The Younger Dryas (YD) impact hypothesis is a recent theory that suggests that a cometary or meteoritic body or bodies hit and/or exploded over North America 12,900 years ago, causing the YD climate episode, extinction of Pleistocene megafauna, demise of the Clovis archaeological culture, and a range of other effects.
The physical evidence interpreted as signatures of an impact event can be separated into two groups. The first group consists of evidence that has been largely rejected by the scientific community and is no longer in widespread discussion…. The second group consists of evidence that has been active in recent research and discussions:…. Over time, however, these signatures have also seen contrary evidence rather than support.
In summary, none of the original YD impact signatures have been subsequently corroborated by independent tests. Of the 12 original lines of evidence, seven have so far proven to be non-reproducible. The remaining signatures instead seem to represent either (1) non-catastrophic mechanisms, and/or (2) terrestrial rather than extraterrestrial or impact-related sources.
The YD impact hypothesis made a big splash at AGU in 2007, and we’ve written about it a few times since. Our assessment was (in 2007), that this would need a lot of confirmatory evidence to get accepted, and even if it was, it did not provide much explanation for other, very similar, abrupt changes in the record. In 2009, we were still skeptical and noted that “the level of proof required for this extraordinary idea will need to be extraordinarily strong”. Unfortunately, as this paper makes clear, neither a lot of confirmatory evidence nor extraordinarily strong proofs have been forthcoming.
This paper is unlikely to the very last word on the subject, but it is likely to be the last time the mainstream paleo-climatologists are going to pay this much heed unless some really big new piece of evidence comes to light.
However, while the specifics of this particular hypothesis and its refutation are interesting in many ways…
The YD impact hypothesis provides a cautionary tale for researchers, the scientific community, the press, and the broader public.
Let’s be specific…
… since there are indeed lessons that can be drawn here:
- ‘Bold’ ideas can get published and get serious people to pay attention. The claims about the YD impact were entirely at odds with mainstream views, yet taken seriously and looked at by a wide variety of other researchers.
- Like most bold ideas that initially raise skeptical eyebrows, the evidence for this one decreased with time. This is not inevitable, but it is not unusual.
- Science is self-correcting because other scientists take the time to look for new evidence backing up or refuting initial ideas, and go back and re-interpret what was previously done.
- Even eventually discarded ideas can provide abundant directions for good science to get done. For instance, a fair amount of research into nanodiamonds has occurred because of the interest in this idea.
- The media loves the ‘radical new idea’ presented by ‘outsider’ scientists (3 documentaries on this so far, a big NYT piece). It fits a lot of the romantic archetype of what science is supposed to be about. It has controversy, narrative and outsize personalities. Whether the ideas are good or not is barely relevant.
- The Feynmanian ideal of a single scientist both proposing and refuting their own new idea is very rare. In practice, the roles of proposing and refuting are far more often done by the scientific community as a whole, not an individual.
- Scientists gain credibility for doing careful work and not going beyond the evidence in interpreting it. This is opposite to what gains readership on blogs. :-)
The Younger Dryas, an extremely abrupt, and still mysterious, interval of climate change, will no doubt continue to excite people across the field of paleo-climate, but we hypothesize that the impact hypothesis has had all the impact it’s going to.