The book ignited a controversy about nuclear conflict because Kahn's thesis was that nuclear war fought with thermonuclear weapons was winnable, thus possibly upping the ante for the nuclear powers. Kahn used many quite rather incomplete arguments to make the not entirely unreasonable point that while such a war would be horrific, it would not mean the end of humanity. The survivors may not necessarily envy the dead. But of course Kahn was speculating based on the then best available scientific data along with his own idiosyncratic biases. One of the biggest effects of a nuclear explosion is to send up debris in the atmosphere, and climate models in the 60s were in a primitive stage to help with predicting any such effects. Also, nuclear effects start wide-ranging fires and, on the rare occasions when the conditions are right, gruesome firestorms; a firestorm is the nearest thing to hell that one can imagine. Fires can account for up to 60% of the damage from a nuclear explosion. While the thermal effects constitute about 35% of the total effects from a typical nuclear air-burst (blast effects constitute about 50%), thermal effects unlike others can naturally sort of self-perpetuate through starting successive fires. According to some analysts, state department officials in the 50s calculating nuclear weapons effects neglected the devastation due to fire, which made their results underestimates. Any realistic simulation of a nuclear explosion has to take into account effects due to fires.
The debate about the effects of a global thermonuclear war was galvanized in the 1980s when Carl Sagan and his colleagues proposed the idea of nuclear winter, in which dimming of sunlight because of the debris from nuclear explosions would lower the average temperature at the surface of the earth. Among other effects, this combined with the resulting darkness would devastate crops, thus bringing about long-term starvation and other catastrophes. Since then, scientists have been arguing about nuclear winter.
What has changed between 1980 and now though is that climate models including general circulation models have vastly improved and computational power to analyze them has exponentially gone up. Although we still cannot predict long-term climate, we now have a reasonably good handle on quantifying the various forcings and factors that affect climate. Thus for the last few years it has seemed worthwhile to predict the effect of nuclear war on our climate. Now scientists working at the University of Colorado and NOAA have come up with a rather disconcerting study in the Proceedings of the National Academy of Sciences indicating the effects of a regional nuclear war on global climate. A typical scenario is a war with 50 warheads of 15 kilotons each (about the yield of the Hiroshima bomb) between India and Pakistan, a conservative estimate. There have been a few such studies published earlier but this one looks at the effects on the ozone layer, the delicate veneer that protects life from UV radiation.
The researchers' main argument is that there is a tremendous mass of soot that is kicked up tens of kilometers into the atmosphere during a nuclear explosion. The study seems to be carefully done, taking into account various factors acting to both reinforce and oppose the effects of this soot. The number they cite for the amount is about 5Tg (teragrams, a teragram being 10^12 grams) which is a huge number. They account for local fallout of the soot through rain as being about 20%. What happens to the remaining 4Tg is the main topic of investigation. According to the model, this enormous plume of soot is intensely heated by sunlight. By this time it has entered the upper layer of the troposphere and snakes up into the lower stratosphere where the ozone layer is situated, it is radiating heat that disrupts the delicate balance of chemical reactions that produce and get rid of ozone, reactions that have now been well-studied for decades. These involve the interaction of radical species of oxygen, nitrogen and halogens with ozone that sap the precious molecule away. The bottom line is that this heat from the hot soot vastly increases the rate of reactions that produce these species and eat up the ozone at that altitude, thus depleting the layer. The soot lingers around since removal mechanisms are slow at that height. The heat also encourages the formation of water vapour and its consequent break up and reaction with ozone, thus further contributing to the breakdown. The researchers also include circulation of water vapour and other gases in the global atmosphere, and how this circulation will be affected by the heat and the flow. Nitrogen oxides generated by natural and human processes have already been shown to deplete ozone, and the heated soot will also intensify the rate of these processes.
