Nuclear winter is a very cold and long-lasting change in Earth's climate that could happen if many cities were destroyed by firestorms during a large nuclear war. Scientists believe this could occur because fires from such events might send soot into the upper part of the atmosphere, where it could block sunlight from reaching Earth. This blocking of sunlight might cause global temperatures to drop for about 10 years, leading to failed crops, a worldwide food shortage, and the loss of many animal species.

Researchers study nuclear winter using computer models and different scenarios. Their findings depend on factors like the size of nuclear weapons used, how many cities are attacked, the amount of flammable materials in those cities, and how firestorms behave in the atmosphere. Examples of firestorms studied include those from World War II bombings of Hiroshima, Tokyo, Hamburg, Dresden, and London, as well as recent wildfires, such as the 2021 British Columbia wildfires.

Studies suggest that a full-scale nuclear war, using thousands of weapons from the largest nuclear arsenals in Russia and the United States, could lower global temperatures by more than 5°C, colder than during the last ice age. These models predict that about five billion people might die from hunger within two years, and 40–50% of animal species could become extinct. Other studies show that a smaller nuclear war, such as one between India and Pakistan using hundreds of weapons, could still cause global temperatures to drop by several degrees, risking the lives of up to two billion people and leading to the extinction of 10–20% of animal species. However, scientists still have many questions about how to model these effects accurately.

General

"Nuclear winter," a term first called "nuclear twilight," became a scientific topic in the 1980s. Earlier ideas about fireball-generated chemicals harming the ozone layer were no longer believed. Scientists then focused on the climate effects of soot from fires. In these models, soot clouds with uncertain amounts of soot were imagined forming over cities, oil refineries, and missile silos. Researchers decide how much soot is used, then model its climate effects. The term "nuclear winter" was created in 1983 by Richard P. Turco, who used a one-dimensional computer model to study the "nuclear twilight" idea. This model predicted that large amounts of soot and smoke would stay in the air for years, causing a sharp drop in global temperatures.

After predictions about the 1991 Kuwait oil fires were proven wrong, no new papers on the topic were published for over a decade. Recently, the same scientists from the 1980s have shared new computer model results. These models show that 100 firestorms, each as intense as those seen in Hiroshima in 1945, could cause a "small" nuclear winter. These firestorms would send soot (black carbon) into the stratosphere, creating an anti-greenhouse effect that lowers Earth’s temperature. In Alan Robock’s model, 100 such firestorms could cool the planet by about 1°C (1.8°F) for two to three years, reducing the effects of human-caused global warming. Robock and his team also modeled food production and found that injecting more than 5 teragrams of soot into the stratosphere could cause years of food shortages. Their model suggests that livestock and aquatic food production would not replace lost crop output in most countries, and reducing food waste would not significantly help.

Nuclear explosions are not needed to start a firestorm, so "nuclear winter" is a misleading name. Most papers on the topic assume nuclear explosions cause firestorms without clear evidence. The only thing modeled in these studies is the climate impact of soot from firestorms, which can be caused by many methods, not just nuclear weapons. Scientists also note that the same "nuclear winter" effects could happen if 100 large conventional firestorms occurred.

In the 1980s, modelers assumed thousands of firestorms would occur, possibly from large-scale nuclear warfare between the U.S. and the Soviet Union. These firestorms were thought to cause severe cooling lasting up to a decade. Summer temperatures in key agricultural areas of the U.S., Europe, and China could drop by 20°C (36°F), and by 35°C (63°F) in Russia. This cooling would result from a 99% drop in sunlight reaching Earth’s surface, gradually improving over decades.

Photographs of tall clouds showed that firestorms can send soot and aerosols into the stratosphere, but how long these aerosols stay there was unknown. In 2006, Mike Fromm of the Naval Research Laboratory found that natural wildfires larger than the Hiroshima firestorm could cause minor, short-lived cooling effects, limited to the hemisphere where the fire occurred. This is similar to volcanic eruptions, which inject sulfates into the stratosphere and cause minor, short-term cooling.

