On the last day of the Cretaceous period, a 7.5-mile-wide asteroid crashed into Mexico’s Yucatán Peninsula and changed the Earth’s arc of life. Sixty-six million years later, scientists developed supercomputers to re-create the destruction of the notorious impact in unprecedented detail. The models help solve the long-standing mystery of what killed all dinosaurs except birds and provides a new insight into how life on Earth reacts to rapid environmental change.
Researchers already understood that the extraterrestrial impactor blew a crater about 120 miles wide into the Earth’s surface, hitting the right spot— and just at the right angle— to fling huge amounts of cooling gases and soot into the upper atmosphere. The sky blackened as gigantic tsunamis tore across the oceans, and wildfires raged for hundreds of miles. Throughout the years, temperatures have plummeted more than 60 degrees Fahrenheit, plunging the earth into a prolonged cold period known as an impact winter that researchers believe destroyed more than three-quarters of life on Earth.
The incident was like “Dante’s Inferno on Earth,” says Alfio Alessandro Chiarenza, research associate at University College London, and leader of a new study published today by the National Academy of Sciences. Just around the same time, the asteroid, known as the Chicxulub impact, struck a massive volcanic complex in what is now Southern India, releasing more than 200,000 cubic miles of lava and pumping climate-altering gasses into the sky.
Although most scientists believe that the asteroid caused the extinction event, experts have long questioned if these volcanoes, called the Deccan Traps, have also contributed to the devastation of life. In the current study, Chiarenza and his colleagues are using simulations to re-create the ancient climate of the Planet, tweaking the variables of various apocalyptic scenarios. Such simulations demonstrate that the asteroid alone has made the Earth uninhabitable for all dinosaurs other than birds.
Perhaps counter-intuitively, the volcanoes of the Deccan Traps may have made Earth more hospitable, not less. “This is, I would think, a nail in the coffin [for the hypothesis that] the Deccan Traps drove the mass extinction,” says paleontologist Anjali Goswami, a research leader at the Natural History Museum, London, who wasn’t involved with the study.
Named for the two geologic periods on either side of the event, the Cretaceous-Paleogen mass extinction occurred at extraordinary speed. Intense cold, persistent darkness, wildfires, tsunamis, intolerable heat in the area of impact, and potential acid rain have mangled the planet. The sudden disruption caused by the extinction event is a chance for scientists today to research how life can react to rapid, extreme stress.
“It gives us an idea of what organisms do when someone pulls the rug out from under them,” says Yale University paleontologist Pincelli Hull, an expert on the extinction of the dinosaurs.
However, to fully understand how mass dinosaur extinction worked out, scientists need to agree on what, precisely, caused it. In the last decade, geologists have reported that the Deccan Traps erupted in multiple pulses in 700,000 years, a period that overlaps with the Chicxulub impact.
Since volcanoes erupted during the extinction event, scientists wondered whether they played a role in killing the animals. Four of the five largest mass extinctions were caused by extreme volcanic carbon dioxide warming, including the very worst of all: 252 million years ago, the Permian-Triassic mass extinction, triggered by ancient eruptions in what is now Siberia, wiped out 96 percent of marine life and around three in every four species on land.
The Deccan Traps could have impacted life 66 million years ago in two main ways. In shorter timescales, the sulfur dioxide released by volcanoes may have cooled the earth and triggered acid rain, bringing the Earth’s oceans— and larger chemical cycles— to disarray. Over time, the large amount of CO2 emitted during the eruptions may have contributed to steady warming, eventually stressing global ecosystems.
Two attempts reported last year, which tried to date the biggest pulse of the Deccan Traps eruptions, disagree by tens of thousands of years — the difference between the biggest eruptions that happened prior to the asteroid impact, when they may have triggered the die-offs, or a short time after when they would have played no part in driving the extinction.
To test the disasters, Chiarenza and his colleague Alexander Farnsworth, climatologist at the University of Bristol, developed computer models of the Earth’s climate 66 million years ago. They simulated 14 different scenarios, including the impact of the asteroid, the Deccan Traps, and the two events combined.
The simulations predicted a CO2 level of between 560 and 1,680 parts per million— as much as four times higher than today. Scientists also dimmed virtual sunlight by between 5% and 20% from pre-impact levels. For some of the simulations, Chiarenza and Farnsworth have modeled the short-term cooling impact of Chicxulub by introducing a hundred times more ash and aerosols than the eruption of Mount Pinatubo that rocked the Philippines in 1991.
To track how disasters affected dinosaurs, Chiarenza mapped out the most ancient animal habitats with another computer model based on ancient climate data and fossil dinosaur locations. All the results showed that the Deccan Traps could not have caused dinosaur die-outs.
The long-term warming triggered by the volcanoes wouldn’t have killed the dinosaurs; if anything, it would have expanded the land they could safely occupy. Even the most severe dimming scenario in the Deccan Traps didn’t wipe out the ecological niche of dinosaurs, the new study reveals. The scenarios of the asteroid impact, however, were really horrific.
In some, the average land temperatures plummeted from more than 68 ° F (20 ° C) to well below zero, and precipitation decreased by between 85 and 95 percent. When the virtual impact of Chicxulub dimmed sunlight by 15% or more, no habitat anywhere on Earth could support non-avian dinosaurs.
Looking at the new data, “it becomes quite reasonable why some things went extinct,” Goswami says. “Actually, it becomes amazing that everything didn’t.”
The research team ‘s models also discovered something unexpected: the Deccan Traps may potentially have helped to restore life, as the CO2 emissions of the volcanoes blunted the impact of winter’s severity. “That’s a great plot twist,” Hull says. “I don’t think anyone was thinking about volcanism making the impact less bad. That’s truly surprising.”
Recent research indicates that Deccan Traps could have erupted in a trickle of activity over hundreds of thousands of years, rather than having a drastic global effect on ecosystems. Last November, a group led by Hull found that Earth’s oceans were rapidly acidifying in the tens of thousands of years after Chicxulub — probably from post-impact acid rain— but ocean pH levels remained constant for the next 100,000 years or so, even as the Deccan Traps had already erupted.
In a follow-up study, Hull found that in the 300,000 years before the impact, global temperatures slowly rose and fell by around 4 degrees Fahrenheit, suggesting rising and dropping levels of CO2, but nothing so drastic as to threaten dinosaurs. There are also signs far from the lava flows in India that suggest the role of Deccan Traps in promoting the recovery of life.
Last October, a group led by Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science, unveiled a number of sites in the Rocky Mountains that document North America ‘s post-asteroid flora and fauna. Lyson’s team found that for 100,000 years after the impact, habitats didn’t have many types of life— but then, animals and plant diversity blossomed in pulses that correspond to mild warming periods, consistent with the CO2 spurts that the Deccan Traps could have given off.
The recent studies, Lyson says, have led to a captivating idea: “Deccan as the creator, versus Deccan as the destroyer.”