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Nuclear winter is the severe global-climatic-cooling-effect, hypothesized to occur after the ignition of a number of firestorms. Such fires, which can inject soot into the stratosphere have historically occurred in a number of cities, with nuclear winter researchers using both Hamburg and the less ferocious Hiroshima firestorms as the principle examples. However, as firestorms are most frequently encountered in much larger area wildfires, these latter fires are the most significant today in assessing both the initial and present-day "nuclear winter" computer models.

Nuclear winter is the severe global-climatic-cooling-effect, hypothesized to occur after the ignition of a number of firestorms. Such fires, which can inject soot into the stratosphere have historically occurred in a number of cities, with nuclear winter researchers using both Hamburg and the less ferocious Hiroshima firestorms as the principle examples. However, as firestorms are most frequently encountered in much larger area wildfires, these latter fires are the most significant today in assessing both the initial and present-day "nuclear winter" computer models.

"Nuclear winter," and its progenitor, "nuclear twilight," both refer to nuclear events as both were formulated at a time when attempts to quantify the climatic effects of large-scale nuclear war scenarios had hit a stumbling block, namely, the prior popular hypotheses that fireball generated NOx emissions would devastate the ozone layer was becoming increasingly unrealistic. It was within this early 1980s context, that the climate effects of soot from fires, was "chanced upon" and became the new focus of the climatic effects of "nuclear war". In these modelled scenarios, various "soot cloud" events, containing uncertain quantities of soot, were assumed to form over cities, oil refineries, and the 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 specifically coined in 1983 by Richard P. Turco to refer to computer model results where this soot/smoke remained in the air for almost a year, and during this time it caused monumentally severe planet-wide temperature drops ("winters")—apocalyptic models that Turco would later temper and distance himself from.

As nuclear devices need not be involved in the ignition of a firestorm, the term is a common misnomer. This is due, in greatest part, to the vast majority of published papers stating, without qualitative justification, that nuclear explosions are the cause of the modeled firestorm effects. The only phenomenon that is scrutinized and computer modeled in the nuclear winter papers is the climate forcing agent of firestorm-soot, a product which can be ignited and formed by a myriad of other, more common, means. Although rarely discussed, the proponents of the hypothesis do state that the same "nuclear winter" effect would occur if 100 conventionally lit firestorms were ignited.

The climate models in the public domain suggest that the ignition of 100 firestorms, comparable in intensity to that observed in Hiroshima in 1945, would produce a "small" nuclear winter. The burning of these firestorms would result in the injection of soot (specifically black carbon) into the Earth's stratosphere, producing an anti-greenhouse effect that lowers 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 would unmistakably cool the global climate by approximately 1 °C (1.8 °F), largely eliminating the magnitude of anthropogenic global warming for two to three years. Robock has not modeled, but speculated, that this would have global agricultural losses as a consequence.

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 result of any large scale employment of countervalue city-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, summer drops in average temperature by about 20 °C (36 °F) in core agricultural regions of the US, Europe, and China, and by as much as 35 °C (63 °F) in Russia. This cooling was 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 several decades.

On the fundamental level, since the advent of photographic evidence of tall clouds were captured, it was known that firestorms can inject soot smoke/aerosols into the stratosphere, more recently however it has been found that each natural occurrence of a wildfire firestorm can "surprisingly frequently" produce minor "nuclear winter" effects, with short-lived, almost immeasurable drops in surface temperatures, confined to the global 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. Information regarding all of these properties is necessary to truly ascertain the length and depth of the cooling effect of firestorms, independent of the nuclear winter computer model projections.

Presently, from satellite tracking data, stratospheric smoke aerosols are removed in a time span under approximately two months. The existence of any hint of a tipping point into a new stratospheric condition where the aerosols would not be removed within this time frame remains to be determined.