Tools like satellites and aircraft are now used to study firestorm soot. Scientists need data on the lifespan, quantity, height, and optical properties of this smoke to understand how long and how severely firestorms could cool the planet, regardless of computer model predictions.

Satellite data suggests that stratospheric smoke aerosols usually disappear within two months. Whether a point exists where these aerosols stay longer and change the stratosphere is still unknown.

Mechanism

The nuclear winter scenario suggests that if 100 or more city firestorms are caused by nuclear explosions, the firestorms could send large amounts of sooty smoke into the upper troposphere and lower stratosphere. This happens because pyrocumulonimbus clouds, which form during firestorms, lift the smoke high into the atmosphere. At heights of 10–15 kilometers (6–9 miles) above Earth, sunlight may heat the soot in the smoke, pushing it even higher into the stratosphere. If rain does not wash the smoke out, it could stay there for years. This layer of tiny particles, called an aerosol, might warm the stratosphere and block sunlight from reaching Earth’s surface, causing temperatures to drop sharply. In this scenario, surface temperatures could become as cold as or colder than winter temperatures for months or years.

Scientists named the layer of hot soot between the troposphere and stratosphere the "Smokeosphere" in a 1988 study by Stephen Schneider and others. This layer is thought to trap heat in the stratosphere, creating an anti-greenhouse effect.

In climate models, firestorms are often considered, but they do not always require nuclear explosions. Conventional sources, like sparks from fires, can also cause firestorms. Before sunlight heats the soot, the height to which smoke is lifted depends on how much energy the firestorm releases, not the size of the initial explosion. For example, the mushroom cloud from the Hiroshima bomb reached six kilometers (middle troposphere) within minutes but dissipated quickly. In contrast, fires in Hiroshima took nearly three hours to form a firestorm, producing a pyrocumulus cloud that reached upper tropospheric heights. Over several hours, these fires released about 1,000 times more energy than the bomb itself.

Nuclear explosions do not create unique fire effects, so experts estimate that the same fire intensity and building damage caused by a 16-kiloton nuclear bomb on Hiroshima could have been achieved using about 1.2 kilotons of conventional incendiary bombs dropped by 220 B-29 bombers across the city.

Historically, firestorms in cities like Dresden, Hiroshima, Tokyo, and Nagasaki occurred in 1945, while a major firestorm in Hamburg happened in 1943. Although these fires were less intense and burned smaller areas, scientists believe they may have released about 5% of the smoke into the stratosphere that modern models predict from 100 nuclear-ignited firestorms. While climate models suggest the soot from 100 firestorms (1 to 5 million metric tons) could have been detected with instruments during World War II, 5% of that amount would not have been measurable at the time.

The length of time soot remains in the stratosphere and its impact on the climate depend on both chemical and physical removal processes. The most important physical removal is "rainout," which occurs during the firestorm’s convective phase, producing "black rain" near the fire, and after the smoke disperses, where rain efficiently washes the soot out. However, some studies, like Robock’s 2007 model, avoid these removal processes by assuming solar heating quickly lifts soot into the stratosphere, separating darker soot particles from the fire clouds’ lighter water droplets.

Once in the stratosphere, soot particles may be removed through physical processes like collisions and coagulation (clumping together) via Brownian motion, or falling out of the atmosphere due to gravity. The "phoretic effect" may also move coagulated particles to lower atmospheric levels, allowing cloud seeding to begin and enabling rain to wash the soot out through wet deposition.

Chemical removal depends on atmospheric reactions that break down the carbon in the smoke, such as those involving ozone and nitrogen oxides, which are present throughout the atmosphere and increase in concentration during high-temperature events.

Historical data on how long aerosols stay in the atmosphere, such as sulfur aerosols and volcanic ash from large eruptions, suggest a one-to-two-year timeframe. However, scientists still do not fully understand how aerosols interact with the atmosphere.

