A new model of the impact of the asteroid that killed the dinosaurs (birds aside) shows just why so few species survived. Not only did temperatures shoot up, the study concludes, but they stayed exceptionally high for 100,000 years.
The effects of a large asteroid smashing into the planet are complex and can pull the climate in contradictory directions. The heat from the impact and burning forests probably raised temperatures, but soot would then have blocked out the Sun for years, creating a “nuclear winter”. On the other hand, all the carbon dioxide released when the forests burned would have stayed in the atmosphere for a far longer period, warming the planet when the Sun came out again.
To establish how all these things came together after the Chicxulub impact, a team led by Professor Ken MacLeod of the University of Missouri examined tiny traces of teeth, scales, and bone from fish in what is now Tunisia. The deposits in which this “fish debris” is found are 3 meters (9.8 feet) thick, allowing for an unusually fine timescale for the period just before the asteroid impact through to 100,000 years afterward.
In Science, MacLeod and colleagues report a 1 percent decrease in oxygen-18 isotopes in this debris, compared to the more common oxygen-16, after the impact. Heavier oxygen isotopes evaporate less easily, so their historic ratio indicates climatic changes. A 1 percent reduction may sound small, particularly since oxygen-18 concentrations were low beforehand, but it indicates a warming of around 5ºC (9ºF), one that proved very sustained.
This, of course, is the average increase in sea temperatures at what was then 20 North. Closer to the poles change was probably greater, and land temperatures were likely prone to larger swings. The paper notes this is roughly in line with an increase in atmospheric carbon dioxide from the Cretaceous 350-500 parts per million to 2,300 ppm.
The authors report: “We see no evidence of an impact winter,” but allowing for the fact that each sample represents more than 1,000 years of deposition add: “Finding evidence for this less than decade-long interval was unlikely.”
The importance of the work, the authors note, lies in what these events can teach us about future warming. Although Chicxulub was a faster and more dramatic event than human-induced greenhouse gas emissions, it provides a contrast to every other previous change in the Earth's climate we can track. All of these were much slower than what we are living through – even the warming at the end of the last Ice Age was glacially slow compared to the last four decades. As such, it can help us predict what is in store if the sharp rise in our own carbon dioxide levels is allowed to continue.