Dyson VS Sagan on Nuclear Winter

I do not wish here to get into a technical argument about the details of nuclear winter. I will merely summarize my own struggles with the technical issues. I spent a few weeks in 1985 trying to make nuclear winter go away. The phrase "go away" here is used in the sense customary among scientists. To destroy a new theory, you try to find a simple situation where the theory predicts that something happens and you can prove that the same something does not happen. Then you say that the thing predicted has gone away and the theory is demolished. I found, after about two weeks of work, that I could not make nuclear winter go away. That is to say, I could not prove the theory wrong. As a result, I now understand the theory better and believe it much more than I did when it was first announced. But I do not yet believe it a hundred percent. Our technical understanding of it is still rudimentary. It is still possible that when we understand it better we will find that it has gone away.

When scientists are arguing about a complicated and controversial problem, it often happens that their technical judgments are influenced more by personal experience than by objective calculation. The arguments about nuclear winter show clearly the influence of personal background. For example, Carl Sagan believes strongly in the reality of nuclear winter whereas I am still skeptical. We both use the same mathematics and both work with the same laws of physics. Why then do we reach different conclusions? Perhaps the difference in our conclusions results from the fact that Carl {263} Sagan spent a large fraction of his life studying Mars while I spent a part of my life living in London. Carl Sagan's intuition about nuclear winter is based on his experience of Martian dust storms. My intuition about nuclear winter is based on my experience of old-fashioned London fog. The main difference between a Martian dust storm and a London fog is that Mars is dry while London is wet.

Carl Sagan has vivid memories of the great dust storm which was raging all over the planet when the Mariner 9 spacecraft came to make the first detailed observations of Mars in 1971. He remembers the temperature on the Martian ground falling as the dust clouds obscured the Sun. I have vivid memories of an orchestral concert in the Albert Hall in London, when I sat in the front row of the circle and heard the music emerging mysteriously from an invisible orchestra behind a wall of fog. I remember that on this and other occasions when London was enveloped in fog, the temperature on the ground under the fog was warm. Carl Sagan's image of nuclear winter is based on the optical properties of dry dust; my image of nuclear winter is based on the optical properties of wet soot. Dry dust or dry soot acts as an optical filter, blocking sunlight from reaching the ground but allowing heat radiation from the ground to escape into space. Wet dust or wet soot acts as a blanket, blocking all radiation from carrying energy either upwards or downwards.

If the atmosphere after a nuclear war is filled with dry soot, the temperature on the ground will fall and the Earth will experience nuclear winter. If the atmosphere is filled with wet soot, the temperature on the ground will stay roughly constant as it used to do under a London fog. The severity of a nuclear winter depends on whether the soot-laden atmosphere is predominantly dry or predominantly wet. In nuclear winter as in the meteorology of normal terrestrial weather, water in its multifarious forms of vapor, cloud, rain and snow plays a dominant role. We live on a water-dominated planet. And the prediction of weather patterns is still, even with all the help {264} we can obtain from large-scale computer simulations, more an art than a science. Meteorologists still must use intuition and experience to make their predictions. Carl Sagan's intuition has the Earth after a nuclear war resembling Mars under a dust storm; my intuition has the Earth resembling London under a fog, and the computers are not clever enough to tell who is right.

These technical arguments about the meteorological consequences of nuclear war should not be taken too seriously. The essential fact, which is true independently of the meteorological uncertainties, is that the consequences of a nuclear war are incalculable and unpredictable. Nuclear winter has only added an additional dimension to the incalculabil-ity and unpredictability of war. Nevertheless, the new dimension of unpredictability is important. No matter how the technical arguments may be settled or remain unsettled, nuclear winter stands as a symbol of the vulnerability of mother Earth to the violence of her human children. Nuclear winter is not primarily a technical problem; it is much more a moral and political problem. It forces us to ask fundamental questions: whether the benefits which we derive from the possession of nuclear weapons are in any way commensurate with the risks; whether the risk of irreparable damage to the fabric of life on Earth can in any way be morally justified.

The nuclear winter issue, like the radioactive fallout issue in the fifties, has the great virtue of being global. It compels the nuclear powers to accept some moral responsibility for the damage we may do to mankind and to our planet as a whole. But our awareness of this responsibility should not depend on whether the technical details of nuclear winter theory turn out to be right or wrong. Quite apart from nuclear winter, a nuclear war would be an unconscionable crime against humanity. The worldwide spread of famine and disease resulting from economic and social disruption may be as murderous as the physical effects of warfare. The discovery of nuclear winter has given a new {265} starkness to our responsibility for the survival of mankind. Even without nuclear winter, we are still responsible.