On a very bad day 66 million years ago, a mountain-sized object from space slammed into Earth, initiating a cascade of calamities that eradicated three-fourths of the species on the planet, including the non-avian dinosaurs.
The buried remnants of the 125-mile-wide (200 km wide) crater have been identified on the Yucatan Peninsula and in the Gulf of Mexico. Scientists have long theorized that an initial pulse of heat was followed by a devastating global winter.
After that, as carbon dioxide in the atmosphere surged, the planet became a hothouse.
A new study published Thursday in the journal Science has produced hard data to support that global warming hypothesis, and it may have unnerving implications for the world we live in today.
The effects of the Chicxulub impact, named for a Yucatan town, produced 5 degrees Celsius (9 degrees Fahrenheit) average warming in a subtropical sea, and this heating persisted for 100,000 years, the researchers concluded.
"This is crocodiles at the poles and large areas of the tropics uninhabitable on land," explained lead author Ken MacLeod, a University of Missouri paleontologist.
The study suggests that even a relatively brief pulse of CO2 can have a lingering effect. That's relevant today given many countries' massive greenhouse-gas emissions, which are creating a spike in atmospheric carbon dioxide and associated global warming.
"The cascading implication of our finding is that carbon dioxide loading would have occurred for just maybe a decade, and the greenhouse warming persisted for 100,000 years," MacLeod said.
"Even if we go back to 1850 levels of CO2 emissions today, we're locked into 100,000 years of the Earth responding to the CO2 we've already put in."
The research is based on fish debris - bones, teeth, scales - retrieved from an outcropping in Tunisia known as El Kef.
It's a famous site, featuring a geological formation with sedimentary layers from the end of the Cretaceous period (when dinosaurs roamed the Earth) and the start of the Paleogene period.
This is known now to scientists as the K/Pg boundary.
The fish debris serves as a kind of thermometer, said Page Quinton, who began the work as a doctoral student with MacLeod and is now a professor at the State University of New York at Potsdam.
The sand-sized fragments of fish contain isotopes of oxygen - atoms that have different numbers of neutrons and different atomic weights.
Water temperature affects the relative abundance of those isotopes. When the fragments show a shift in the isotopic ratio, that signals a change in temperature, Quinton said.
Preliminary investigation of some samples from El Kef produced a "wow" moment four years ago, with clear indication of long-term global warming, she said.
The researchers then obtained more samples and continued scrutinizing the debris, and the pattern initially detected held up over time.
"We're providing the first empirical evidence that there's actually warming after the impact," she noted.
It appears the dinosaurs and much of life on Earth died out in a triple whammy, or maybe a quadruple whammy, depending on how you're counting.
First came the impact itself, with shock waves and tsunamis. In the minutes and hours that followed, the frictional heating from debris falling back into the atmosphere was so intense that "the sky became an oven," MacLeod said.
Wildfires broke out globally.
Then came years of cold and darkness as sulfates, dust and soot in the upper atmosphere blocked light from the sun.
"The first six months, it was almost a blackout," said NASA planetary scientist Adriana Ocampo, who was not involved in the new study.
If not for the warmth retained in Earth's vast oceans, she said, "our planet would have frozen."
MacLeod painted a bleak picture: "Anything that's not killed by the thermal heat pulse likely had to deal with years of very little, if any, vegetation, and anything that survived that then had to survive 100,000 years of quite substantial greenhouse conditions."
This is a contentious scientific field, and the new paper quickly generated pushback from Gerta Keller, a Princeton geologist who has long argued that the end-Cretaceous mass extinction was triggered by volcanism in India - a huge flood of basaltic lava that created a vast geological formation known as the Deccan Traps.
Keller, who read the study in advance of publication, said her interpretation of the El Kef formation and the sedimentation rate that created it suggests a much longer period of greenhouse warming, about 500,000 years.
That could not have been caused by a single injection of carbon dioxide, such as by the Chicxulub impact, but would be consistent with a protracted volcanic era, she said.
Paul Renne, a geologist with the Berkeley Geochronology Center who has argued that the shock wave from the Chicxulub impact may have intensified the Deccan Traps volcanism, said in an email: "It is most extraordinary that the authors don't even mention volcanism. That is really bizarre."
But Brian Huber, a research geologist at the Smithsonian's National Museum of Natural History, said he was impressed by the new report. "It's a pretty tight study that's telling us a pretty important story about the longevity of CO2," he said.
"The lesson is here for us with regard to future warming, and what burning fossil fuels at the rate we're doing is doing to the atmosphere."
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