the lives of many of our ancestors, perhaps for generations (not to mention that it likely killed many people). 

The much smaller (about 500 kilotons) Chelyabinsk airburst in 2013 caused over a thousand injuries and material damage on the ground, such as shattered windows and partial collapses. It left a lasting memory for those who experienced it. 

Cosmic impacts have affected the Earth and those who live on it, and will continue to do so in the future. They can happen at any time, and they can have significant consequences. After all, we live in a big uncaring universe, and we are mere specks on a speck, orbiting a flaming speck of cosmic dust.

The problem with the YDIH is not even the colourful personalities highlighted in the New York Times Magazine article, the political fights, or the allegations of scientific fraud, or the links of some of the researchers with New Age movements or religious organizations. 

In the end, while those make good copy, they are all quite irrelevant to the fundamental critique of the hypothesis. There are much better reasons to question it.

The Younger Dryas is not unique

The first and most serious problem with the YDIH is that the Younger Dryas, the sudden cooling period it seeks to explain, doesn’t need a cosmic impact to explain it. It isn’t a unique event, and it isn’t even that unusual toward the end of glacial periods in Earth’s recent history.

There are good reasons to think that cooling events such as the Younger Dryas are normal for deglaciations, and are the result of the internal dynamics of Earth’s long-term climate cycle. In an extensive simulation study, Sima et al. 2004 conclude that “Younger Dryas-type events seem to be an intrinsic feature of climate change during glacial–interglacial cycles.” In fact, the emptying of proglacial lakes caused by the warming at the end of ice ages could itself be the cause of the sudden cold snaps such as the Younger Dryas, before the lasting warming of interglacials.

There have been many such events in the past couple of million years. As Hodell et al. (2023) observe in their study of glacial cycles of the past 1.5 million years, “Almost all glacial periods end with a strong terminal stadial event that marks the start of deglaciation, with some terminations containing additional millennial events during deglaciation (e.g. Bolling–Allerod and Younger Dryas oscillations).”

If events like the Younger Dryas are quite a normal part of the glacial cycle, then why should this one in particular, need a special explanation outside the normal progression of glaciation and deglaciation? The impact is simply not needed. Which doesn’t mean there wasn’t one, but it does mean we need strong evidence to establish that this seemingly purely elective impact actually did happen.

The evidence that supports the YDIH is not necessarily related to an impact

I’ll focus here on the type of evidence claimed in support of the YDIH with which I am most familiar:  the black mat layer. But as Holliday et al. (2023) clearly show, the same argument applies to all categories of YDIH evidence.

Black mats are organically enriched layers of soil that are identified by supporters of the YDIH as evidence of intense fires related to the impact. They are supposed to be a continent-wide layer dated to about 12900 years ago, at the start of the younger Dryas.

In fact, there are many different kinds of black mat layers in the archaeological and geological records that have many different kinds of causes and that date to many different periods, all through the Holocene, which directly follows the Younger Dryas and continued until more or less recently, depending on whom you ask. 

To quote Holliday et al., “Some [black mats] are algal mats, others aggrading wetland deposits or lowland soils, or lacustrine deposits including white to light gray diatomites, and still others are well-drained upland soils.”

It isn’t surprising that there would be black mats in archaeological and geological locations just at the start of the Younger Dryas, and they don’t need a cosmic explanation. The initial warming that preceded the Younger Dryas would have favoured the development of soils and plant communities (i.e. pedogenesis) and yes, even increased frequency of brush and forest fires in many places, resulting in organically enriched layers in many local stratigraphies. Even evidence of increased fire at the end of the last ice age is not surprising. In fact, it is expected, even if there was no cosmic impact.

The rapid cooling of the Younger Dryas would have slowed those processes down (or stopped them entirely in some places) and the layers immediately above would be less enriched than the “black mat.” And that’s just one of the many different ways in which those dark layers can develop in a stratigraphy, even absent a cosmic catastrophe.  These mats would be widespread in the Americas just before the Younger Dryas, and in many places they would show up clearly in a stratigraphy.

So even if there are black mat layers in some places that immediately precede the Younger Dryas, I am not shocked, and I don’t need a cosmic impact to explain them. Sure, they could have been created by a cosmic impact, but they don’t need to have been, and their presence is not surprising at all. This is true of the rest of the evidence claimed for the impact.

As for the changes in human societies and lifeways that have unfolded since the Younger Dryas, which include a significant settling down, reliance on domesticates, and integration into ever larger and more complex polities, we have to consider that this was the first time that fully modern humans (i.e. “us”, rather than “our ancestors”) were going through a deglaciation.

Modern humans have reacted differently to this latest deglaciation than our predecessors reacted to previous ones. Again, that is not surprising.

None of this means that there wasn’t a cosmic impact that coincided with the start of the Younger Dryas. It doesn’t mean that catastrophic events like comet impacts are not important in human history. We know that these kinds of events affected our ancestors, although as John Hawks has noted, that impact can be difficult to see at the scale of the archaeological record. We hope that cosmic impacts won’t directly affect us as badly as they did our ancestors, but we also know that sadly, this is hope in vain.

All of this does mean that we don’t need a cosmic impact to explain Younger Dryas cooling, and it does mean that we have little (if any) evidence that it actually happened.

References

Hawks J 2023. Ancient apocalypses are so disappointing. https://johnhawks.net/weblog/ancient-apocalypses-are-so-disappointing

Hodell DA et al. 2023. A 1.5-million-year record of orbital and millennial climate variability in the North Atlantic, Clim. Past 19: 607–636, https://doi.org/10.5194/cp-19-607-2023 https://cp.copernicus.org/articles/19/607/2023

Holliday VT et al. 2023. Comprehensive refutation of the Younger Dryas Impact Hypothesis (YDIH), Earth Science Reviews 247. https://www.sciencedirect.com/science/article/pii/S0012825223001915#s0135

Sima A et al. 2004. The Younger Dryas—an intrinsic feature of late Pleistocene climate change at millennial timescales, Earth and Planetary Science Letters 222: 741-750. https://www.sciencedirect.com/science/article/pii/S0012821X04002109?via%3Dihub