What is nuclear winter?
Over time, global nuclear powers developed more and more powerful nuclear weapons — and with them, more concerning effects on our atmosphere.
Nuclear winter theory emerged properly into the scientific world in the 1980s, thanks to the famous, pioneering work done by TTAPS (Turco, Toon, Ackerman, Pollack, and Sagan) and Crutzen and Birks, but its ancestry stretches back into premonitory science fiction as early as the 1940s. Twilight World (1961) by physicist-turned-writer Poul Anderson, based on a short story co-authored with F.N. Waldrop in 1947, warns of a Fimbulwinter caused by the dust launched into the atmosphere by a nuclear war blocking out sunlight.1 In 1957, “Torch” by C. Anvil described how the ignition of an oil field from nuclear war “screened out the sun’s radiation” and resulted in Arctic temperatures for the Northern Hemisphere. Even Philip K. Dick’s Do Androids Dream of Electric Sheep? (1967) mentions massive amounts of dust and fallout following the nuclear “World War Terminus” blocking out sunlight and leading to mass extinctions.
Let’s see how close they came to the truth.
1. The Atmosphere
A 200-kiloton nuclear bomb, about 10 times as powerful as the “Fat Man” the US dropped on Nagasaki, would have most of the resulting dust, gas, and radioactivity washed out of the middle and upper troposphere within days to a month — a devastating environmental impact to be sure, but nothing compared to a one megaton explosion. With a 1M bomb, so far successfully constructed by the US, Russia, and China, the debris would be immediately carried into the stratosphere, where it would remain in our atmosphere potentially for years. This is also possible, although less likely, with any nuclear yield greater than “a few hundred kilotons,” accomplishable by any nuclear power today.
Nuclear winter arises from the temperature reduction resulting from massive clouds of smoke and dust produced by these explosions, as well as the smoke from combustion of cities, forests, crops, and fuel caused by the blasts. Large petroleum storage and refining facilities would likely be completely destroyed as strategic targets, greatly contributing to smoke, pollution, and potentially creating a fuel shortage. If so, they might burn for months after ignition. Confidence in contemporary estimations is supported by studies of volcanic eruptions, forest fires, and urban firestorms from conventional warfare, although these comparisons are likely to underestimate the magnitude of fires resulting from a nuclear blast due to the destruction of functional and effective urban firefighting services; in fact, for a variety of reasons, when past climate models have been found to be incorrect, they’ve largely been too optimistic.
Given a sufficient amount of smoke, the amount of sunlight reaching the northern hemisphere could be reduced by up to 97% in places and up to 50% on average. After initial days of near-total darkness at the areas of impact, this creates the low-light phenomenon of “twilight at noon,” as witnessed by victims of the Hiroshima and Nagasaki bombings. While the density of the smoke clouds will decrease as they disperse across a greater area, light levels are still estimated as similar to a “heavily overcast day, even at noon” anywhere in the Northern Hemisphere between the latitudes 30 and 70 degrees, “where most of the World’s population lives and where most of our crops are grown.” Contrary to the primary danger of nuclear winter (cold temperatures), warming in the stratosphere from the injection of hot smoke would actually accelerate the spread to the Southern Hemisphere compared to average atmospheric winds, lowering temperatures across the world.
2. Plants, Animals, Agriculture
The “twilight at noon” effect could continue over major crop-growing areas for up to two weeks or even two months. Growing plants and crops would essentially starve to death after only a fraction of this time. The Greenhouse Effect may reverse as sunlight would no longer reach Earth, but the infrared radiation from the sun would still be able to escape. This would lead to a massive reduction in average global temperature by up to 9 degrees Celsius on land and 3 degrees Celsius in the ocean. Scientists have been less divided on the specific temperature variations than they are on humanity’s ability to recover from the world’s breadbasket latitudes losing anywhere between 10 and 40 degrees Celsius from their average temperature. Wildfires and volcanic eruptions give us a good idea of the effects of tropospheric smoke; after Mt. St. Helen’s eruption, daytime temperatures beneath the smoke cloud dropped between 5-8 degrees Celsius. However, it is crucial to note that depending on the size of the nuclear war, historic temperature variation due to volcanic eruption, which has caused slight global cooling and severe crop failure and famine, might reflect “less than one-twentieth of what might be expected.” Volcanic eruptions also led to noticeable average temperature decreases for “only a few years,” but nuclear war is estimated to affect the global climate for between 6 and 10 years.
Temperature reductions pose a serious threat to crops and other plants for years after light levels have returned to near-normal. A decrease of even one degree Celsius could reduce average wheat production by half; two degrees is calculated to reduce production by up to 94%; at three degrees, wheat production is impossible. Rice, soybean, and corn production would be reduced by 20% and remain decreased after a decade. Workarounds such as planting later in the season or greater reliance on species of winter wheat are not useful due to continued low temperatures, although auto irrigation and additional fertilizer use would apparently reduce the impact of the nuclear war somewhat. This is, however, not accounting for radioactive pollution of freshwater supplies and possible toxicity, for disruptions in supply chains which could inhibit the creation and transportation of that fertilizer, or for the resulting mass deaths of pollinator species.