Sooty aerosols vary in size, shape, and properties, making it hard to predict how they affect sunlight. The conditions during soot formation influence its final properties. Soot from efficient burning is nearly pure "elemental carbon black," while less efficient burning produces more partially burned fuel, forming tar balls and brown carbon. These can coat black carbon particles. However, the most important soot for climate effects is that injected high into the stratosphere by firestorms fed by strong winds. Under these conditions, most soot is likely more oxidized black carbon.

Consequences

A study shared at a meeting of the American Geophysical Union in December 2006 showed that even a small nuclear war could harm the world's climate for many years. In a situation where two countries in the subtropics used 50 nuclear weapons, each similar in size to the one dropped on Hiroshima (about 15 kilotons each), on major cities, scientists predicted that up to five million tons of soot would be released. This soot would cause temperatures to drop by several degrees in large parts of North America and Eurasia, including areas where crops are grown. The cooling would last for years and could greatly harm farming and food supplies, especially in countries near the poles.

Nuclear explosions create large amounts of nitrogen oxides by mixing elements in the air around them. These gases rise into the sky through heat. When they reach the stratosphere, they can break down ozone, a gas that protects Earth from harmful sunlight. This would allow more dangerous ultraviolet radiation to reach Earth's surface.

A 2008 study by Michael J. Mills and others, published in the Proceedings of the National Academy of Sciences, found that a nuclear conflict between Pakistan and India using their current weapons could create a near-global hole in the ozone layer. This would harm human health and damage the environment for at least a decade. The study used computer models to examine a war involving 50 Hiroshima-sized weapons from each country. This would cause huge fires and send up to five million metric tons of soot into the stratosphere, about 50 miles (80 km) high. The soot would absorb sunlight, heating the air and speeding up the breakdown of the ozone layer. This could lead to up to 70% ozone loss in northern high latitudes.

A "nuclear summer" is a possible situation that could happen after a "nuclear winter." A nuclear winter occurs when particles in the atmosphere block sunlight, lowering temperatures. After these particles settle, a "nuclear summer" might happen because heat-trapping gases like carbon dioxide and methane from burning and decaying organic matter would increase temperatures. This could raise surface temperatures rapidly, killing much of the life that survived the cold. Nuclear explosions would release carbon dioxide and other greenhouse gases from fires and from the decay of dead plants and animals. They would also send nitrogen oxides into the stratosphere, which would destroy the ozone layer.

Other possible scenarios suggest that a nuclear winter could lead to a nuclear summer. The extreme heat from nuclear explosions could destroy ozone in the middle stratosphere.

History

In 1952, scientists were worried that the Ivy Mike bomb test, which was 10.4 megatons, might cool the Earth. This test happened a few weeks before it was carried out on Elugelab Island. Major Norair Lulejian from the USAF and astronomer Natarajan Visvanathan studied this possibility. They reported their findings in a book called Effects of Superweapons Upon the Climate of the World. This book was not widely shared. A 2013 report by the Defense Threat Reduction Agency called this study the first one about the "nuclear winter" idea. The study said there was no real chance that the explosion would change the Earth's climate.

A 1957 report on The Effects of Nuclear Weapons, edited by Samuel Glasstone, described the effects of high-yield hydrogen bomb tests, like Ivy Mike in 1952 and Castle Bravo in 1954. A section titled "Nuclear Bombs and the Weather" explained that volcanic eruptions, like the one at Krakatoa in 1883, can reduce sunlight reaching Earth. However, the report said that even the largest nuclear explosions would not put more than about 1% of that dust into the air. It also said that no nuclear test so far had caused any noticeable change in sunlight. In 1956, the US Weather Bureau thought that a large nuclear war with many megaton-range explosions might cause enough dust to lead to a new ice age.

In 1966, a RAND Corporation report by E. S. Batten studied how nuclear explosions might affect the weather. It said that besides dust, fires caused by nuclear bombs could change weather patterns. However, more research was needed to understand these effects fully.

A 1975 book by the US National Research Council, Long-Term Worldwide Effects of Multiple Nuclear-Weapons Detonations, said that a nuclear war using 4,000 megatons of weapons would probably put less dust into the stratosphere than the Krakatoa eruption. It believed that the dust and nitrogen oxides might cause slight cooling, but this would likely be within normal climate changes. It also said that more dramatic changes could not be ruled out.

