Nuclear winter revisited

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.

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 found that the smoke in the stratosphere causes a 20% reduction in Earth's protective ozone layer, with losses of 25-45% over the mid-latitudes where the majority of Earth's population lives, and 50-70% ozone loss at northern high latitude regions such as Scandinavia, Alaska, and northern Canada. A massive increase in ultraviolet radiation at the surface would result, capable of causing widespread and severe damage to plants and animals. Thus, even a limited nuclear exchange could trigger severe global climate change capable of causing economic chaos and widespread starvation.


Figure 2. Top: Time variation of global average surface air temperature and precipitation for a limited nuclear exchange between Pakistan and India of 100 Hiroshima-sized weapons, assuming they inject 5 Tg of Black Carbon (BC) into the stratosphere. The global average precipitation is 3 mm/yr, so the changes in years 2-4 represent a 9% global average reduction in precipitation. Bottom: Time variation of sunlight (shortwave radiation) at the surface, in watts per meter squared, due to the 1991 eruption of Mt. Pinatubo in the Philippines (blue line) and the limited nuclear war between India and Pakistan (black line). The effects of a limited nuclear war are far more severe and long lasting than the eruption of Pinatubo, the greatest eruption of the 20th century. Image credit: "Climatic consequences of a regional nuclear conflict" by Robock et al., Atmospheric chemistry and Physics, 7, 2003-2012, 2007.

Climate change and the Doomsday Clock
It is sobering to realize that the nuclear weapons used in the study represented only 0.3% of the world's total nuclear arsenal of 26,000 warheads. Fortunately, significant progress was made in the 1990s and 2000s to reduce the threat of nuclear war. If the 2002 Strategic Offensive Reductions Treaty (SORT) is fully implemented by the U.S. and Russia as planned, by 2012 the world's stockpile of nuclear weapons will be just 6% of the 70,000 warheads that existed at the peak of the cold war in 1986. However, the threat of a more limited regional nuclear war has increased in recent decades, since more countries have been joining the nuclear club--an average of one country every five years. The 2007 move by the Bulletin of the Atomic Scientists to move the hands of their Doomsday Clock two minutes closer to midnight--the figurative end of civilization--helped call attention to this increased threat. In addition, they also mentioned climate change for the first time as part of the rationale for moving the clock closer to midnight. I believe that climate change does not pose an immediate threat to civilization--at least for the next 20 years or so--and there is still time to significantly reduce the threat of "doomsday" levels of climate change to civilization if strong action is taken in the next 20 years to cut carbon emissions. Thus, setting the hands of the clock closer to midnight because of climate change is probably premature. However, climate change triggered by a limited nuclear war is a whole different situation. The twin disasters of a limited nuclear war, coupled with the devastating global climate change it could wreak, should remind us that there is no such thing as a small scale nuclear war. Even a limited nuclear war is a huge threat to Earth's climate. Thus, there is no cause more important to work for than peace, so, this Easter weekend, I plan on making myself--and thus the world--more peaceful.

Jeff Masters