The asteroid impact that wiped out the dinosaurs caused temperatures to rise by 5°C. Earth stayed that hot for 100,000 years.
That’s much hotter than expected, which could mean we are underestimating how much the planet will warm in the next few centuries.
“The implication is that the amount of warming that we are likely to see is greater than current predictions,” says Ken MacLeod of the University of Missouri in Columbia.
The asteroid that ended the reign of the dinosaurs around 66 million years ago slammed into rocks rich in carbonates, releasing immense quantities of carbon dioxide into the atmosphere. It also triggered vast wildfires, releasing even more C02.
So it has long been thought that after a few years of an “impact winter” caused by dust blocking sunlight, there was rapid global warming much like what is happening today. There have been countless other times in Earth’s history when rising CO2 levels warmed the planet, but it has usually happened over many thousands of years.
“The warming would likely have happened even faster than warming today,” says MacLeod. “It’s a much closer natural experiment to what we are doing today.”
But how much did the planet warm? For decades, researchers have been hunting for direct evidence. MacLeod’s team finally found it in rocks in Tunisia.
These rocks formed from sediments deposited before, during and after the time of the impact, in shallow waters in what was then the Tethys Sea. The team extracted hundreds of tiny fish teeth, bones and scales from the rock and analysed the ratio of oxygen isotopes in them. This is a standard way to work out past water temperature.
But the findings pose a puzzle. Climate models suggest that CO2 levels must have soared to around 2300 parts per million to cause such an increase in temperatures. But studies of fossil soils, and of how much CO2 would have been released by the impact, suggest CO2 levels were less than half this.
Only one 2004 study of a few fossil leaves suggests CO2 levels could have been as high as 2300 ppm, and even the authors of this study acknowledge that this could be wrong. “Our estimate based on stomata was provisional, as we were at pains to emphasize,” says David Beerling of the University of Sheffield, UK.
So if CO2 levels did not rise much above 1000 ppm and yet temperatures really rose 5°C, the implication is that CO2 causes more warming than the current “most likely” estimates. And that’s reasonable, says MacLeod, because there is a lot of uncertainty about how strongly the climate responds to higher CO2 levels.
A series of unfortunate events
It is also possible that the team have overestimated the temperature rise. For instance, if there were fewer bottom-dwelling fish after the impact, this would skew the temperature record, says Johan Vellekoop of KU Leuven in Belgium. But there is no particular reason to think this is the case, he says.
The rapid warming after the impact may have been the final nail in the coffin for many of the plants and animals that survived the fires, tsunamis and years of darkness and acid rain. “It’s always the combination of the factors,” says Vellekoop.
Some researchers argue that massive volcanic eruptions shortly before the impact played a part in the extinction of the dinosaurs. These eruptions covered a huge part of what is now India in lava – the Deccan traps. MacLeod hopes to extend his temperature record to cover this time period, so see if the eruptions had a big impact on climate.
But Matthew Huber of Purdue University thinks the evidence is already clear. “It was all the impact,” he says. “People need to move on.”
TunisianMonitorOnline (New Scientist)