Sagan and a team of atmospheric scientists proposed that along with radiation and blast damage, a nuclear war would create enormous firestorms in cities struck by large bombs.
These would be so intense that they would inject smoke not just into the lower atmosphere, where it would eventually be removed by rainfall, but all the way into the stratosphere, where it would linger for years, block sunlight, and plunge much of the world into an extended, deadly cold snap.
Even a relatively limited nuclear exchange – such as a war between India and Pakistan – could affect regions far removed from the combat.
“Following a war in which as few as 100 of the smallest nuclear weapons are used in modern cities, urban firestorms would build for hours, producing millions of tonnes of black, sooty smoke,” says Mike Mills, of the National Centre for Atmospheric Research, US.
“Studies have shown that such smoke would cause global temperatures to plummet, creating risks of global famine.”
A critical aspect of the theory, however, was the prediction that smoke particles, once in the stratosphere, wouldn’t settle back to the lower atmosphere, but would continue to rise higher.
That would occur, Sagan’s team predicted, because smoke particles would not only block sunlight, but absorb it.
This would heat not only the particles themselves, but the surrounding air, causing it to rise in a self-lofting effect that would carry the entire smoke plume with it.
Thankfully, there haven’t been any nuclear wars in which scientists could see this effect in action. But occasionally, forest fires become intense enough to mimic the effects Sagan’s team predicted for burning cities.
That happened in British Columbia (BC), Canada, on 12 August 2017, when wildfires briefly became so intense that they created pyrocumulonimbus clouds – basically, fire-induced thunderstorms – that acted like giant chimneys that funnelled smoke directly into the stratosphere.
In a new study published in the journal Science, a team led by Pengfei Yu, an atmospheric scientist now at Jinan University, China (but who did this research at the University of Colorado, US) turned a combination of high-tech instruments on the plume, including satellites, balloons, and observations from the International Space Station.
Initially, Yu says, the plume was at elevation of 12 kilometres, slightly higher than the flight paths of most jet airliners. But in the next two months, it rose to 23 kilometres, and spread across the entire Northern Hemisphere.
“It validates the nuclear winter theory,” he says. “For the first time, we can observe that the plume is rising.”
And, he says, it persisted for about eight months before the smoke eventually dropped back into the lower atmosphere and was removed by rain.
Smoke plumes in the stratosphere, he adds, can do more than block sunlight. They can also damage the ozone layer, exposing the Earth’s surface to increased cancer-inducing ultraviolet radiation – something that might happen if climate change causes fires like those that occurred in BC to become substantially larger and more common.
But the most important finding, Yu says, is that even though the amount of smoke produced by the BC fires was 1000 to 10,000 times smaller than what would be produced by a nuclear war, it was enough to confirm that nuclear winter is indeed possible. “This validates the nuclear winter study,” he says.
Mills agrees. “This tells us that nuclear arsenals continue to represent the greatest risk to our planet.”
Nuclear winter researcher Alan Robock, a geophysicist at Rutgers University, US, notes that the 2017 Nobel Peace Prize went to the International Campaign to Abolish Nuclear Weapons (ICAN) for its efforts to get the United Nations to promulgate the 2017 Treaty on the Prohibition of Nuclear Weapons (now signed by 25 countries).