In 1985, a report called The Effects on the Atmosphere of a Major Nuclear Exchange said that a 1-megaton nuclear explosion might inject about 0.3 teragrams of dust into the stratosphere, with 8% being very small particles. In 1992, a US National Academy of Sciences report estimated that about 10 teragrams of dust would be needed to cool the Earth by 2°C, which is the same as reducing the warming from doubled carbon dioxide levels.

In 1969, Paul Crutzen discovered that nitrogen oxides (NOx) could destroy the ozone layer in the stratosphere. In the 1970s, studies on NOx from supersonic planes led John Hampson to suggest in 1974 that nuclear explosions might also destroy the ozone layer, exposing Earth to harmful ultraviolet radiation for a year or more. In 1975, the US National Research Council reported that a nuclear war with many large explosions might reduce ozone levels by 50% or more in the northern hemisphere.

However, a 1973 study in Nature found no connection between nuclear tests and ozone changes. A 1976 study also found that nuclear tests did not deplete ozone, contradicting earlier models. A 1981 paper confirmed that models and physical measurements disagreed, as no ozone destruction was observed.

Between 1945 and 1971, about 500 megatons of nuclear weapons were tested in the atmosphere, with the most testing happening in 1961–1962. During this time, about 300 megatons of energy were released by the US and Soviet Union. This released about 5,000 tons of nitric oxide into the stratosphere. A 2.2% drop in ozone was noted in 1963, but scientists believe this was caused by other weather factors.

In 1982, journalist Jonathan Schell wrote The Fate of the Earth, suggesting that nuclear explosions could destroy the ozone layer so badly that crops would fail and life would be at risk. That same year, Australian physicist Brian Martin reviewed Hampson’s work and said that ozone loss from nuclear war was unlikely to be a serious problem. He called Schell’s claims about crop failure and extinction highly unlikely.

More recent studies in the 1990s, by Robert P. Parson, said that about 1.2 million tons of stratospheric NOx are naturally and human-made each year. This is much more than earlier estimates, showing that nuclear explosions might not have as big an impact on ozone as once thought.

The first published idea that a nuclear war could cause climate cooling appears to have been introduced in the 1970s.

Recent modeling

Between 1990 and 2003, experts noted that no scientific papers about "nuclear winter" were published in peer-reviewed journals.

In 2007 and 2008, some of the scientists who first studied nuclear winter shared new research. They suggested that as few as 100 firestorms caused by nuclear explosions could lead to a nuclear winter. This idea was not new, as similar conclusions were reached in the 1980s.

Compared to climate changes over the past 1,000 years, even the smallest nuclear war modeled would cause global temperatures to drop below those of the Little Ice Age (1600–1850 AD). This cooling would happen quickly, harming agriculture. More smoke from larger nuclear exchanges would cause even greater climate changes, making farming impossible for years. New climate models show these effects could last more than 10 years.

A study published in 2007 used modern climate models to examine the effects of a global nuclear war involving most or all of the world’s nuclear weapons. The researchers used a model called ModelE from NASA, which had been tested for climate studies. They simulated two scenarios: one with 150 teragrams (Tg) of smoke from a full nuclear arsenal and another with 50 Tg from one-third of the arsenal. In the 150 Tg case, global temperatures would drop by −7 °C to −8 °C for years, with cooling still at −4 °C after a decade. This would be the coldest average temperature in human history, far colder than during the last ice age. Land areas, especially in North America and Eurasia, would experience extreme cooling, with temperatures falling below −20 °C and −30 °C, respectively.

The study also found that global rainfall would decrease by about 45%, weakening the water cycle. In the 50 Tg case, the effects were similar but less severe. The researchers noted that previous studies suggested a year without food production could lead to widespread starvation, and their results showed this period would last much longer, worsening the impact of a nuclear winter.

