Researchers say they’ve used a world-class computer model to learn that Earth was plunged into darkness for nearly two years, following an asteroid impact 66 million years ago, at the end of Earth’s Cretaceous Period. The National Center for Atmospheric Research (NCAR) led the study, with support from NASA and the University of Colorado Boulder. An August 21, 2017 statement from NCAR/UCAR, written by Laura Snider (@lauracsnider), said:
This would have shut down photosynthesis, drastically cooled the planet, and contributed to the mass extinction that marked the end of the age of dinosaurs.
Details of the study were published August 21 in the peer-reviewed journal Proceedings of the National Academy of Sciences.
It’s thought that more than three-quarters of all species on Earth, including all non-avian dinosaurs, disappeared at the boundary of the Cretaceous-Paleogene periods, an event known as the K-Pg extinction.
Evidence suggests that the extinction occurred at the same time that a large asteroid hit Earth in what is now the Yucatán Peninsula. The collision would have triggered earthquakes, tsunamis, and even volcanic eruptions.
Scientists also calculate that the force of the impact would have launched vaporized rock high above Earth’s surface, where it would have condensed into small particles known as spherules. As the spherules fell back to Earth, they would have been heated by friction to temperatures high enough to spark global fires and broil Earth’s surface. A thin layer of spherules can be found worldwide in the geologic record.
NCAR scientist Charles Bardeen, who led the study, said:
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.
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 that might have been produced by global wildfires by measuring soot deposits still preserved in the geologic record. For the new study, Bardeen and his colleagues used the NCAR-based Community Earth System Model (CESM) to simulate the effect of the soot on global climate going forward. They 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 the simulations, soot heated by the sun was lofted higher and higher into the atmosphere, eventually forming a global barrier that blocked the vast majority of sunlight from reaching Earth’s surface. Brian Toon of the University of Colorado at Boulder, who co-authored the study, said:
At first it would have been about as dark as a moonlit night.
While the skies would have gradually brightened, photosynthesis would have been impossible for more than a year and a half, according to the simulations. Because many of the plants on land would have already been incinerated in the fires, the darkness would likely have had its greatest impact on phytoplankton, which underpin the ocean food chain. The loss of these tiny organisms would have had a ripple effect through the ocean, eventually devastating many species of marine life.
The research team also found that photosynthesis would have been temporarily blocked even at much lower levels of soot. For example, in a simulation using only 5,000 million tons of soot — about a third of the best estimate from measurements — photosynthesis would still have been impossible for an entire year.
In the simulations, the loss of sunlight caused a steep decline in average temperatures at Earth’s surface, with a drop of 50 degrees Fahrenheit (28 degrees Celsius) over land and 20 degrees Fahrenheit (11 degrees Celsius) over the oceans.
While Earth’s surface cooled in the study scenarios, the atmosphere higher up in the stratosphere actually became much warmer as the soot absorbed light from the Sun. The warmer temperatures caused ozone destruction and allowed for large quantities of water vapor to be stored in the upper atmosphere. The water vapor then chemically reacted in the stratosphere to produce hydrogen compounds that led to further ozone destruction. The resulting ozone loss would have allowed damaging doses of ultraviolet light to reach Earth’s surface after the soot cleared.
The large reservoir of water in the upper atmosphere formed in the simulations also caused the layer of sunlight-blocking soot to be removed abruptly after lingering for years, a finding that surprised the research team. 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 — cooling causing precipitation that caused more cooling — the thinning soot layer disappeared in just a few months.
While the scientists think the new study gives a robust picture of how large injections of soot into the atmosphere can affect the climate, they also caution that the study has limitations.
For example, the simulations were run in a model of modern-day Earth, not a model representing what Earth looked like during the Cretaceous Period, when the continents were in slightly different locations. The atmosphere 66 million years ago also contained somewhat different concentrations of gases, including higher levels of carbon dioxide.
Additionally, the simulations did not try to account for volcanic eruptions or sulfur released from the Earth’s crust at the site of the asteroid impact, which would have resulted in an increase in light-reflecting sulfate aerosols in the atmosphere.
Bottom line: A team led by researchers at the National Center for Atmospheric Research used computer modeling to learn that Earth was plunged into darkness for nearly two years, following the massive asteroid strike that ended the age of the dinosaurs, 66 million years ago.
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