Younger Dryas impact hypothesis

Purported evidence and rebuttals

The 2007 paper resulted in cross-disciplinary relevant scientific rebuttals, including from many geologists, astronomers, archaeologists, and paleoecologists. This led to a back-and-forth of rebuttals in which proponents of YDIH maintain that the opposition to the hypothesis is a minority group of skeptics and conspiratorial claims of a mainstream coverup, unusual for academic conversations.^[4] Geologist Nicholas Pinter and other experts found no evidence for any such impact, failed to replicate many key evidentiary claims, and found more plausible explanations for scattered extraterrestrial material in the absence of evidence of an impact.^[14] In 2011, Pinter and collaborators set out to review and test YDIH and to present the results of such tests. They concluded that the hypothesis was rejected on the basis that the results of the 2007 study could not be reproduced and were instead a misinterpretation of data,^[5] primarily due to the fact that many of them were caused by normal geologic processes on Earth.^[4]

One of the key claims of YDIH was the finding of high concentrations of nanodiamonds purported to be caused by a bolide impact; they speculated that an air burst caused by a comet hitting the atmosphere over North America would have caused the formation of the nanodiamonds.^[15] This remained the "last hope" for the hypothesis, despite the lack of an impact crater, until a study led by physicist Tyrone Daulton concluded no such nanodiamonds could be found. Another study by geophysicist Jay Melosh showed that an air burst from such an impact could not produce the amount of pressure necessary to produce nanodiamonds.^[16] Instead, a 2010 study led by paleobotanist Andrew C. Scott found fossil evidence that the materials found were fungi or insect fecal pellets.^[17] Additional studies in 2016 replicated the refutation of the existence of the nanodiamond evidence.^[15]

Purported evidentiary support for YDIH includes a spike of platinum in an ice core drilled out from Greenland in 2013 dated to around 13,000 years ago,^[11] and the 2015 discovery of the Hiawatha impact structure. However, in 2018 Melosh argued that it was statistically unlikely the impact was recent given that an object the size of the projectile would only collide with Earth once every few million years. He also said that it was too small to cause a catastrophic extinction event in the continental United States.^[13] A 2025 study led by volcanologist Charlotte E. Green found that the elevated platinum occurred 45 years after the onset of YD, and thus too late to support YDIH, and lasted for 14 years, negating an instantaneous event, but rather a sustained condition. Green and her co-authors concluded instead that the platinum spike was due to fissure eruptions in Iceland, which was consistent with several other previous studies.^[11][18]

Other evidence cited in support of YDIH includes black mats, or strata of organic-rich soil that have been identified at several archaeological sites across North America. However, a 2009 study led by paleoecologist Jennifer Marlon examined lake sediments from from a period of 10,000 to 15,000 years ago to document charcoal and pollen data during that 5,000 period, which included the onset of YD at around its midpoint. However, no spike was found aligning with a YD boundary, and a rise in black mats were found to have preceded the onset of YD. West argued that Marlon's radiocarbon dating calibration was wrong, which Marlon dismissed due to her use of multiple sites, none of which showed any spike as required by YDIH.^[10] Similarly, a 2012 study of 13 sites in the Atacama Desert led by geologist Jeffrey Pigati found the black mats at various sites regardless of their age and location, many of which were much older than the YD.^[19] Both Marlon and Pigati argued that various natural processes unrelated to an impact could explain their presence.^[10][19] Firestone criticized Pigati's methods, to which Pigati accepted and responded that he stood by the findings, he said "We admit in our paper that we can't disprove the impact hypothesis," and "Our point is that some of the spherules and other markers [cited in the 2007 report] aren't uniquely produced by impacts."^[19]

Comet Research Group

Skepticism increased in 2011 when Rex Dalton published his finding in the magazine Pacific Standard that consultant Allen West (born Alan Whitt),^[4] one of the lead authors of the 2007 paper and co-author of the 2006 book, had been convicted in California of fraud "for masquerading as a state-licensed geologist," discovered by Dalton under West's given name.^[4][20] Afterward, several co-authors of the paper distanced themselves from YDIH.^[4]

In 2016, West and some others formed Comet Research Group Inc. (CRG), a division of Rising Light Group, an Arizona that "promotes public awareness and tolerance in a variety of fields, including religion, philosophy and science."^[4] CRG states that its mission is to "find evidence about comet impacts and raise awareness about them before your city is next."^[21] Philanthropist and early Googler Eugene Jhong donated $1.25 million across two universities to fund CRG's work. The group publishes research about real geological phenomenons with alternative explanations that diverge from scientific consensus. Since its forming, CRG has grown in collaborators and, since 2021, published several papers it claimed confirmed YDIH, arguing it was no longer a hypothesis, but instead a scientific theory, including a 2021 paper authored by theoretical physicist Martin Sweatman, who rejected all rebuttals of his work as "poorly constructed."^[4]

CRG members also claim that papers have been rejected by editors of scientific journals despite favorable peer reviews, leading them to publish their own journal Airbursts and Cratering Impacts, which they claim is held to the same peer-reviewed standards as their other journals. They claim that publishing their own journal was necessary as their work is suppressed by "very influential scientists." Astrophysicist Malcolm LeCompte alleged one such scientist to be Holliday, who in 2023 was the lead author on a comprehensive refutation of YDIH, and a co-author, impact physicist Mark Boslough.^[4] Holliday, Boslough, and collaborators concluded that:^[22][1]

Evidence and arguments purported to support the YDIH involve flawed methodologies, inappropriate assumptions, questionable conclusions, misstatements of fact, misleading information, unsupported claims, irreproducible observations, logical fallacies, and selected omission of contrary information.

Also in 2023, Boslough was the author of an article in Skeptic magazine in which he cited more than 50 scientific publications to support his assertion that YDIH has never been accepted by any relevant experts. He wrote:^[23]

Plagued by self contradictions, logical fallacies, basic misunderstandings, misidentified impact evidence, abandoned claims, irreproducible results, questionable protocols, lack of disclosure, secretiveness, failed predictions, contaminated samples, pseudoscientific arguments, physically impossible mechanisms, and misrepresentations, the YDIH has never been accepted by experts in any related field.