In 2014, a study by Michael J. Mills and others used models from the US National Center for Atmospheric Research to simulate a regional nuclear war involving 100 small nuclear weapons. The model predicted global ozone losses of 20–50% over populated areas, the worst in human history. This would increase harmful ultraviolet (UV) radiation, damage crops and ecosystems, and reduce growing seasons by 10–40 days for five years. Surface temperatures could remain lower for over 25 years, threatening global food supplies and causing a nuclear famine.

Researchers at Los Alamos National Laboratory studied the climate effects of a regional nuclear exchange. Unlike earlier studies, they found that very little black carbon (soot) would reach the stratosphere, leading to smaller long-term climate impacts. They concluded that a "nuclear winter" effect was unlikely in such a scenario. However, later studies disputed this finding.

A 2019 study in the journal Safety suggested that no country should have more than 100 nuclear warheads due to the risk of "nuclear autumn," which could harm the aggressor nation’s population.

In 2019, two studies expanded on earlier models, exploring new scenarios of nuclear winter from smaller nuclear exchanges. One study by Coupe et al. modeled a nuclear war between the United States and Russia, releasing 150 Tg of black carbon. This amount exceeds all volcanic eruptions in the past 1,200 years but is less than the asteroid impact that caused a mass extinction 66 million years ago. The study used a more advanced model called WACCM4, which showed black carbon particles grow larger in the stratosphere, absorbing more sunlight and causing greater cooling. This model predicted faster recovery to normal temperatures compared to the 2007 study, but temperatures would drop more than 20K below normal for six years, leading to freezing conditions.

Criticism and debate

The nuclear winter concept has faced criticism over five main areas. The initial 1983 TTAPS model, which predicted extreme global cooling, was widely reported in the media. However, later models showed less severe cooling, though some still suggested harmful global cooling if many fires started in spring or summer. Starley L. Thompson’s 1980s 3D model led him to use the term "nuclear autumn" to describe the climate effects of soot more accurately, as he criticized earlier models for being overly dramatic.

A major criticism came from Cresson Kearny’s 1987 book Nuclear War Survival Skills, which questioned the assumption that large amounts of soot would reach the stratosphere. Kearny cited a Soviet study suggesting modern cities would not burn as firestorms because flammable items would be buried under rubble. He also argued that the TTAPS model overestimated non-urban wildfires. The TTAPS authors responded that cities would not be intentionally destroyed, but fires could start in undamaged suburbs. Dr. Richard D. Small disagreed with TTAPS’s 1990 estimate of 5,075 Tg of material burned in a US-Soviet war, citing analysis showing a maximum of 1,475 Tg could realistically burn.

Kearny believed future models would show even smaller temperature drops, noting that firestorms might not occur as reliably as assumed. In Nuclear Winter Reappraised (1986), Starley Thompson and Stephen Schneider estimated cooling would last only a few days, suggesting the initial nuclear winter idea had low scientific support. However, a 1988 article by Brian Martin noted that while Thompson and Schneider described effects as "nuclear autumn," they did not fully reject the nuclear winter concept. A 2007 study by Alan Robock and others mentioned that the term "nuclear autumn" led some to think the nuclear winter theory was exaggerated.

In 2006, Schneider supported findings that a limited nuclear war in tropical regions (like between Pakistan and India) could cause significant cooling due to smoke in areas with strong sunlight. He emphasized discouraging nuclear use.

Early models assumed smoke from vegetation near missile silos would be large, but Bush and Small (1987) found this contribution was minimal. Vegetation would only burn if very close to a nuclear fireball, which would also experience strong winds. This was supported by earlier studies and observations from nuclear tests in the 1950s–1960s.

A 2010 US Department of Homeland Security report stated that while firestorms could theoretically form after a city attack, modern city designs likely prevent them. For example, Nagasaki’s bombing did not create a firestorm. Earlier studies (1986–1988) found that models overestimated fuel availability in cities, leading to smoke lofting higher than 4 km in real-world conditions.

Russell Seitz, a Harvard researcher, argued that nuclear winter models used worst-case assumptions to produce alarming results. In 1986, he studied the 1915 Siberian fire, which caused 8°C cooling but no severe frosts. He criticized nuclear winter models for assuming unlikely, extreme events, comparing it to finding a rare card in a game. He also cited Carl Sagan, noting that nuclear winter models may exaggerate risks in realistic scenarios.

