Nuclear winter

Nuclear winter is a severe and prolonged global climatic cooling effect that is hypothesized to occur after widespread urban firestorms following a large-scale nuclear war. The hypothesis is based on the fact that such fires can inject soot into the stratosphere, where it can block some direct sunlight from reaching the surface of the Earth. It is speculated that the resulting cooling, typically lasting a decade, would lead to widespread crop failure, a global nuclear famine, and an animal mass extinction event.

Climate researchers study nuclear winter via computer models and scenarios. Results are highly dependent on nuclear yields, how many cities are targeted, their flammable material content, and the firestorms' atmospheric environments, convections, and durations. Firestorm case studies include the World War II bombings of Hiroshima, Tokyo, Hamburg, Dresden, and London, and modern observations from large-area wildfires such as the 2021 British Columbia wildfires.

Studies suggest that a full-scale nuclear war, expending thousands of weapons in the largest arsenals in Russia and the United States, could cool global temperatures by more than 5 °C, exceeding the last ice age. According to these models, five billion people would die from famine within two years, and 40–50% of animal species would go extinct. Studies of a regional nuclear war involving hundreds of weapons, such as between India and Pakistan, could also cause cooling of a few degrees, threatening up to two billion people and making 10–20% of animal species extinct. However, many gaps remain in the understanding and modeling the effects of nuclear war.