A nuclear arms race is heating back up again — and with it, talk of the cold, dark nuclear winter that could follow on the heels of a nuclear war.

Within the past week, the US and Russia pulled out of a critical arms control agreement. The US is ramping up production of a new mini-nuke that could change the landscape of nuclear conflict, according to Defense News. And North Korea doesn’t appear willing to get rid of its nuclear weapons any time soon. As nuclear tensions start rising again, the threat of a nuclear winter is coming back into the frame.

It’s a subject worth talking about says Richard Turco, a professor emeritus at UCLA and one of the authors of the 1983 scientific paper that first proposed the idea. “Although there is a relatively low probability of nuclear winter happening, the potential consequences would be catastrophic — namely the destruction of human civilization,” Turco says in an email to The Verge.

“The potential consequences would be catastrophic.”

The idea is that a global nuclear war might set entire cities on fire, as Alex Ward describes for Vox. The soot from the conflagration could waft all the way into a part of the upper atmosphere called the stratosphere. There, the theory goes, the soot will shade the Earth from the sun — dropping temperatures, destroying crops, drying up the rain, and damaging the ozone layer. “It wouldn’t take very long for people to starve to death,” says Alan Robock, a professor of environmental sciences at Rutgers University who has been studying nuclear winter since the 1980s.

It’s a dark prediction for a post-apocalyptic world, and scientists are still figuring out just how bad it could get. After all, no one has dropped a nuke on a city since the US bombed Hiroshima and Nagasaki. That means there’s little real-world data for researchers like Robock to go on. “This theory is not one we want to actually test outdoors,” he says.

“This theory is not one we want to actually test outdoors.”

So scientists rely on simulations and events like forest fires and volcanic eruptions to validate their models. Different models disagree — although Robock doesn’t like to characterize the discrepancies as a debate, calling them instead an area of active research. For example, Robock and his colleagues estimate that if India and Pakistan began nuking each other, the conflict could churn out enough soot to make global temperatures plummet. But another study published in 2018 by scientists at Los Alamos National Laboratory argues that not enough soot would make it into the upper atmosphere to cause major shifts in the climate. (The study’s corresponding author did not respond to multiple requests for comment.)

“Now we come to the scientific issue that’s at the heart of the controversy,” says Kerry Emanuel, a professor of atmospheric science at MIT who wrote about the concept of a nuclear winter in the 1980s. “Are the fires hot enough, or big enough to get material up into the stratosphere?”

“Are the fires hot enough, or big enough?”

Robock and Julie Lundquist, an associate professor in the department of atmospheric and oceanic sciences at the University of Colorado Boulder, are trying to find out. We know that smoke can get into the stratosphere, Lundquist says. Researchers studying the 2017 wildfires in British Columbia, for example, discovered that storm clouds that formed because of the smoke, called pyrocumulonimbus clouds, helped deliver soot particles into the stratosphere. That’s where soot would need to go if it were going to cause long-term climate changes: soot in the lower atmosphere settles out of the air quickly, often falling to the ground with rain.

But Lundquist and her colleagues don’t know how often those pyrocumulonimbus clouds or their younger siblings, pyrocumulus clouds, are likely to form over a nuked and burning city. The conditions need to be just right, with calm winds and enough humidity. Then there’s the amount of smoke likely to rise from a nuked city, which would vary from city to city depending on the available fuel. That’s another major source of uncertainty: most fire-modeling studies have focused on wildland fires rather than major urban areas.

That’s something Lundquist is trying to change with her new models. She expects cities to produce more smoke than, say, a forest fire because of the sheer density of things that can burn. “Think about the carpets, think about papers, think about books, think about the furniture,” she says. “There’s more combustible stuff per square unit area in a city or in a suburban area.”

“Even if it’s not very likely, it could be devastating.”

Lundquist and Robock haven’t published the findings from this new research yet, but in the meantime, they want to continue the conversation about nuclear war and the possibility of a nuclear winter. “I think it’s important to let policy makers know that there could be very big consequences to political choices,” she says. “So even if it’s not very likely, it could be devastating.”

No matter how the science shakes out, there are lots of other consequences to a nuclear conflict that would be felt long before we even get to a nuclear winter. “Is that the main reason you’d want to avoid having a nuclear war? I don’t know,” says MIT’s Emanuel. “Maybe it’s a reason, but there are plenty of other reasons you don’t want to go down that road.”