Policy implications

During the Cuban Missile Crisis, Fidel Castro and Che Guevara asked the USSR to launch a nuclear attack on the US if the US invaded Cuba. In the 1980s, Castro encouraged the Soviet government to take a stronger stance against the US under President Ronald Reagan, even suggesting the use of nuclear weapons. Because of this, a Soviet official went to Cuba in 1985 with a group of experts who explained the environmental effects of nuclear attacks on the US. After this, Castro stopped supporting the idea of using nuclear weapons. In 2010, Alan Robock visited Cuba to help Castro share a new belief that nuclear war would cause Armageddon. Robock’s 90-minute lecture was later shown on Cuba’s national television.

However, Robock said that his efforts to influence US government policies about nuclear weapons have not been successful. In 2009, he and Owen Toon spoke to the US Congress, but no action was taken. The science adviser to the US president at the time, John Holdren, did not reply to their requests in 2009 or later.

In a 2012 article in the Bulletin of the Atomic Scientists, Robock and Toon argued that the effects of nuclear winter, which they believe would cause extreme global cooling, require replacing the idea of "mutually assured destruction" (MAD) with a new concept called "self-assured destruction" (SAD). They said that regardless of which country starts a nuclear war, the results would be extremely harmful. In 1989, Carl Sagan and Richard Turco wrote a paper suggesting that both the US and USSR should reduce their nuclear weapons to a small number, like 100–300 warheads each, to lower the risk of a nuclear winter.

A secret US government report from 1984 said that both the US and USSR were already making nuclear weapons smaller, more accurate, and less powerful in the 1970s and 1980s. For example, the US replaced older warheads like the B28 and W31 with newer ones like the W68, W76, and B61. These changes allowed more warheads to be placed on missiles and reduced the harm to nearby countries. Smaller warheads also reduced the chance of large fires starting, which could lead to nuclear winter.

In 1983, a study called the TTAPS paper described a nuclear attack using 3,000 megatons of energy. However, scientists later said that smaller, more accurate warheads like the W76 could achieve the same goal with only 3 megatons of energy. They explained that using smaller warheads and detonating them closer to the ground would create less smoke and fallout, reducing the risk of nuclear winter.

The 1984 US report also said that military planners could avoid nuclear winter by considering factors like the flammability of targets, the height of explosions, and timing. Using surface or underground bursts instead of airbursts would create more localized fallout but less global cooling.

Scientists like Michael Altfeld and Stephen Cimbala argued that believing in nuclear winter might make nuclear war more likely, as it could encourage the development of smaller, more accurate weapons. They said that using weapons like the Robust Nuclear Earth Penetrator (RNEP) could reduce the risk of nuclear winter. These weapons, which were still being planned in the 1980s, were described by some experts as making it easier to use nuclear weapons in conflicts.

In 2000, Mikhail Gorbachev, the leader of the Soviet Union from 1985 to 1991, said that models showing the dangers of nuclear winter helped inspire efforts to stop the arms race. However, the 1984 US report was more skeptical, saying that the nuclear winter idea was not scientifically proven. It predicted that the USSR would continue its nuclear strategy and use the idea for propaganda, not for real policy changes. The report also suggested that the USSR might demand strong scientific proof of nuclear winter before taking it seriously, which could be hard to provide without testing.

Mitigation techniques

Many ideas have been suggested to reduce the possible damage of a nuclear winter if it happens. People have tried solving the problem in two ways: some methods aim to stop fires from growing, which would reduce the amount of smoke that enters the sky, and other methods focus on growing food even when sunlight is limited, based on the idea that the worst predictions about nuclear winter are correct and no other solutions are used.

A 1967 report described several ways to control fires caused by nuclear events. These included using liquid nitrogen, dry ice, or water to cool fires. The report also suggested creating firebreaks by removing flammable materials from areas, possibly even using nuclear weapons to do this. It also mentioned using controlled fires to reduce the risk of large wildfires. One of the most promising ideas was using special techniques to make rain fall from large storm clouds that form over fires.

