The asteroid that wiped out the dinosaurs could have thrust Earth into darkness for two years

After the massive asteroid struck 66 million years ago – once the earthquakes, tsunamis and fires died away – Earth’s remaining inhabitants would have been left clinging to life as the planet was cast into darkness, according to new research.

Computer models show that this darkness would have lasted a staggering two years, during which photosynthesis would have been impossible, temperatures would have plummeted, and marine life would have perished as a key cycle in the food chain was wiped out.

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Not only does this hint at why so many creatures disappeared during the Cretaceous-Paleogene period’s K-Pg extinction, it also goes some way to explaining how a small minority survived.

At the end of the Cretaceous period, scientists have previously estimated that more than three-quarters of all species on Earth, including all non-avian dinosaurs, disappeared at the same time a large asteroid hit in what is now the Yucatán Peninsula.

The collision would have triggered earthquakes, tsunamis and volcanic eruptions, and is believed to have launched vapourised rock and soot high above Earth’s surface, where it condensed into particles known as spherules. As these spherules fell back to Earth, they would have been heated by friction and caused global fires that grilled Earth’s surface.

The latest research picks up where this story left off.

“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,” said lead author Charles Bardeen from the National Center for Atmospheric Research (NCAR). “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.”

In past studies, researchers have estimated the amount of soot produced by global wildfires in the aftermath of the extinction event by measuring deposits in rocks. Bardeen and his colleagues Rolando Garcia, Andrew Conley and University of Colorado Boulder’s Owen Brian Toon used the NCAR-based Community Earth System Model (CESM) to instead simulate the effect of the soot on global climate going forward.

The impact on Earth’s surface

The researchers used the most recent estimates of the amount of fine soot found in the layer of rock left after the impact (15,000 million tons), as well as larger and smaller amounts, to quantify the climate’s sensitivity to more or less extensive fires.

In their simulations, soot heated by the sun eventually formed a barrier that blocked the vast majority of sunlight from reaching Earth’s surface. “At first it would have been about as dark as a moonlit night,” Toon said.

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During this time, there wouldn’t have been enough sunlight for photosynthesis to occur for more than a year and a half. The majority of plants on land would have been incinerated in the fires so any that survived, as well as phytoplankton in the oceans, would have struggled to survive in these harsh conditions. The loss of these tiny organisms would have had a ripple effect through the ocean, eventually devastating many species of marine life.

This simulated loss of sunlight would have additionally led to a steep decline in average temperatures at Earth’s surface, with a drop of 28°C (50°F) over land and 11°C (20°F)  over the oceans. What’s more, these results would have still applied even at the lower estimates of soot levels – around 5,000 million tons. These levels would have blocked photosynthesis for at least a year.

The impact on Earth’s atmosphere

Beyond Earth, while our planet’s surface temperature dropped, the atmosphere higher up in the stratosphere would have warmed as the soot absorbed light from the sun. These warmer temperatures destroyed the ozone, causing large quantities of water vapor to be stored in the upper atmosphere, which reacted in the stratosphere to produce hydrogen compounds and fuel the destruction further. A loss of ozone would have let damaging doses of ultraviolet light to reach Earth’s surface after the soot cleared.

To the scientists’ surprise, this water vapour did conversely help the situation by removing the soot from the atmosphere. As the soot began to settle out of the stratosphere, the air began to cool. This cooling, in turn, caused water vapor to condense into ice particles, which washed even more soot out of the atmosphere. As a result of this feedback loop, the thinning soot layer disappeared in just a few months.

The “nuclear winter” scenario

There are limitations to these findings, though. While the scientists believe their study gives a robust picture of how large injections of soot into the atmosphere can affect the climate, the simulations were run in a model of modern-day Earth.

This model doesn’t accurately represent what Earth looked like during the Cretaceous Period, when the continents were in slightly different locations. The atmosphere 66 million years ago also contained different concentrations of gases, including higher levels of carbon dioxide.

Additionally, the simulations didn’t try to account for volcanic eruptions or sulphur released from the Earth’s crust at the site of the asteroid impact, which would have resulted in an increase in light-reflecting sulphate aerosols in the atmosphere.

“An asteroid collision is a very large perturbation – not something you would normally see when modelling future climate scenarios,” Bardeen said. “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 200˚C.”

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That said, these findings could be useful to scientists when modelling a “nuclear winter” scenario. Like global wildfires millions of years ago, the explosion of nuclear weapons could similarly inject large amounts of soot into the atmosphere, which could lead to a temporary global cooling.

“The amount of soot created by nuclear warfare would be much less than we saw during the K-Pg extinction,” Bardeen said. “But the soot would still alter the climate in similar ways, cooling the surface and heating the upper atmosphere, with potentially devastating effects.”

These new details about how the climate could have dramatically changed following the impact of a 10km-wide asteroid have been published in the Proceedings of the National Academy of Sciences. The study was carried out with support from NASA and the University of Colorado Boulder.