The theorized global effects of a nuclear war actually don’t arise from nuclear explosions, themselves, but rather from the all-consuming fires they would ignite. Dropping one hundred nukes on dense urban areas would give rise to firestorms that would send seven trillion grams of soot into the atmosphere. These particles would gradually rise into the stratosphere and linger for years, blocking the sun’s rays and lowering global temperatures. Scientists have observed this process on smaller, more localized scales with large wildfires.
According to Pearce and Denkenberger, “This would be more than sufficient to produce the lowest temperatures Earth has experienced in the past 1,000 years—lower than during the post-medieval Little Ice Age or in 1816, the so-called ‘Year Without a Summer’. It would result in a 20% drop in sunlight and lead to a 19% drop in global precipitation.”
Needless to say, a nuclear exchange of 1,000 missiles would be far more devastating. In this scenario, the authors estimate that 140,000 Americans would perish from global food shortages, in addition to the direct deaths from nuclear strikes and billions more starvation deaths around the world.