NUCLEAR WINTER

14.6. NUCLEAR WINTER

Nuclear winter is a term used to describe a catastrophic atmospheric effect that might occur after a massive exchange of nuclear firepower between major powers. 8 The heat from the nuclear blasts and from resulting fires would cause powerful updrafts carrying sooty combustion products to stratospheric regions. This would result in several years of much lower temperatures and freezing temperatures even during summertime. There are several reasons for such an effect. First of all, the highly absorbent, largely black particulate matter would absorb solar radiation high in the atmosphere so that it would not reach earth’s surface. Cooling would also occur from a phenomenon opposite to that of the greenhouse effect. That is because outgoing infrared radiation from particles high in the atmosphere would have to penetrate relatively much less of the atmosphere and, therefore, would be exposed to much less infrared-absorbing water vapor and carbon dioxide gas. This would deprive the lower atmosphere of the warming effect of outgoing infrared radiation and would mean that less infrared would be re-radiated from the atmosphere back to earth’s surface. The cooling would also inhibit the evaporation of water, thereby reducing the amount of infrared-absorbing water vapor in the atmosphere and slowing the process by which particulate matter is scavenged from the atmosphere by rain.

Conditions similar to those of a nuclear winter occurred in 1816, “the year without a summer,” following the astoundingly massive Tambora, Indonesia volcanic explosion of 1815. Brutally cold years around 210 B.C. that followed a similar volcanic incident in Iceland were recorded in ancient China. The June 1991 explosion of the Philippine volcano Pinatubo, which blasted millions of tons of material, including 15-30 million tons of sulfur dioxide, into the atmosphere resulted in an approximately 0.5˚C cooling the following year. In addition to the direct suffering caused, massive starvation would result from crop failures accompanying years of nuclear winter. The incidents cited above clearly illustrate the climatic effects of huge quantities of particulate matter ejected high into the atmosphere.

Evidence exists to suggest that military explosives can result in the introduction of large quantities of particulate matter into the atmosphere. For example, carpet Evidence exists to suggest that military explosives can result in the introduction of large quantities of particulate matter into the atmosphere. For example, carpet

An idea of the potential climatic effect resulting from a full-scale nuclear exchange may be obtained by considering the magnitude of the blasts that might be involved. Only two nuclear bombs have been used in warfare, both dropped on cities in Japan in 1945. The Hiroshima fission bomb had the explosive force of 12 kilotons of TNT explosive. Its blast, fireball, and instantaneous emissions of neutrons and gamma radiation, followed by fires and exposure to radioactive fission products, killed about 100,000 people and destroyed the city on which it was dropped. By comparison with this 12-kiloton bomb, modern fusion bombs are typically rated at 500 kilotons, and 10-megaton weapons are common. A full-scale nuclear exchange might involve a total of the order of 5,000 megatons of nuclear explosives. As a result, unimaginable quantities of soot from the partial combustion of wood, plastics, paving asphalt, petroleum, forests, and other combustibles would be carried to the stratosphere. At such high altitudes, tropospheric removal mechanisms for particles are not effective because there is not enough water in the stratosphere to produce rainfall to wash particles from the air, and convection processes are very limited. Much of the particulate matter would be in the µm size range in which light is reflected, scattered, and absorbed most effectively and settling is very slow. Therefore, vast areas of the earth would be overlain by a stable cloud of particles and the fraction of sunlight reaching the earth’s surface would be drastically reduced, resulting in a dramatic cooling effect. There would be other effects as well. The extreme heat and pressure in the fireball would result in fixation of nitrogen by the following reaction:

O 2 + N 2 → 2NO (14.6.1)

The timing and location of nuclear blasts are very important in determining their climatic effects. Atmospheric testing of nuclear weapons, including a 58-megaton monster detonated by the Soviet Union, have had little atmospheric effect. Such tests were carried out at widely spaced intervals on deserts, small tropical islands, and other locations with minimal combustible matter. In contrast, military use of nuclear weapons would involve a high concentration of firepower, both in time and in space, on industrial and military targets consisting largely of combustibles. Furthermore, destruction of hardened military sites requires blasts that disrupt large quantities of soil, rock, and concrete, which are pulverized, vaporized, and blown into the atmos- phere.

On a hopeful note, the East-West conflict that dominated world politics and threatened nuclear war from the mid-1900s until 1990 has now abated and the probability of nuclear warfare seems to have diminished. However, the outbreak of armed conflict in Kosovo and Yugoslavia in 1999, nuclear proliferation as mani- fested by tests of nuclear bombs by both India and Pakistan in 1998, disintegration of great powers with vast nuclear arsenals, racial hatred accompanied by a determination to perform “ethnic cleansing,” and a “trigger-happy” state of mind among even educated people who should know better should still cause concern with respect to the prospect of “nuclear winter.”

Visitors from Space

Of all the possible atmospheric catastrophes that can occur, arguably the most threatening would be one caused by collision of a large asteroid with earth. Con- vincing evidence now exists that mass extinctions of species in the past have resulted from earth being hit by asteroids several kilometers in diameter. Such an event would cause much the same effects as those from “nuclear winter,” though with a large asteroid the effects would be much more pronounced. 9