The asteroid smash that finished off the dinosaurs also cranked up Earth’s thermostat by 5 degrees for 100,000 years, according to shards of fish fossils found in Tunisia.
And even though the asteroid collided with Earth around 66 million years ago, the sudden global warming that followed is relevant to what’s happening today, according to Ken MacLeod, the lead author of a study outlining the findings, published in Science on Friday.
“The rate at which the planet system was perturbed by the impact is quite comparable to human perturbations over the past 200 years,” said Professor MacLeod, who is a palaeoclimatologist at the University of Missouri.
“If we turn off our fossil fuel lifestyle tomorrow, it’ll still take 100,000 years for the changes we’ve already imposed upon the Earth to come back down to something like the pre-industrial revolution state.”
To find out what the climate was like when the dinosaurs died, Professor MacLeod and his colleagues looked to a region in northern Tunisia called El Kef.
Even though it’s a good 9000 kilometres from the Chicxulub crater in Mexico — all that’s left of the asteroid impact — El Kef is renowned in palaeontology circles for its record of the Cretaceous-Palaeogene, or K-Pg, boundary.
The K-Pg boundary is a thin layer of rust-coloured sediment that’s dated to around 66 million years ago. It’s associated with the mass extinction event that famously destroyed all dinosaurs, apart from some birds.
The sedimentary layers are particularly clear at El Kef because the area, which was seabed at the time, was shielded from waves and on stable ground, away from the warping effects of earthquakes.
Professor MacLeod’s team analysed the chemical composition of sand-grain-sized fragments of fish fossils embedded in the sedimentary layers below and above the Cretaceous-Paleogene boundary, signifying the 50,000 years preceding and 300,000 years after the collision.
When fish build minerals in their teeth, bones and scales, the temperature of the surrounding water dictates how much of a specific type or isotope of oxygen, called oxygen-18, is laid down.
In the El Kef fish fossils, he and his team found the signature of a rapid, intense rise in temperature from around 20 to 25 degrees Celsius, which was sustained for 100,000 years before dropping back to pre-collision levels.
And because of polar amplification, which tends to produce bigger climate swings near the poles than the equator, it’s likely the polar regions warmed even more than the 5 degrees calculated at El Kef, Professor MacLeod said.
Didn’t the Earth cool after the collision?
You might have been taught that when the asteroid that killed the dinosaurs smashed into the Earth, fine particles were blasted into the atmosphere, blocking the sun and causing global cooling.
Even though this may have been the case in some parts, particularly close to the impact site, any local effects would have soon been swamped by overall global warming, said Andrew Glikson, a palaeoclimatologist at the Australian National University, who was not involved with the study.
“The impact could cause a rise of dust, fragments, aerosols, sulfur dioxide and so on. This is the normal process,” he said.
Where particles might hang in the atmosphere for tens or hundreds of years, he added, carbon dioxide has real staying power, sticking around for millennia.
A major source of carbon dioxide likely came from burning forests, said Chris Turney, a palaeoclimatologist at the University of New South Wales, who was also not involved in the study.
“There would have been global fires, arboreal forests bursting into flames. That amount of carbon locked in vegetation and dumped into the atmosphere almost instantaneously would have a massive effect,” Professor Turney said.
Greenhouse gases belching from the rattled ground, too, probably played a big role during the post-asteroid global warming.Vast stores of subterranean methane could have been shaken loose by the impact’s shockwave.
Methane has more than 20 times the greenhouse effect of carbon dioxide. And even though methane doesn’t last as long in the atmosphere, it breaks down into carbon dioxide, Professor Turney said.
Some palaeoclimatologists suggest the asteroid shouldn’t shoulder all responsibility for the mass extinction that followed. Rather, vast volcanic eruptions were already underway when the asteroid hit, and these got the extinction ball rolling.
The Deccan Traps in India, for instance, comprise 500,000 square kilometres of ancient volcanic flows more than 2 kilometres thick in parts.
They pre-date the asteroid impact by a couple of hundred thousand years.
Next, Professor MacLeod will analyse more fish fossils from deeper underground at El Kef to investigate the warming effects of volcanic activity.
“We’d like to extend to 2 million years before the K-Pg boundary to cover the interval where volcanism occurred to see if there’s a warming pulse [before the collision],” he said.
This, Professor MacLeod added, “would really help us calibrate our fish-based palaeo-thermometer”.