Things move around in the story queue here, but occasionally a particular item almost gets past me before I remember to cover it. Such is the recent work on the possible impact event some 12,900 years ago, which Richard Firestone (Lawrence Berkeley Laboratory) and colleagues have argued would have contributed to the extinction of such large mammals as woolly mammoths and mastodons, not to mention causing continent-wide wildfires that could have brought about the end of the Clovis culture in North America.
The period in question comes at the beginning of the Younger Dryas, a 1300-year cold spell whose termination saw the temperature of Greenland warm by over 5°C in just a few decades (see comments below). We’ve speculated about the possibility of an asteroid or comet impact on Centauri Dreams (the most recent story is here), but new analysis casts doubt on the theory. Sandy Harrison (University of Bristol) has gone to work on charcoal and pollen evidence to study how wildfires affected North America between 15,000 and 10,000 years ago. The team used charcoal and pollen accumulation in lake sediments to see whether fire regimes continent-wide showed a response to the rapid warming.
The result: There is no evidence for fires on a continent-wide scale. There are clear changes in fire frequency, however, whenever the climate changes abruptly. In fact, the authors find increases in biomass burning during periods of rapid climate change not only at 13,900 years ago, but again at 13,200 years ago and 11,700 years ago, with the timing of these changes not coincident with any changes in human populations or megafauna extinctions. The notion that a comet exploded over North America to trigger an extinction event thus appears less likely. If it did occur, it did not lead to a single, continent-wide fire event.
I dig into all this because those of us who argue that a space-based infrastructure is critical for planetary survival have an interest in publicizing the dangers of asteroid and comet impacts. But possible impacts have to be subject to the same critical methods as any other investigations, and the evidence for a Younger Dryas impact is now rendered less credible. Richard Firestone disagrees, commenting about Harrison’s work to the BBC:
“Their data is too low resolution to say much about what happened 12,900 years ago. The paper merely shows that fires increased near the onset of the Younger Dryas and continued for some time. These results are in complete agreement with what we observed.”
The more we learn about impact events, the better, and that includes finding out when extinctions are actually the result of climate change or other causes. Good science goes for the truth, wherever it may lead. The paper is Marlon et al., “Wildfire responses to abrupt climate change in North America,” Proceedings of the National Academy of Science (February 3, 2009 — published online before print). The abstract is here; the BBC story referenced above is also available online.