The Younger Dryas impact hypothesis (YDIH) is a disputed idea that is not widely accepted by scientists. It suggests that a sudden flood of freshwater into the ocean caused changes in ocean currents, leading to a cold period called the Younger Dryas around 12,900 years ago. However, many studies by experts in geology, astronomy, and other fields have shown there is no evidence to support this idea. These studies found the hypothesis to be contradictory, incomplete, and sometimes inconsistent with scientific laws. Scientists often debate YDIH because its main supporters claim that most scientists reject it unfairly and accuse mainstream science of hiding the truth.
YDIH gained attention through documentaries and books by author Graham Hancock, who promotes ideas that are not supported by scientific evidence. A 2022 episode of the Netflix series Ancient Apocalypse discussed Hancock’s theories about YDIH. In 2024, The New York Times described YDIH as "The Comet Strike Theory That Just Won't Die," noting that it continues to attract public interest despite being disproven. Supporters of YDIH often label scientists who oppose it as part of a secretive group that dismisses public opinions. People who believe in conspiracy theories are more likely to support YDIH, especially if they already distrust scientific experts.
Supporters of YDIH created a non-profit organization called Comet Research Group Inc. to study impacts like the one proposed by YDIH. One of their most famous studies, published in 2021, claimed that a site called Tell el-Hammam was the ruins of the Biblical city of Sodom, destroyed by God as described in the Book of Genesis. However, this study was later found to contain altered or incorrect data and was retracted in 2025. This was the second retraction of a study by Comet Research Group Inc. from the journal Scientific Reports.
History
In the 19th century, a theory suggesting that a comet struck North America at the end of the last ice age was first proposed by Ignatius Donnelly, a politician and writer. He introduced this idea in his 1883 book Ragnarok as an alternative explanation for certain geological features, such as clay and gravel deposits, and claimed the event may have led to the destruction of a fictional ancient civilization called Atlantis.
The Younger Dryas Impact Hypothesis (YDIH) is a scientific idea that attempts to explain the sudden cooling of the Earth during the Younger Dryas period, which occurred at the end of the last ice age. This theory was first formally presented in 2007 by nuclear physicist Richard Firestone and his colleagues in a scientific journal called Proceedings of the National Academy of Sciences. Before this, Firestone and archaeologist William Topping shared an earlier version of the theory in 2001 in a publication called Mammoth Trumpet.
In their 2006 book The Cycle of Cosmic Catastrophes, Firestone and others expanded on the hypothesis. They suggested that the comet impact caused a massive lake, Lake Agassiz, to flood the Atlantic Ocean, disrupting ocean currents and leading to a sudden cooling event. They also claimed the impact destabilized an ice sheet, caused wildfires, and led to the extinction of large animals and the Clovis culture, a group of early humans. However, genetic studies show that the Clovis people moved south and adapted to changing conditions. Some scientists, like Vance T. Holliday, argue that there is no clear break in the archaeological record and that Clovis people did not stay in one place for long.
The widely accepted explanation for the sudden cooling is the release of meltwater from retreating ice sheets, as proposed in a 1989 study by geochemist Wallace Smith Broecker and others. James Kennett, a paleoceanographer, worked on both the 1989 study and the 2007 YDIH paper.
The 2007 study faced criticism from scientists in various fields, including geology, astronomy, and archaeology. Some supporters of YDIH argue that opposition to the theory comes from a small group of critics who claim there is a cover-up, which is unusual in scientific discussions. Geologist Nicholas Pinter and others found no evidence of the impact, could not reproduce key findings, and suggested other natural explanations for the materials studied. In 2011, Pinter and colleagues tested YDIH and concluded the hypothesis was not supported because the 2007 study’s results could not be repeated and were likely misinterpreted.
A key claim of YDIH was the discovery of high levels of nanodiamonds in certain materials, which supporters believed were caused by a comet impact. However, physicist Tyrone Daulton later found no such nanodiamonds, and geophysicist Jay Melosh showed that an air burst from a comet could not create the pressure needed to form them. A 2010 study by paleobotanist Andrew C. Scott found that the materials were likely from fungi or insect waste, not an impact. Additional studies in 2016 confirmed these findings.
