Global warming Weather effects

In the 1980s and early 1990s, a series of scientific papers published by Soviet scientists and Western scientists (including prominent 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. 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.

Even a limited nuclear exchange can cause a climate disaster
Well, it turns out that this portrayal of nuclear winter was overly optimistic, according to a series of papers published over the past few years by Brian Toon of the University of Colorado, Alan Robock of Rutgers University, and Rich Turco of UCLA. Their most recent paper, a December 2008 study titled, "Environmental Consequences of Nuclear War", concludes that "1980s predictions of nuclear winter effects were, if anything, underestimates". Furthermore, they assert that even a limited nuclear war poses a significant threat to Earth's climate. 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 (Tg) 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 the cities burned up by a limited nuclear war involving 100 Hiroshima-sized bombs in the region. India and Pakistan are thought to have 109 to 172 nuclear weapons of unknown yield.

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 between Pakistan and India 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 conflict" by Robock et al., Atmospheric chemistry and Physics, 7, 2003-2012, 2007.

The intense heat generated by the burning cities in the models' simulations lofted black smoke high into the stratosphere, where there is no rain to rain out the particles. The black smoke absorbed far more solar radiation than the brighter sulfuric acid aerosol particles emitted by volcanic eruptions. This caused the smoke to heat the surrounding stratospheric air by 30°C, resulting in stronger upward motion of the smoke particles higher into the stratosphere. As a result, the smoke stayed at significant levels for over a decade (by contrast, highly reflective volcanic aerosol particles do not absorb solar radiation and create such circulations, and only stay in the stratosphere 1-2 years). The black soot blocked sunlight, resulting in global cooling of over 1.2°C (2.2°F) at the surface for two years, and 0.5°C (0.9°F) for more than a decade (Figures 1 and 2). Precipitation fell up to 9% globally, and was reduced by 40% in the Asian monsoon regions.

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