The Younger Dryas impact hypothesis
Did a comet trigger a sudden cold snap at the end of the last ice age? A fair look at a fierce debate.
This page describes a scientific hypothesis that is still argued over. It sets out the case for it, the case against, and where the evidence stands today.
Key facts
- When
- proposed for about 12,900 years ago
- Where
- Proposed: North America, Europe and the Middle East
- What hit
- proposed: a fragmenting comet, with many airbursts rather than one crater
- Era
- In the age of humans
Not measured: supporters suggest many airbursts spread over continents; critics argue there is no clear sign that any impact happened.
No crater has been found. Most researchers do not accept the hypothesis.
What happened
About 12,900 years ago, as the last ice age was ending, much of the northern hemisphere was plunged back into near-glacial cold for about 1,200 years. This cold spell is called the Younger Dryas, after a small Arctic flower whose pollen appears in lake sediments of that time.
Most climate scientists explain it with a change in ocean currents. As the great ice sheets melted, huge amounts of fresh water flowed into the North Atlantic, weakening the currents that carry warm water north. Geological evidence for these floods exists, although their exact routes are still argued over.
In 2007 a group of researchers proposed a different trigger: a comet that broke up and exploded over North America and beyond, setting fires and changing the climate. They suggested it might also explain the disappearance of mammoths and other large animals in the Americas, and changes in the Clovis culture of early Americans.
Effects
Supporters point to a layer at the start of the Younger Dryas at many sites that they say contains peaks in tiny magnetic and glassy spherules, nanodiamonds, platinum, soot and melted glass. They have argued that a settlement at Abu Hureyra in Syria shows signs of such an event. Critics reply that many of these markers are not unique to impacts and form in ordinary fires or settle out of space all the time; that other teams could not reproduce several findings; that the layers at different sites are not all the same age; and that the large animals died out over thousands of years, not in one moment. A comprehensive review in 2023 concluded the hypothesis is not supported, and supporters have published detailed rebuttals.
The evidence
There is no crater, no shocked quartz widely accepted by impact specialists, and no layer that everyone agrees on. The Hiawatha crater beneath the Greenland ice was briefly suggested as a candidate, but in 2022 it was dated to about 58 million years old. For now the Younger Dryas impact hypothesis remains a minority view: unproven, but still being tested.
Did you know?
The Younger Dryas ended even more suddenly than it began: parts of Greenland warmed by several degrees within a few decades.
A 2021 claim that an airburst destroyed a Bronze Age city in Jordan, linked by its authors to the same ideas, drew heavy criticism from other scientists.
Related pages
How is impact energy measured? See energy explained, or try the impact calculator.