More than 20 years ago, climate scientists led by the renowned Carl Sagan coined the term “nuclear winter” to describe the global ecological destruction that would result from a massive nuclear exchange between the US and the former Soviet Union. Applying climate model simulations available to them at the time, the scientists concluded that smoke and dust produced by a catastrophic nuclear war would cause rapid drops in temperature and precipitation, block sunlight, and threaten agriculture worldwide for at least a year.
Using sophisticated, modern climate models that have been developed to study global warming, some of these same scientists and their colleagues have recently returned to the question of nuclear winter and have reexamined the climate consequences of a range of nuclear wars. These new studies have reconfirmed that a nuclear war involving the large arsenals of the US and Russia would result in a nuclear winter even more long lasting than previously thought.
The scientists looked at the effects over a 10-year period of two different scenarios that are possible today—a nuclear war injecting 150 teragrams [Tg] of smoke into the upper troposphere over a one-week period, and one producing 50 Tg of smoke. One important difference between now and 20 years ago, which they looked at closely, has been the growth of cities and, consequently, larger smoke emissions from the same targets.
They calculated that roughly 150 Tg of smoke would be emitted by the use of the entire current global nuclear arsenal with a yield of 5,000 megatons. If one-third of the current arsenal were used, 50 Tg of smoke would be emitted.
In the 150 Tg scenario, black carbon particles spread quickly across the upper stratosphere and produce “a long-lasting climate forcing” that would last for more than a decade and affect both the Northern and Southern hemispheres. Among the effects would be a 45% global average reduction in precipitation and a global average surface cooling of –7°C to -8°C, which would persist for years. By comparison, the scientists remind us, “the global average cooling at the depth of the last ice age 18,000 years ago was about 5°C,” which would be “a climate change unprecedented in speed and amplitude in the history of the human race.” At the extremes, people in North America and Eurasia would experience cooling of more than -20°C and -30°C respectively.
The reductions in temperature and precipitation in the 50 Tg scenario were about half those for 150 Tg, over the same timescale. While not cold enough to be classified as “nuclear winter,” according to the scientists, such climate forcing would still be “severe and unprecedented.”
Perhaps the most extreme and lethal impact would be the collapse of agriculture. The earlier nuclear winter studies concluded that food production would cease entirely around the world for at least a year, leading to death by starvation for most of the human population. The results of the new studies paint an even grimmer picture: “this period of no food production needs to be extended by many years, making the impacts of nuclear winter even worse than previously thought.”
Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences. Alan Robock, Luke Oman, George. L. Stenchikov. 2008. Journal Of Geophysical Research 2008;112 [www.pnas.org/content/105/14/5307].