For the last few decades, Exhibit A in catastrophic climate change scenarios has been the Younger Dryas event that occurred about 12,000 years ago, when a sharp cold snap in Europe and North America evidently resulted from the shut-down of North Atlantic currents. A year ago an international team reported evidence that the shutdown may itself have resulted from the explosion of a meteor or meteors over Canada. Wallace Broecker, father of the big chill scenario, commented that he might be convinced if more direct evidence were found, perhaps in the form of buckyballs, nanodiamonds, or iridum. Now in fact the discovery of nanodiamands is reported, in an article that appeared this week in Science magazine.
Douglas Kennett and colleagues report finding nanodiamands in sediments across North America that mark the onset of the Younger Dryas period. "Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval," says the abstract. "These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America."
We repeat the question we raised at the end of our report last year: Might the new scenario have some influence on some of the better informed northern Europeans who have been worrying that runaway greenhouse gas emissions could melt the Arctic ice and plunge them into an ice age? At a time when concerns are rising in Europe about the costliness of carbon reduction measures, it might indeed.
But let me now add: it really shouldn't. This is because, as Richard Alley observed last year, something had to have initiated the vast infusion of fresh water into the North Atlantic that resulted in the conveyor current's shut-down. In Broecker's scenario that event was the postulated breaking of an ice dam in the St. Lawrence river system that sent fresh waters from a vast inland lake hurtling toward the North Atlantic. In terms of the final catastrophic outcome, it doesn't really matter whether the initiating event was an ice dam break, meteors, or rising greenhouse gas emissions that capture heat and melt the Arctic and Greenland ice caps.