On the last day of the Cretaceous period, a 7.5-mile-wide asteroid slammed into Mexico’s Yucatán Peninsula and changed the arc of life on Earth. Sixty-six million years later, scientists have used supercomputers to re-create the devastation of that infamous impact in unprecedented detail. The models are helping to solve a longstanding mystery about what killed off all dinosaurs except birds and offering a new glimpse at how life on Earth responds to rapid environmental change.
Scientists already knew that the extraterrestrial impactor blew a crater roughly 120 miles wide into Earth’s crust, hitting in just the right spot—and at just the right angle—to fling colossal amounts of cooling gases and soot into the upper atmosphere. Skies blackened as massive tsunamis tore across the oceans, and wildfires raged for hundreds of miles around. Within years, temperatures plummeted more than 60 degrees Fahrenheit, plunging the world into a prolonged cold period known as an impact winter that scientists believe killed off more than three-quarters of life on Earth. (Find out more on how the dinosaurs went extinct.)
The event was like “Dante’s Inferno on Earth,” says Alfio Alessandro Chiarenza, a research associate at University College London, and leader of a new study published today in the Proceedings of the National Academy of Sciences.
Around the same time the asteroid struck—known as the Chicxulub impact—a large volcanic complex in what’s now Southern India was erupting, releasing more than 200,000 cubic miles of lava and pumping climate-altering gases into the sky. While most scientists agree that the asteroid triggered the extinction event, researchers have long wondered whether these volcanoes, called the Deccan Traps, also contributed to the devastation of life.
In the new study, Chiarenza and his colleagues use models to re-create Earth’s ancient climate, tweaking the variables of various apocalyptic scenarios. These simulations reveal that the asteroid alone rendered the planet uninhabitable for all dinosaurs other than birds. Perhaps counterintuitively, the volcanoes of the Deccan Traps may have actually 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-Paleogene mass extinction happened with remarkable speed. Intense cold, constant darkness, wildfires, tsunamis, unbearable heat in the impact area, and eventual acid rain mangled the planet. The sudden destruction caused by the extinction event presents an opportunity for today’s scientists to study how life may respond to rapid, severe stresses.
“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.
To truly understand how the mass dinosaur extinction played out, though, scientists need to agree on what, exactly, caused it.
In the last decade, geologists have confirmed that the Deccan Traps erupted in several pulses over 700,000 years, a period that overlaps with the Chicxulub impact. Because the volcanoes were erupting during the extinction event, scientists have wondered if they played a part in killing the animals. Two of the five biggest mass extinctions were caused by severe warming from volcanic carbon dioxide, including the worst of all: The Permian-Triassic mass extinction 252 million years ago, triggered by ancient eruptions in what’s now Siberia, wiped out 96 percent of marine life and about three of every four species on land.
The Deccan Traps could have affected life 66 million years ago in two major ways. On shorter timescales, sulfur dioxide released by the volcanoes could have cooled the planet and fostered acid rain, throwing Earth’s oceans—and broader chemical cycles—into disarray. Over time, the vast amount of CO2 released in the eruptions could have led to steady warming, potentially stressing global ecosystems.
Two efforts published last year that tried to date the biggest pulse of eruptions from the Deccan Traps disagree by tens of thousands of years—a difference between the biggest eruptions occurring before the asteroid impact, when they could have affected the die-offs, or a short time after, when they would have played no role in driving the extinction.
To put the disasters to the test, Chiarenza and his colleague Alexander Farnsworth, a climatologist at the University of Bristol, built computer models of Earth’s climate as it was 66 million years ago. They ran 14 different scenarios that included the asteroid impact, the Deccan Traps, and the two events combined. The simulations assumed CO2 levels of between 560 and 1,680 parts per million—as much as four times higher than today. The scientists also dimmed the virtual sunlight by between 5 percent and 20 percent from pre-impact levels.
In some of the simulations, Chiarenza and Farnsworth also modeled the Chicxulub impact’s short-term cooling effects by injecting a hundred times more ash and aerosols than the Mount Pinatubo eruption that rocked the Philippines in 1991. To track how the disasters affected dinosaurs, Chiarenza mapped out the ancient animals’ likeliest habitats with another computer model based on ancient climate data and dinosaur fossil locations.
All of the models showed the Deccan Traps could not have caused dinosaur die-outs. The long-term warming the volcanoes caused wouldn’t have eliminated the dinosaurs; if anything, it expanded the land area they could comfortably inhabit. Even the most extreme dimming scenario from the Deccan Traps didn’t wipe out dinosaurs’ ecological niche, the new study shows.
The asteroid impact scenarios, however, were downright horrific. In some, average land temperatures plummeted from more than 68°F (20°C) to well below zero, and precipitation declined by between 85 percent and 95 percent. When the virtual Chicxulub impact dimmed sunlight by 15 percent or more, no habitat anywhere on Earth could support non-avian dinosaurs. (Find out more about how birds survived the mass extinction.)
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 revealed something unexpected: The Deccan Traps may have actually helped life recover, as the volcanoes’ CO2 emissions blunted the impact 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 work suggests the Deccan Traps likely erupted in a trickle of activity over hundreds of thousands of years, rather than dealing a sharp global blow to ecosystems. Last November, a group led by Hull found that Earth’s oceans rapidly acidified in the tens of thousands of years after Chicxulub—likely from post-impact acid rain—but ocean pH levels were stable during the 100,000 years or so before the impact, even while the Deccan Traps were already erupting.
In a follow-up study, Hull showed that in the 300,000 years before the impact, global temperatures gradually rose and fell by about 4 degrees Fahrenheit, indicative of rising and falling CO2 levels, but nothing so extreme that it would threaten the dinosaurs.
There are also signs far from the lava flows in India that hint at the Deccan Traps’ role in facilitating life’s recovery. Last October, a group led by Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science, unveiled several sites in the Rocky Mountains that record North America’s post-asteroid flora and fauna. Lyson’s team found that for 100,000 years after the impact, ecosystems didn’t have many types of life—but then, mammal and plant diversity blossomed in pulses that correlate with mild warming periods, consistent with 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.”