This is because smoke from the enormous nuclear firestorms created by even a “successful” US nuclear first-strike would cause catastrophic disruption of global climate and massive destruction of the Earth’s protective ozone layer, leading to global famine.
Recent peer-reviewed studies, done by atmospheric scientists Alan Robock (Rutgers), Brian Toon (University of Colorado-Boulder), Richard Turco (UCLA) and colleagues, predict that such an attack would create immense firestorms that would quickly surround the planet with a dense stratospheric smoke layer.
The black smoke would be heated by the sun, lofted like a hot air balloon, and would remain in the stratosphere for at least 10 years. There it would block and prevent a large fraction of sunlight from reaching the Earth’s surface. The sharp reduction of warming sunlight would rapidly produce global Ice Age weather conditions. This would eliminate or dramatically reduce growing seasons for a decade and would likely cause the starvation of most or all humans.
Along with other effects – including prolonged destruction of the ozone layer – most complex life on Earth could be destroyed. Scientists say the process would be similar to when an asteroid hit the Earth some 65 million years ago, raising a global dust cloud that reduced sunlight, lowering temperatures and killing vegetation. That caused the extinction of the dinosaurs and 70 percent of the Earth’s species.
The cause of extinction in our case would not be an external, celestial event, but rather the launching of thermonuclear weapons we had created by our own cleverness, supposedly for our own security.