The ability of modern society to rely mostly on stored food until agriculture can be reorganized is contentious. Food stocks in an average town might last 2-4 weeks, and government stores in most “developed Western countries” deplete within four months. More optimistic estimates suggested that with total reallocation to human consumption, the U.S. population (in 1985) could be fed a vegetarian diet relying on grain and beans for over two years – this is, however, not accounting for loss of stored food due to rodents, pests, or humidity; potential unavailability of processing plants or difficulty in transportation to said plants; or the logistic and political questions involved in the seizure and distribution of said food. Globally, tropical roots such as cassava and yams make up the staple diet of more than 600 million people, and yet are extremely vulnerable to decreases in temperature, which would likely come to pass in this scenario, leading to massive food loss. Estimates for Chinese food security are similar to optimistic U.S. estimates – most likely exhausted within two years. Currently, up to 1/3 of the world’s food is lost or wasted, which would need to change immediately. In the end, even if stored food is available, famine may not be averted if it is not accessible and affordable.
Unlike the destructive downpours after volcanic eruptions, plants would suffer from a lack of precipitation following a nuclear catastrophe. Turco estimated an average reduction in rainfall by up to 75% in the main northern grain-growing latitudes for the first few months following the attacks and up to 50% in seemingly unrelated areas like Australia for up to 1-3 years; multiple authors forecast the failure of the monsoon season in Asia, crucial for certain ecosystems and agricultural regimes. Even though temperature fluctuations would be highest in temperate regions, smaller decreases in tropical regions could still lead to irreversible damage to rainforests, including the extinction of species unable to adapt. The estimated effects of decreased oxygen production from forest fires and loss of plant life to deaths from fire and cold combined with increased atmospheric CO2 are largely unknown. The one blessing is that soil erosion would therefore be limited to that caused by the blast itself, as opposed to resulting rains as well. On the other hand, hydroelectric power might suffer significantly from this decrease in average precipitation combined with the more frequent freezing temperatures.
The reduction in fish catch would be less than the reduction in land agriculture, but most fishing occurs around coastal waters, which would receive the radiation and toxins flushed in from rivers and thereby contaminate the fish, shrimp, and mollusks that make up significant portions of human protein consumption around the world to dangerous levels. It is difficult to estimate the number of people who might be exposed to dangerous levels of radiation. Outside of pollution, there will be significant deaths of phytoplankton and zooplankton in the Northern Hemisphere, in some areas potentially a majority of these populations would die off depending on the reduction in sunlight. Even those which remain living would be unable to grow or reproduce as long as the twilight phenomenon continued. The fish and other marine animals that feed on these species would then go hungry and, in turn, likely fall in numbers significantly, especially in the event of a summer war when fat stores are lower. All in all, ocean productivity could drop by up to 20% immediately and not restabilize for 6-9 years.
3. Implications for Humanity
Nuclear winter carries the immense weight of great human suffering, and poses a potential threat to human civilization. There is historical precedent in the Tambora Crisis for consecutive ruined harvests which threaten large portions of the Northern Hemisphere with food insecurity, having cascading effects into food exports to the Southern Hemisphere, and causing widespread famines. According to the Food and Agriculture Organization in 2013, roughly half of global calorie intake comes from cereals (although this percentage is shrinking), which would be threatened globally by catastrophic nuclear exchange; 18% comes from meat and fish, fed by these grains and fostered by quickly contaminated waters. China, on which most of the modern agricultural research into the effects of nuclear winter has been performed, is better positioned than most of the world in terms of food security to reestablish self-sufficiency and would potentially only suffer a 10% average caloric reduction; others, such as the estimated 805 million undernourished people in the world, would not be so lucky and would likely risk starvation.
Reestablishing food security was politically at the top of the list for the U.S. occupation of Japan after World War 2 to prevent civil unrest leading to greater chaos, but it’s unlikely in a large-scale nuclear winter scenario that there would be a nation in the position of the U.S. to distribute even what little food and medical aid they offered at that time. Political and social ramifications of this instability, including the potential for greater local and international violence, remain as of yet unstudied but would doubtlessly be significant, as they were in the aftereffects of the Tambora eruption. In the worst-case scenarios where most plant and animal life in the Northern Hemisphere is wiped out, we might see a breakdown in capacity for governance or rule of law.
There is much more to say on this subject, and I intend to continue next week with a discussion of specialized foods that would be available even in nuclear winter scenarios: resilient foods. If you’d like to be notified, you know what to do.
If you find this subject interesting, I recommend On the 8th Day — a documentary by the BBC featuring Carl Sagan and other nuclear winter theorists exploring nuclear winter and its impact on agriculture.
Fimbulwinter itself comes from Norse mythology and makes up part of the Earthly apocalypse. It’s said that at the end of the world, three successive winters with no intervening summer will wipe out life on Earth, except for two surviving humans. It’s argued that this legend is inspired by the real events of AD 536, when a volcanic eruption caused a global climatic downturn at the beginning of the Late Antique Little Ice Age and led to massive societal changes in Scandinavia.