Supporters of YDIH also pointed to a spike in platinum found in a Greenland ice core from around 13,000 years ago and the discovery of the Hiawatha impact structure in 2015. However, geophysicist Jay Melosh argued in 2018 that the Hiawatha impact was unlikely to have caused a major extinction event and was too small to have affected North America. A 2025 study by volcanologist Charlotte E. Green found that the platinum spike occurred 45 years after the start of the Younger Dryas and lasted only 14 years, which does not support a sudden, catastrophic event. Green concluded that the platinum spike was likely caused by volcanic eruptions in Iceland.
Other evidence cited for YDIH includes "black mats," which are layers of organic-rich soil, and magnetic spherules found at archaeological sites. A 2009 study by paleoecologist Jennifer Marlon found no spike in charcoal or pollen data that aligned with the start of the Younger Dryas. A 2012 study by geologist Jeffrey Pigati found similar black mats at sites of varying ages, suggesting natural processes, not an impact, could explain them. Firestone criticized Pigati’s methods, but Pigati stood by his findings, stating that some materials cited in the 2007 study were not unique to impacts. A 2009 study by anthropologist Todd A. Surovell found no spike in magnetic minerals at Younger Dryas boundary sites, and later research by Pinter and colleagues showed that the magnetic spherules were likely from Earth-based sources.
Skepticism grew in 2011 after a report revealed that Allen West, a co-author of the 2007 study, had been convicted of fraud for falsely claiming to be a licensed geologist. Some co-authors of the study later distanced themselves from the hypothesis.
Public interest
YDIH became widely known because it was featured in documentaries on the National Geographic Channel, History Channel, and the PBS program NOVA. In 2024, The New York Times (NYT) described YDIH as "The Comet Strike Theory That Just Won't Die" in an article about its history and why people are drawn to it. People often believe YDIH even though scientists disagree because of a process called "epistemic vigilance," which helps individuals tell real facts from false ones. Psychologists have found that when people hear conflicting information from experts, they usually side with the information that matches their own beliefs or their political or cultural background. Psychologist Spencer Mermelstein explained that YDIH is appealing because it provides a simple explanation that fits with what people know about Earth's geological past, such as "one big cause, one big outcome."
Graham Hancock wrote in his 2015 book Magicians of the Gods that the Younger Dryas comet caused a major disaster on Earth and was linked to the Noahide flood myth. He compared this myth to flood stories from other cultures, suggesting it was common worldwide. These claims were criticized by experts like Jason Colavito, Michael Shermer, and Marc J. Defant, who called them incorrect. Hancock expanded on these ideas in his 2019 book America Before: The Key to Earth's Lost Civilization, where he claimed the Younger Dryas catastrophe erased evidence of a complex Ice Age civilization in North America. The Society for American Archaeology (SAA) released an open letter stating that Hancock's ideas promoted "dangerous racist thinking."
The SAA's letter urged Netflix to reclassify Hancock's 2022 docuseries, Ancient Apocalypse, as science fiction. The series focused on Hancock's belief in a lost civilization, including the fictional Atlantis, and discussed YDIH in episode 8. Impact physicist Mark Boslough wrote in Skeptic magazine that many parts of the series were based on unscientific ideas. Articles in The Guardian, Slate, The Nation, and other left-leaning publications criticized the series, while conservative outlets praised it and called its critics "left-wing propaganda." Political activist Tucker Carlson described the SAA as an "elitist, closed-minded cabal" connected to the "collapse of the American idea."
On YouTube, many videos have increased public interest in YDIH. Similar divisions about accepting or rejecting YDIH appear on the platform as in mainstream media. YouTubers who support YDIH often label skeptics as part of a "scientific cabal" or victims of groupthink. Despite scientific evidence against it, YDIH continues to grow in popularity because of its connection to personal and group identity, not scientific research. It especially appeals to people who believe in conspiracy theories that reject scientific knowledge.