Checked · The Younger Dryas
Did a cosmic impact cause the Younger Dryas?
The claim
A comet or asteroid struck or exploded over the northern hemisphere around 12,900 years ago, triggering the Younger Dryas cold snap and contributing to the megafaunal extinctions.
Our verdict
Under reviewUnresolved, and properly so. It is a live scientific controversy with four papers arguing it out in one journal between 2021 and 2024, both sides peer-reviewed and both still publishing. Neither side is fringe. Anybody telling you it is settled, in either direction, is telling you about themselves.
Around 12,900 years ago the northern hemisphere, warming out of the last ice age, went back to near-glacial cold and stayed there for over a thousand years. That the Younger Dryas happened is not in dispute. It is in the Greenland ice cores and in European lake sediments, and its abruptness at both ends is what makes it strange.
What caused it is disputed, and the dispute is unusually well documented.
The proposal
In 2007 Richard Firestone and colleagues proposed in PNAS that a major cosmic impact around 12,900 years ago triggered the cooling and contributed to the megafaunal extinctions. The evidence offered was geochemical: a set of markers at the Younger Dryas boundary across at least four continents, including excess platinum, quench-melted materials, and nanodiamonds.
That is the proposal. It is a real, published, peer-reviewed hypothesis, and everything since has been an argument about whether the markers hold.
The early failures to reproduce
Two papers in PNAS in 2009 reported that they could not find the signal.
Surovell and colleagues analysed magnetic minerals and microspherules from seven sites of similar age, two of which Firestone and colleagues had examined themselves, and reported being unable to reproduce any of the results. The detail worth keeping is the two sites. "Could not reproduce" is a much weaker statement than "could not reproduce, at your sites, using your method".
Surovell, T. A. et al. (2009), An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis, PNAS 106(43), pages 18155 to 18158. DOI 10.1073/pnas.0907857106.
Paquay and colleagues, the same year, found no extraterrestrial platinum-group-element enrichment across North America and in Belgium. That one was answered directly the following year by Bunch and colleagues, in the same journal, arguing the geochemical data does not refute the impact.
The exchange, which is the real story
Between 2021 and 2024 the whole argument ran through one journal, in order, in public.
| Year | Paper | Side |
|---|---|---|
| 2021 | Sweatman, review of the impact evidence, Earth-Science Reviews 218 | For |
| 2023 | Holliday and colleagues, comprehensive refutation, Earth-Science Reviews 247 | Against |
| 2024 | Sweatman, Powell and West, rejection of the refutation, Earth-Science Reviews 258, article 104960 | For |
| 2024 | Holliday and colleagues, rebuttal of the rejection, Earth-Science Reviews 258, article 104961 | Against |
Titles escalating from review to refutation to rejection of the refutation to rebuttal of the rejection. You can follow the entire dispute in one place, which is rare and which is the single most useful thing we can point a reader at.
Both cases, at comparable strength
The case for
A boundary layer with an unusual geochemical signature has been reported at sites on multiple continents, by multiple groups, over nearly two decades.
The timing coincidence is real and it is tight. An abrupt climate reversal, a megafaunal extinction pulse and a proposed impact horizon all land in the same narrow window.
An airburst rather than a ground impact would leave no crater, and airbursts are a real phenomenon with a real physics.
The hypothesis is published and defended in a mainstream Earth-science journal by named researchers, which is what a live controversy looks like rather than a fringe one.
The case against
The central markers have repeatedly failed independent reproduction, including at the proponents own sites using their own method.
The nanodiamond marker specifically has been attacked by two separate teams, and that argument has its own page.
No crater has been identified at the right date, and the airburst explanation is an argument for why the missing evidence is expected rather than evidence itself.
Alternative explanations for the cooling, principally a meltwater disruption of ocean circulation, do not require a new mechanism.
Two traps in the material
Both of these will catch a reader skimming for a conclusion, and both caught us.
Trap one. Sweatman 2021 states that after 14 years the overwhelming consensus of research suggests the claims should be accepted. That is the author's characterisation, inside a paper arguing for the hypothesis. It is not a neutral report of where the field stands, and Holliday and colleagues maintain the opposite in the same journal. Quoting the first without the second is exactly the misrepresentation each side accuses the other of.
Trap two. Vance Holliday is an author on the 2009 Surovell paper and leads the 2023 refutation. Philippe Claeys appears on Surovell 2009, Paquay 2009 and Tian 2011. A proponent reads that as the same small group publishing against the hypothesis for 15 years. A sceptic reads it as consistency and accumulated expertise. Both readings come off the same author lists, and picking one without saying so is not analysis.
Why we are not calling it
Our field is a laptop and a lidar tile. We have no standing to adjudicate impact geochemistry, and a verdict from here would be worth nothing except to whichever side it flattered.
What we can do is tell you how to read it. Four papers, one journal, two sides, all of them open to anybody who wants to check. That is not a field suppressing an inconvenient finding. It is a field arguing, slowly and in writing, which is the only way these things ever get settled.
Where this leaves things
The five labels, applied to this question. Nothing is left unlabelled.
The Younger Dryas happened, around 12,900 years ago, and its abruptness is recorded in ice cores and lake sediments.
Whether a cosmic impact caused it. Four peer-reviewed papers across three years, unresolved.
The individual geochemical markers. Each has a reproduction history worth reading on its own terms.
Any claim that the question is settled. Both sides were still publishing in late 2024.
Everything the retelling attaches to it about a lost civilisation. That is a separate proposition and it has its own page.
Sources
- Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Firestone, R. B. et al., PNAS 104(41), pages 16016 to 16021, 2007.
- An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Surovell, T. A. et al., PNAS 106(43), pages 18155 to 18158, 2009.
- Absence of geochemical evidence for an impact event at the Bolling-Allerod / Younger Dryas transition. Paquay, F. S. et al., PNAS 106, pages 21505 to 21510, 2009.
- Geochemical data reported by Paquay et al. do not refute Younger Dryas impact event. Bunch, T. E. et al., PNAS 107, 2010.
- The Younger Dryas impact hypothesis: Review of the impact evidence. Sweatman, M. B., Earth-Science Reviews 218, article 103677, 2021.
- Comprehensive refutation of the Younger Dryas Impact Hypothesis (YDIH). Holliday, V. T. et al., Earth-Science Reviews 247, article 104502, 2023.
- Rejection of Holliday et al.'s alleged refutation of the Younger Dryas impact hypothesis. Sweatman, M. B., Powell, J., West, A., Earth-Science Reviews 258, article 104960, 2024.
- Rebuttal of Sweatman, Powell, and West's rejection of Holliday et al.'s alleged refutation of the Younger Dryas Impact Hypothesis. Holliday, V. T. et al., Earth-Science Reviews 258, article 104961, 2024.
Related
Where this is argued in full
Answering the Architect Question
Both cases at the same length, measured and printed at the back. The argument runs across four papers in one journal and you can read all of it.
Both cases at the same length, measured and printed at the back. It is written and not on sale, and advance readers can read it first.