Free The Nuclear Winter by Carl Sagan Page A

awaiting orders.
    The World Health Organization, in a recent detailed study chaired by Sune
    K. Bergstrom (the 1982 Nobel laureate in physiology and medicine), concludes that 1.1 billion people would be killed outright in such a nuclear war, mainly in the United States, the Soviet Union, Europe, China
    and Japan. An additional 1.1 billion people would suffer serious injufles
    and radiation sickness, for which medical help would be unavailable. It thus seems possible that more than 2 billion people-almost half of all the
    humans on Earth-would be destroyed in the immediate aftermath of a global
    thermonuclear war. This would represent by far the greatest disaster in the history of the human species and, with no other adverse effects, would
    probably be enough to reduce at least the Northern Hemisphere to a state
    of prolonged agony and barbarism. Unfortunately, the real situation would
    be much worse. In technical studies of the consequences of nuclear weapons
    explosions, there has been a dangerous tendency to underestimate the results. This is partly due to a tradition of conservatism which generally
    works well in science but which is of more dubious applicability when the
    lives of billions of people are at stake. In the Bravo test of March 1, 1954, a 15-megaton thermonuclear bomb was exploded on Bikini Atoll. It had
    about double the yield expected, and there was an unanticipated last-minute shift in the wind direction. As a result, deadly radioactive
    fallout came down on Rongelap in the Marshall Islands, more than 200
    kilometers away. Most all the children on Rongelap subsequently developed
    thyroid nodules and lesions, and other long-term medical problems, due to
    the radioactive fallout.
    Likewise, in 1973, it was discovered that high-yield airbursts will chemically burn the nitrogen in the upper air, converting it into oxides
    of nitrogen; these, in turn, combine with and destroy the protective ozone
    in the Earth's stratosphere. The surface of the Earth is shielded from deadly solar ultraviolet radiation by a layer of ozone so tenuous that, were it brought down to sea level, it would be only 3 millimeters thick.
    Partial destruction of this ozone layer can have serious consequences for
    the biology of the entire planet.
    These discoveries, and others like them, were made by chance. They were largely unexpected. And now another consequence -- by far the most dire
    --
    has been uncovered, again more or less by accident.
    The U.S. Mariner 9 spacecraft, the first vehicle to orbit another planet,
    arrived at Mars in late 1971. The planet was enveloped in a global dust storm. As the fine particles slowly fell out, we were able to measure temperature changes in the atmosphere and on the surface. Soon it became
    clear what had happened:
    The dust, lofted by high winds off the desert into the upper Martian atmosphere, had absorbed the incoming sunlight and prevented much of it from reaching the ground. Heated by the sunlight, the dust warmed the adjacent air. But the surface, enveloped in partial darkness, became much
    chillier than usual. Months later, after the dust fell out of the atmosphere, the upper air cooled and the surface warmed, both returning to
    their normal conditions. We were able to calculate accurately, from how much dust there was in the atmosphere, how cool the Martian surface ought
    to have been.
    Afterwards, I and my colleagues, James B. Pollack and Brian Toon of NASA's
    Ames Research Center, were eager to apply these insights to the Earth.
    In
    a volcanic explosion, dust aerosols are lofted into the high atmosphere.
    We calculated by how much the Earth's global temperature should decline after a major volcanic explosion and found that our results (generally a
    fraction of a degree) were in good accor4 with actual measurements.
    Joining forces with Richard Turco, who has studied the effects of nuclear
    weapons for many years, we then began to turn our attention to the climatic effects of nuclear war.