In the book Feeding Everyone No Matter What, the authors describe ways to grow food during a nuclear winter. These include using bacteria that eat natural gas, such as Methylococcus capsulatus, which is already used to feed fish. Another idea is making bread from tree bark, a food used during past famines in Scandinavia. The book also suggests growing mushrooms, like honey fungi, which can grow on wood without sunlight. It also mentions using biofuels made from wood or plants, which create sugars that could be eaten. One author, David Denkenberger, says mushrooms might be enough to feed everyone for three years. Seaweed and plants like dandelions or tree needles could provide vitamins, and bacteria could supply other nutrients. Crops like potatoes might still grow near the equator if they receive enough sunlight.

To help people survive a nuclear winter, large amounts of food would need to be stored before the event. These food supplies should be kept underground, in high places, and near the equator to protect them from harmful radiation and sunlight. They should also be placed near areas where people are more likely to survive the initial disaster. One challenge is deciding who would pay for the food storage. Some people might be wealthy enough to fund it, but they might not be the ones most in need. The smallest amount of wheat needed to feed the world for a year is about two months’ worth of storage.

Climate engineering

Although the term "nuclear winter" is used, actual nuclear events are not required to create the climate changes modeled in scientific studies. Scientists are exploring quick and low-cost ways to address global warming, which could lead to at least 2°C of surface temperature rise if carbon dioxide levels in the atmosphere double. One approach involves solar radiation management, a type of climate engineering. This method has led researchers to examine the potential of the "nuclear winter" effect. Common ideas include injecting sulfur into the upper atmosphere to mimic the cooling effects of volcanic eruptions. Another proposal, suggested by scientists like Paul Crutzen, involves releasing specific types of soot particles to create mild "nuclear winter" conditions. Computer models show that injecting 1 to 5 trillion grams of soot from firestorms into the lower stratosphere could cause the stratosphere to warm while cooling the lower atmosphere. This process might reduce global temperatures by 1.25°C for two to three years. After 10 years, temperatures could still be 0.5°C lower than before the soot was released.

Potential climatic precedents

Historical supervolcano eruptions caused climate changes similar to "nuclear winter," as they sent sulfate particles high into the stratosphere, a phenomenon called a volcanic winter. The cooling effect of smoke in the atmosphere, known as an "antigreenhouse" effect, is similar to the hazy atmosphere of Titan. Scientists like Pollack and Toon studied Titan's climate in the late 1980s, around the same time they researched nuclear winter models.

Extinction-level comet or asteroid impacts are also thought to have caused "impact winters" by creating large amounts of fine rock dust. If the impact hits sulfate-containing rock, it can produce volcanic winter effects by sending sulfate particles into the air. Additionally, the heat from heavy rock fragments ejected during the impact could start widespread forest fires, similar to nuclear winter effects.

The "global firestorm winter" hypothesis, supported by scientists like Wendy Wolbach, H. Jay Melosh, and Owen Toon, suggests that tiny sand-grain-sized debris from massive impacts could re-enter Earth's atmosphere, creating a hot layer of debris high in the sky. This layer might have burned all above-ground plant material, including rainforests, explaining the severity of the Cretaceous–Paleogene extinction event. This event was caused by an asteroid about 10 kilometers wide, but the initial impact's energy alone was not enough to explain the level of extinction.

However, scientists like Claire Belcher, Tamara Goldin, and Melosh later questioned the firestorm hypothesis between 2003 and 2013. They raised concerns about the low amount of soot found in sediment layers near asteroid dust, the duration and intensity of re-entry heating, and how cooled debris might have reduced the heat reaching Earth's surface.

Owen Toon and others argued in 2013 that the Cretaceous period had higher oxygen levels than today, which might have influenced the firestorm hypothesis. It is challenging to determine how much soot from plants and fossil fuels contributed to the geological record, just as it is hard to measure how much material was directly ignited by the meteor impact.