The Younger Dryas impact hypothesis (YDIH) is a disputed theory that many scientists do not agree with. 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, scientists in fields like geology and astronomy have rejected this idea for decades because there is no strong evidence to support it. Studies have shown the theory to be inconsistent, missing important information, and even contradicting basic scientific principles. Some supporters of YDIH claim that most scientists who disagree with it are part of a small group that hides the truth, but this is not supported by evidence.
YDIH became well-known through documentaries on ancient history and books by author Graham Hancock, who promotes unscientific theories. A 2022 episode of the Netflix series Ancient Apocalypse discussed Hancock’s ideas 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. Some supporters and conservative media describe scientists who oppose YDIH as part of a secretive group of experts. YDIH is especially appealing to people who believe in conspiracy theories and distrust scientific evidence.
Supporters of YDIH created a non-profit group called Comet Research Group Inc. to study impacts like the one proposed by the theory. One of the group’s 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 However, this study was found to contain false data and was retracted in 2025 by the journal Scientific Reports. This was the second retraction of a study by Comet Research Group Inc. from the same journal.
History
The first idea that a comet hit North America at the end of the last ice age was proposed in the 19th century by Ignatius Donnelly, a politician and believer in secret theories. He wrote a book in 1883 called Ragnarok and suggested that a comet impact caused clay and gravel found across North America. He also claimed this event destroyed the fictional lost civilization of Atlantis.
The Younger Dryas Impact Hypothesis (YDIH) is an unusual theory that tries to explain why the Earth suddenly cooled at the end of the Last Glacial Period, known as the Younger Dryas (YD) period. This idea was first shared in 2007 by Richard Firestone, a nuclear physicist, and his team in a scientific journal called Proceedings of the National Academy of Sciences of the United States of America. Their paper was titled "Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling." Earlier, in 2001, Firestone and William Topping, an archaeologist, published a version of the theory in a newsletter called Mammoth Trumpet.
In their 2006 book The Cycle of Cosmic Catastrophes, Firestone and others expanded on the YDIH. However, one of the book’s co-authors said the book was not meant to be scientific. The authors argued that the comet impact broke an ice dam holding a large lake called Lake Agassiz, which then flooded the Atlantic Ocean with freshwater. This disrupted the North Atlantic Conveyor, a current that affects Earth’s climate, and caused a sudden cooling. They also claimed the impact destabilized the Laurentide ice sheet, leading to wildfires, the extinction of large animals, and the disappearance of the Clovis culture, a group of early humans. However, genetic evidence shows the Clovis people moved south and adapted to environmental changes. Anthropologist Vance T. Holliday noted that there is no clear break in the archaeological record and that Clovis tools were not used in one place for long. Folsom points, a type of tool, appeared at the same time as the decline of megafauna hunting tools.
The widely accepted explanation for the sudden influx of freshwater into the ocean is melting ice from retreating ice sheets, as described in a 1989 study in the journal Nature by Wallace Smith Broecker and others. James Kennett, a paleoceanographer, co-authored both the 1989 study and the 2007 YDIH paper.
The 2007 YDIH paper led to scientific debates, with many geologists, astronomers, archaeologists, and paleoecologists challenging the theory. Supporters of YDIH argue that opposition comes from a small group of critics and claims of a coverup, which is rare in scientific discussions. Geologist Nicholas Pinter and others found no evidence of an impact, could not reproduce key claims, and suggested natural Earth processes better explain scattered extraterrestrial materials. In 2011, Pinter and collaborators tested YDIH and concluded the hypothesis was rejected because the 2007 study’s results could not be reproduced and were likely misinterpretations of data caused by normal geological processes. They wrote:
In all cases, rare but common materials were incorrectly reported as unusual signs of an impact at the start of the YD. No clear signs of an impact were found, leading to new, conflicting, and sometimes impossible explanations. The YD impact hypothesis serves as a warning for scientists, the public, and the media.
A key claim of YDIH was the discovery of high levels of nanodiamonds in carbon-rich spherules, which were thought to form from a comet hitting the atmosphere. Scientists suggested an air burst from a comet could create these diamonds. This became the last hope for the theory, despite no impact crater being found. However, a 2010 study by Andrew C. Scott found the materials were actually fungi or insect waste. Later studies in 2016 confirmed no nanodiamonds existed.
Other evidence cited for YDIH includes a spike in platinum found in Greenland ice cores from around 13,000 years ago and the 2015 discovery of the Hiawatha impact structure. However, in 2018, Jay Melosh argued the impact was unlikely to be recent, as objects of that size collide with Earth only every few million years. He also said the impact was too small to cause major extinctions in North America. A 2025 study by Charlotte E. Green found the platinum spike occurred 45 years after the YD began and lasted 14 years, which does not support a sudden event. Instead, the spike was linked to volcanic activity in Iceland.
Other evidence includes "black mats," layers of rich soil found at archaeological sites, and magnetic spherules. A 2009 study by Jennifer Marlon found no spike in charcoal or pollen data aligning with the YD boundary, and black mats appeared before the YD cooling. A 2012 study by Jeffrey Pigati found black mats at sites of different ages, suggesting natural processes, not impacts, explain them. Firestone criticized Pigati’s methods, but Pigati stood by his findings, noting that some markers cited in the 2007 study are not unique to impacts. A 2009 study by Todd A. Surovell found no magnetic mineral spike at YD boundary sites. Pinter and others later found the magnetic spherules were more likely iron-rich grains from Earth, not impacts.
Public interest
YDIH became well-known because it was included in documentaries on channels like National Geographic, the History Channel, and PBS's NOVA. In 2024, The New York Times described YDIH as "The Comet Strike Theory That Just Won't Die" in an article about its history and why people are interested in it. People without scientific training often accept YDIH even though scientists disagree with it. This happens because of a mental process called "epistemic vigilance," which helps people tell real facts from fake ones. Psychologists have found that when people hear conflicting information from experts, they often choose the side that matches their beliefs, political views, or cultural background. Psychologist Spencer Mermelstein explained that people may support YDIH because it offers a simple explanation that fits with what they know about Earth's history, such as "one big cause, one big outcome."
Graham Hancock wrote a book in 2015 called Magicians of the Gods, in which he claimed that the Younger Dryas comet caused a major disaster on Earth and was linked to the Noahide flood myth. He compared this story to flood myths from other cultures, suggesting the myth was common worldwide. These claims were criticized by experts like Jason Colavito, Michael Shermer, and Marc J. Defant. In a later book, America Before: The Key to Earth's Lost Civilization (2019), Hancock argued that the Younger Dryas event destroyed evidence of an advanced Ice Age civilization in North America. The Society for American Archaeology (SAA) said in an open letter that Hancock's ideas promoted "dangerous racist thinking."
The SAA asked 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. Episode 8 discussed YDIH. Physicist Mark Boslough wrote in Skeptic that many parts of the series were pseudoscience. Articles in The Guardian, Slate, The Nation, and other left-leaning publications criticized the series, while conservative outlets praised it and called criticism "left-wing propaganda." Political activist Tucker Carlson called the SAA an "elitist, closed-minded cabal" connected to the "collapse of the American idea."
On YouTube, many videos discuss YDIH, increasing its visibility. Similar divisions appear online, with people who support YDIH often calling skeptics "villains" or "part of a scientific cabal" or "victims of groupthink." Despite scientific disagreement, YDIH continues to grow in popularity because of its appeal to people who believe in conspiracy theories and reject scientific expertise.