In an exchange in which each side uses only 50 Hiroshima-size bombs—just 0.3 percent of the world's arsenal—the initial explosions could kill more than 20 million people, the scientists calculate.
But more far-reaching would be the resulting fires, which would fill the upper atmosphere with soot—destroying the Earth's ozone layer, blocking sunlight, and reducing average global temperatures by 2 degrees Fahrenheit (1.25 degrees Celsius), said co-author Alan Robock of Rutgers University in New Jersey.
The effect would persist for several years and be stronger at mid-latitudes, including the U.S. and Europe
The soot would absorb sunlight before it reached Earth's surface, reducing temperatures and causing the soot to rise dozens of miles higher.
Eventually the soot would settle into the upper layer of the atmosphere known as the stratosphere, where it would block sunlight for many years.
This would have a devastating impact on agriculture, causing a 10 percent reduction in rainfall and shrinking the growing season in some parts of the globe by as much as 30 days.
Stephen Schneider of Stanford University in Palo Alto, California, said, "Nobody can use these things without the effects spilling over to the rest of the planet."