Nuclear war between the superpowers in the spring or summer would not only contaminate the land with radioactive fall-out, but the nuclear forces expended would drive hundreds of millions of tons of soot, smoke and dust into the atmosphere and, by blocking sunlight, drop the temperature 10 to 25 deg C throughout North America, Europe and Northern Asia, destroying most crops immediately. A nuclear war in January could blacken the sky until June, kill all livestock and induce famine for over a year. There would be few survivors: civil defence plans for food stores do not take into account such a nuclear winter.
Theories about a nuclear winter are speculative. Much nuclear-made soot would be dispersed by winds, and high soot clouds would disperse because of their absorbed heat. The Gulf Stream and other natural, warm thermal currents would help raise local temperatures. Pollution of the upper atmosphere can also create the greenhouse effect of trapping heat; a nuclear winter in the summer, for example, could be followed, eventually, by a frost-free year-round climate. Some scientists have allowed their anxiety about nuclear war to influence their professional evaluation of its implications.