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.. "the injection height of the smoke is controlled by the energy release from the burning fuel not from the nuclear explosion"., "...smoke plumes deep within the stratosphere over Florida that had originated a few days earlier in Canadian fires, implying that the smoke particles had not been significantly depleted during injection into the stratosphere (or subsequent transport over thousands of kilometers in the stratosphere)."
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.Potential Environmental Effects of Nuclear War, National Academy of Sciences, June 2025.
General
"Nuclear winter", or as it was initially termed, "nuclear twilight", began to be considered as a scientific concept in the 1980s after it became clear that an earlier hypothesis predicting that fireball generated NOx emissions would devastate the ozone layer was losing credibility. It was within this context that the climatic effects of soot from fires became the new focus of the climatic effects of nuclear war. In these model scenarios, various soot clouds containing uncertain quantities of soot were assumed to form over cities, oil refineries, and more rural missile silos. Once the quantity of soot is decided upon by the researchers, the climate effects of these soot clouds are then modeled. The term "nuclear winter" was a neologism coined in 1983 by Richard P. Turco in reference to a one-dimensional computer model created to examine the "nuclear twilight" idea. This model projected that massive quantities of soot and smoke would remain aloft in the air for on the order of years, causing a severe planet-wide drop in temperature.
After the failure of the predictions on the effects of the 1991 Kuwait oil fires that were made by the primary team of climatologists that advocate the hypothesis, over a decade passed without new published papers on the topic. More recently, the same team of prominent modellers from the 1980s has begun again to publish the outputs of computer models. These newer models produce the same general findings as their old ones, namely that the ignition of 100 firestorms, each comparable in intensity to that observed in Hiroshima in 1945, could produce a "small" nuclear winter. These firestorms would result in the injection of soot (specifically black carbon) into the Earth's stratosphere, producing an anti-greenhouse effect that would lower the Earth's surface temperature. The severity of this cooling in Alan Robock's model suggests that the cumulative products of 100 of these firestorms could cool the global climate by approximately 1 °C (1.8 °F), largely eliminating the magnitude of anthropogenic global warming for the next roughly two or three years. Robock and his collaborators have modeled the effect on global food production, and project that the injection of more than 5 Tg of soot into the stratosphere would lead to mass food shortages persisting for several years. According to their model, livestock and aquatic food production would be unable to compensate for reduced crop output in almost all countries, and adaptation measures such as food waste reduction would have limited impact on increasing available calories.
As nuclear devices need not be detonated to ignite a firestorm, the term "nuclear winter" is something of a misnomer. The majority of papers published on the subject state that without qualitative justification, nuclear explosions are the cause of the modeled firestorm effects. The only phenomenon that is modeled by computer in the nuclear winter papers is the climate forcing agent of firestorm-soot, a product which can be ignited and formed by myriad means. Although rarely discussed, the proponents of the hypothesis state that the same "nuclear winter" effect would occur if 100 large scale conventional firestorms were ignited.. "...fires occurred within a few months of each other in 1945, the Hamburg mass fire occurred in 1943. These five fires potentially placed 5% as much smoke into the stratosphere as our hypothetical nuclear fires. The optical depth resulting from placing 5 Tg of soot into the global stratosphere is about 0.07, which would be easily observable even with techniques available in WWII."
A much larger number of firestorms, in the thousands, was the initial assumption of the computer modelers who coined the term in the 1980s. These were speculated to be a possible result of any large scale employment of counter-value airbursting nuclear weapon use during an American-Soviet total war. This larger number of firestorms, which are not in themselves modeled, are presented as causing nuclear winter conditions as a result of the smoke inputted into various climate models, with the depths of severe cooling lasting for as long as a decade. During this period, summer drops in average temperature could be up to 20 °C (36 °F) in core agricultural regions of the US, Europe, and China, and as much as 35 °C (63 °F) in Russia. This cooling would be produced due to a 99% reduction in the natural solar radiation reaching the surface of the planet in the first few years, gradually clearing over the course of several decades.