The frightening thing about the study is the magnitude of the predicted ozone loss due to these accelerated processes; about 20% globally, 40% at mid latitudes and up to 70% at high latitudes. Also, these losses would last for at least five years or so after the war. These are horrifying numbers. The ozone layer has evolved in a synergistic manner over hundreds of millions of years to wrap up life in a protective blanket and keep it safe. What would the loss of 40% of the ozone layer entail? The steep decline would allow low wavelength UV radiation which is currently almost completely blocked out to penetrate the biosphere. This deadly UV radiation would have large-scale devastating effects including rapid increases in cancer and perhaps irreversible changes in ecosystems, especially aquatic ones. The DNA effects documented by the researchers are appalling- up to 213% increases in DNA damage with respect to normal levels, with plant damage up to 132%. In addition, the increased UV light would hasten the normal decomposition of organic material, further contributing to the natural balance of the biosphere. The phenomenon would indeed be a global phenomenon. Decomposition of the soot is thought to be negligible.
Now I am no atmospheric scientist, but even if we assume that some of these estimates end up a little exaggerated, it still seems to me that effects on the ozone layer could be pretty serious. If I had to guess, I would think that there could be uncertainty in estimating how much soot is produced, how much goes up and to what altitude, and how long it stays there. What seems more certain are the effects on the well-studied radical reactions that deplete ozone. Some elementary facts seem to reinforce this in my mind- carbon has a very high sublimation point and can get heated up to high temperatures, the energy radiated by a hot body goes as the fourth power of the temperature, and from college chemistry I do remember the rule of thumb that on an average, the rate of a reaction doubles with a 10 degrees centigrade temperature rise. The estimates of rate increases made by the authors seem reasonable to me.
What is most disconcerting about the study is that it involves a rather "small" nuclear exchange that takes place in a localized part of a continent, and yet whose effects can affect the entire world. "Globalization" acquires a new and portentous meaning in this context. India and Pakistan can both easily field 50 weapons each of 15 kilotons yield, if not now, in the near future. In addition to this global-scale devastation of the ozone layer, it would be unthinkable to imagine the more than 10 million people dying in such a conflict, as well as total devastation of public systems and the food supply. Herman Kahn might have thought that nuclear war is "survivable". Well, maybe not exactly...
Reference and abstract for those who are interested:
Mills, M.J., Toon, O.B., Turco, R.P., Kinnison, D.E., Garcia, R.R. (2008). Massive global ozone loss predicted following regional nuclear conflict. Proceedings of the National Academy of Sciences, 105(14), 5307-5312. DOI: 10.1073/pnas.0710058105
"We use a chemistry-climate model and new estimates of smoke produced by fires in contemporary cities to calculate the impact on stratospheric ozone of a regional nuclear war between developing nuclear states involving 100 Hiroshima-size bombs exploded in cities in the northern subtropics. We find column ozone losses in excess of 20% globally, 25–45% at midlatitudes, and 50–70% at northern high latitudes persisting for 5 years, with substantial losses continuing for 5 additional years. Column ozone amounts remain near or <220 Dobson units at all latitudes even after three years, constituting an extratropical "ozone hole." The resulting increases in UV radiation could impact the biota significantly, including serious consequences for human health. The primary cause for the dramatic and persistent ozone depletion is heating of the stratosphere by smoke, which strongly absorbs solar radiation. The smoke-laden air rises to the upper stratosphere, where removal mechanisms are slow, so that much of the stratosphere is ultimately heated by the localized smoke injections. Higher stratospheric temperatures accelerate catalytic reaction cycles, particularly those of odd-nitrogen, which destroy ozone. In addition, the strong convection created by rising smoke plumes alters the stratospheric circulation, redistributing ozone and the sources of ozone-depleting gases, including N2O and chlorofluorocarbons. The ozone losses predicted here are significantly greater than previous "nuclear winter/UV spring" calculations, which did not adequately represent stratospheric plume rise. Our results point to previously unrecognized mechanisms for stratospheric ozone depletion.