In the 1980s and early 1990s, a series of scientific papers published by Soviet scientists and Western scientists (including "rock star" scientists Dr. Carl Sagan, host of the PBS "Cosmos" TV series, and Nobel Prize winner Paul Crutzen) laid out the dire consequences on global climate of a major nuclear exchange between the U.S. and Soviet Union. The nuclear explosions would send massive clouds of dust high into the stratosphere, blocking so much sunlight that a nuclear winter would result. Global temperatures would plunge 20° C to 40° C for several months, and remain 2-6° C lower for 1-3 years. Up to 70% of the Earth's protective stratospheric ozone layer would be destroyed, allowing huge doses of ultraviolet light to reach the surface. This UV light would kill much of the marine life that forms the basis of the food chain, resulting in the collapse many fisheries and the starvation of the people and animals that depend it. The UV light would also blind huge numbers of animals, who would then wander sightlessly and starve. The cold and dust would create widespread crop failures and global famine, killing billions of people who did not die in the nuclear explosions.

What about a small-scale nuclear war?
The "nuclear winter" papers were widely credited with helping lead to the nuclear arms reduction treaties of the 1990s, as it was clear that we risked catastrophic global climate change in the event of a full-scale nuclear war. But even a limited nuclear war poses a significant threat to Earth's climate, according to a paper presented at the American Geophysical Union meeting in December 2006 by scientists at Rutgers University and the University of Colorado. The scientists used a sophisticated atmospheric/oceanic climate model that had a good track record simulating the cooling effects of past major volcanic eruptions, such as the Philippines' Mt. Pinatubo in 1991. The scientists injected five terragrams of soot particles into the model atmosphere over Pakistan in May of 2006. This amount of smoke, they argued, would be the likely result of a limited nuclear war involving 100 Hiroshima-sized bombs in the region.


Figure 1. Global average temperature departure from normal since 1880 (top) and A.D. 1000 (bottom) in black, and those projected after a limited nuclear exchange of 100 Hiroshima-sized weapons in 2006 (in red). Temperatures are forecast to plunge 1.2° C (2.2° F) after such a war, reaching levels colder than anything seen in the past 1000 years. The 1815 eruption of Tambora in Indonesia produced a similar cooling, and led to the notorious "Year Without a Summer". Image credit: "Climatic consequences of a regional nuclear conflicts" by Robock et al., 2006.

The black smoke, they found, absorbed far more solar radiation than the brighter sulfuric acid particles emitted by volcanic eruptions. This allowed the smoke to heat the surrounding air to much higher temperatures, resulting in stronger upward motion of the smoke particles higher into the stratosphere. Once the smoke reached the stratosphere, where there is no rain to rain out the soot particles, it stayed at significantly high levels for over a decade. The black soot blocked sunlight, resulting in global cooling of over 1.2° C (2.2° F) for two years, and 0.5° C (0.9° F) for more than a decade.

This magnitude of this cooling would bring about the coldest temperatures observed on the globe in over 1000 years (Figure 1). The growing season would shorten by 10-30 days over much of the globe, resulting in widespread crop failures. The effects would be similar to what happened after the greatest volcanic eruption in historic times, the 1815 Tambora eruption in Indonesia. This cooling from this eruption triggered the infamous Year Without a Summer in 1816 in the Northern Hemisphere, when killing frosts disrupted agriculture every month of the summer in New England, creating terrible hardship. Exceptionally cold and wet weather in Europe triggered widespread harvest failures, resulting in famine and economic collapse. However, the cooling effect of this eruption only lasted about a year. Cooling from a limited nuclear exchange would create two to three consecutive "Years Without a Summer", and over a decade of significantly reduced crop yields. The authors anticipated that the smoke in the stratosphere would partially destroy Earth's protective stratospheric ozone layer as well, but did not model how large of an impact this would have. Clearly, even a limited nuclear exchange could trigger severe global climate change capable of causing economic chaos and widespread starvation.