What you won't learn about in Oppenheimer: the potential effects of a nuclear winter

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This week:

The movie Oppenheimer, which is approaching half a billion dollars at the global box office, has renewed interest in the history of the atomic bomb. 

The Trinity test, which was the first atomic bomb exploded and the centrepiece of Christopher Nolan's film, took place more than 78 years ago. But the possibility of nuclear warfare remains to this day. For example, earlier this week, former Russian prime minister Dmitry Medvedev threatened the use of nuclear weapons if Ukraine's current counteroffensive threatens Russian territory.

One potential effect of the atom bomb wasn't understood until years after the death of J. Robert Oppenheimer, the director of the Manhattan Project. Specifically, the concept of nuclear winter, which was first brought to the world's attention by astrophysicist and author Carl Sagan in 1983. 

Virtually every modern climate model has confirmed the initial findings: nuclear war would cool the planet.

"Nuclear weapons dropped on cities and industrial areas would produce fires, the fires would produce smoke, and that smoke would be lofted up into the stratosphere in a giant thunderstorm," Alan Robock, a climate scientist at Rutgers University in New Jersey, said in a recent interview.

Such fire-driven storms are known as pyrocumulonimbus clouds, and one resulted from the U.S. bombing of Hiroshima, Japan, on Aug. 6, 1945. There wasn't one after Nagasaki three days later because of uneven terrain that prevented a firestorm from forming.

Since burning major world cities is an experiment no one on Earth wants to run, climate scientists look to similar conditions to study nuclear winter.

"The basic physics are very simple: if you block out the sun, it gets cold at the ground," Robock said. "We have analogues of that. We have nighttime, we have winter."

We can also look at volcanic eruptions. For example, in 1815, Mount Tambora in Indonesia erupted, spewing aerosols that made their way into the stratosphere. These aerosols travelled all over the world, blocking sunlight and cooling the planet by an average of 3 C. The climate effects lasted at least three years. As a result, 1816 was known as the "year without a summer" in Europe and North America. Crops died because of a lack of sunlight or frost.

Why has the smoke from wildfires not caused global cooling? Unless smoke particles reach the stratosphere, they get washed out of the lower atmosphere by precipitation.

The 2017 B.C. and northwestern U.S. wildfires formed a pyrocumulonimbus event that did reach the stratosphere, but it was only about 100,000 to 300,000 tonnes of smoke. The smoke from the Hiroshima firestorm also wasn't enough to have a significant cooling effect.