The Younger Dryas — a sudden 1,200-year cold period that began 12,800 years ago — has been attributed to a cosmic impact for over a decade. The impact hypothesis proposed that a comet or asteroid struck the Laurentide ice sheet, destabilizing Atlantic circulation and plunging the Northern Hemisphere into near-glacial conditions.
New analysis of Greenland ice cores points to a different trigger: volcanic eruptions. The timing, chemical signatures, and distribution of sulfate aerosols in the ice record are consistent with a cluster of major eruptions that injected sulfur dioxide into the stratosphere, reflecting sunlight and cooling the planet.
The distinction matters mechanistically. A cosmic impact is a single event — instantaneous, unique, unrepeatable. Volcanic eruptions are recurring processes with known dynamics. If the Younger Dryas was volcanic, it belongs to a class of events that can happen again. If it was an impact, it was a statistical anomaly.
The impact hypothesis persisted partly because it was dramatic — a single catastrophic explanation for a dramatic climatic event. The volcanic explanation is less narratively satisfying. Multiple eruptions whose individual effects compounded into a sustained cooling episode lacks the clean causality of a single strike.
But climate systems do not respect narrative preferences. The evidence increasingly supports compound causation — multiple moderate forcing events that individually would not be remarkable but collectively pushed the climate system past a threshold. The Younger Dryas may not have had a single cause. It may have had a cluster of causes that happened to align, which is harder to tell a story about but easier to defend with data.