If you were lucky enough to be in the solar eclipse's path of totality this week, you might have seen a few minutes of darkness in the middle of the day. But that's nothing compared to 65 million years ago, when the sun may have disappeared for a year and a half. The huge asteroid that crashed into Earth back then may have wiped out the dinosaurs, but the force of the impact alone wasn't the problem. New simulations show that particles thrown into the atmosphere would have blocked sunlight for up to 18 months, disrupting the photosynthesis of plants and cascading into one of the worst mass extinction events the world has ever seen.
An impact by a massive asteroid, at least 10 km (6.2 miles) wide, is the leading theory on what caused the Cretaceous-Paleogene (K-Pg) extinction event, thanks to abnormal layers in the geologic record and the huge Chicxulub crater in the Gulf of Mexico, which dates back to the same time. While the dinosaurs are the most famous victims, scientists estimate that it also wiped out 93 percent of mammals alive at the time, and devastated most other species.
The death toll wasn't limited to those animals unlucky enough to be in the direct firing line. NASA says that a collision by any space rock wider than 1 km (0.6 miles) would be a global catastrophe, triggering earthquakes, tsunamis, volcanic eruptions and widespread fire storms. And even after those settled down, the effects would have been felt for thousands of years. A new study led by the National Center for Atmospheric Research (NCAR) modeled how soot from the global wildfires would have affected the climate.
"The extinction of many of the large animals on land could have been caused by the immediate aftermath of the impact, but animals that lived in the oceans or those that could burrow underground or slip underwater temporarily could have survived," says Charles Bardeen, lead researcher on the study. "Our study picks up the story after the initial effects — after the earthquakes and the tsunamis and the broiling. We wanted to look at the long-term consequences of the amount of soot we think was created and what those consequences might have meant for the animals that were left."
To start their simulations, the NCAR researchers worked with the most recent estimates of soot levels, as found in the geological record: some 15 billion tons. From there, they ran tests with higher and lower amounts to study how it might affect the climate differently.
The study found that soot would have gathered in the upper atmosphere, forming a thick blanket over the planet that blocked most of the sunlight. Immediately after the impact, the researchers say the day would have been as dark as a moonlit night. Over time the skies slowly brightened back to normal, but it would have taken a full 18 months for sunlight to return to a level that would allow plants to photosynthesize.
Plants that weren't already destroyed by fire would have been done in by the darkness. That includes marine plants like phytoplankton, the one-celled organisms at the bottom of the house-of-cards that is the food chain. When they were devastated, the rest of the marine life alive at the time toppled too, leading to the extinction of giant marine reptiles like plesiosaurs and mosasaurs.
Even when the researchers ran simulations with a conservative 5 billion tons of soot, they found that photosynthesis would still have been disrupted for a full year, dealing a damaging blow to the food web. Temperatures dropped drastically too, with the model saying they may have fallen by as much as 28º C (50º F) on land and 11º C (20º F) over the oceans.
Surprisingly, the simulations suggest that once the soot started to clear, it would have done so fairly quickly. Full of ash, the stratosphere warmed dramatically, but as it settled back down to Earth, the high atmospheric air cooled. Then, water vapor condensed into ice and washed more of the soot back to the ground, clearing the dusty layer in a matter of months.
While it has helped the team understand the run-on effects of a large asteroid impact, the researchers are careful to say that the simulation isn't perfect. To start with, it's based on the Earth as it is today, which has a very different atmosphere and geography to the Cretaceous period. The model also wasn't designed to account for a curveball as large as an asteroid impact.
"An asteroid collision is a very large perturbation — not something you would normally see when modeling future climate scenarios," says Bardeen. "So the model was not designed to handle this and, as we went along, we had to adjust the model so it could handle some of the event's impacts, such as warming of the stratosphere by over 200º C (392º F)."
Taking this into account, the simulation might be more useful for similar scenarios today, like studying the climatic effects of nuclear winter.
"The amount of soot created by nuclear warfare would be much less than we saw during the K-Pg extinction," says Bardeen. "But the soot would still alter the climate in similar ways, cooling the surface and heating the upper atmosphere, with potentially devastating effects."
The research was published in the journal Proceedings of the National Academy of Sciences.
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