Since the advent of photography that captured evidence of tall clouds,London 1906, San Francisco Fire and others. it has been known that firestorms could inject soot smoke and aerosols into the stratosphere, but the longevity of this slew of aerosols was a major unknown. Independent of the team that continue to publish theoretical models on nuclear winter, in 2006, Mike Fromm of the Naval Research Laboratory, experimentally found that each natural occurrence of a massive wildfire firestorm, much larger than that observed at Hiroshima, can produce minor "nuclear winter" effects, with short-lived, approximately one month of a nearly immeasurable drop in surface temperatures, confined to the hemisphere that they burned in. This is somewhat analogous to the frequent volcanic eruptions that inject sulfates into the stratosphere and thereby produce minor, even negligible, volcanic winter effects.
A suite of satellite and aircraft-based firestorm-soot-monitoring instruments are at the forefront of attempts to accurately determine the lifespan, quantity, injection height, and optical properties of this smoke.
Currently, from satellite tracking data, it appears that stratospheric smoke aerosols dissipate in a time span under approximately two months. The existence of a tipping point into a new stratospheric condition where the aerosols would not be removed within this time frame remains to be determined.
Mechanism
The nuclear winter scenario assumes that 100 or more city firestorms "50 Hiroshima-size (15 kt) bombs could generate 1–5 Tg of black carbon aerosol particles in the upper troposphere, after an initial 20% removal in "black rains" induced by firestorms..." & "the 1 to 5 Tg soot source term derives from a thorough study of the smoke produced by firestorms...". "the injection height of the smoke is controlled by the energy release from the burning fuel not from the nuclear explosion". are ignited by nuclear explosions, and that the firestorms lift large amounts of sooty smoke into the upper troposphere and lower stratosphere by the movement offered by the pyrocumulonimbus clouds that form during a firestorm. At above the Earth's surface, the absorption of sunlight could further heat the soot in the smoke, lifting some or all of it into the stratosphere, where the smoke could persist for years if there is no rain to wash it out. This aerosol of particles could heat the stratosphere and prevent a portion of the sun's light from reaching the surface, causing surface temperatures to drop drastically. In this scenario it is predicted that surface air temperatures would be the same as, or colder than, a given region's winter for months to years on end.
The modeled stable inversion layer of hot soot between the troposphere and high stratosphere that produces the anti-greenhouse effect was dubbed the "Smokeosphere" by Stephen Schneider et al. in their 1988 paper.
Although it is common in the climate models to consider city firestorms, these need not be ignited by nuclear devices; more conventional ignition sources can instead be the spark of the firestorms. Prior to the previously mentioned solar heating effect, the soot's injection height is controlled by the rate of energy release from the firestorm's fuel, not the size of an initial nuclear explosion. For example, the mushroom cloud from the bomb dropped on Hiroshima reached a height of six kilometers (middle troposphere) within a few minutes and then dissipated due to winds, while the individual fires within the city took almost three hours to form into a firestorm and produce a pyrocumulus cloud, a cloud that is assumed to have reached upper tropospheric heights, as over its multiple hours of burning, the firestorm released an estimated 1000 times the energy of the bomb.
As the incendiary effects of a nuclear explosion do not present any especially characteristic features, it is estimated by those with strategic bombing experience that as the city was a firestorm hazard, the same fire ferocity and building damage produced at Hiroshima by one 16-kiloton nuclear bomb from a single B-29 bomber could have been produced instead by the conventional use of about 1.2 kilotons of incendiary bombs from 220 B-29s distributed over the city.
While the firestorms of Dresden and Hiroshima and the mass fires of Tokyo and Nagasaki occurred within mere months in 1945, the more intense and conventionally lit Hamburg firestorm occurred in 1943. Despite the separation in time, ferocity and area burned, leading modelers of the hypothesis state that these five fires potentially placed five percent as much smoke into the stratosphere as the hypothetical 100 nuclear-ignited fires discussed in modern models. While it is believed that the modeled climate-cooling-effects from the mass of soot injected into the stratosphere by 100 firestorms (one to five million metric tons) would have been detectable with technical instruments in WWII, five percent of that would not have been possible to observe at that time.
Aerosol removal timescale
The exact timescale for how long this smoke remains, and thus how severely this smoke affects the climate once it reaches the stratosphere, is dependent on both chemical and physical removal processes.