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Since the 1940s, the mean global surface temperature has fallen only about 1° F. But even this small drop has trimmed a week to ten days from the growing season in the middle latitudes that are the earth's breadbasket. Continued cooling could lead to agricultural disasters. The vaunted "miracle" wheat and rice of the Green Revolution were specifically created by plant geneticists to thrive under the optimum growing conditions of recent years. They are particularly vulnerable to vagaries of weather. A decline in moisture can significantly reduce their yields; they can also become susceptible to blights and pests. It was a bout of wet, chilly air during the growing season that apparently touched off the Middle Ages' outbreaks of St. Anthony's fire —excruciatingly painful convulsions and gangrenous hands and feet that are caused by a fungus which grows on rye in cold, damp weather. Some changes, to be sure, could be beneficial. The Midwest's corn growers expect harvests to go up in slightly chillier weather (because the cold reduces water losses through evaporation). But in most cases, any changes in climate mean trouble for farmers.
Scientists disagree sharply about the cause of the earth's cooling and whether it will continue. But a flood of observations by weather satellites and other new instruments show its major effect: a gradual expansion in recent years of the so-called circumpolar vortex—the great icy winds that whip around the top and bottom of the world. Those winds move generally from west to east, but the outer edge of the vortex twists and bends, like the bottom of a large, swirling skirt. In the U.S. Far West, for instance, the winds bring down cold, dry Arctic air; thus winters there have been unusually bitter. Conversely, in such normally chilly regions as New England and Scandinavia, winters have been uncharacteristically warm because the vortex has pulled up warm air from the south. At times, great air masses of differing temperature and humidity can collide, creating unusually violent storms, like last spring's tornadoes in the Midwest.
The most devastating influence of the circumpolar vortex has been felt in a broad tropical belt stretching round the globe. As the edge of the great wind system reaches closer to the planet's midriff, it has blocked moisture-laden equatorial winds. No longer have they been able to bring needed rain to such diverse areas as India, parts of Central America and West Africa's Sahel. Already suffering from years of overgrazing, the Sahel has dried up so badly that the Niger River can be forded by foot for the first time in centuries. In effect, the Sahara has edged south.
Bryson, for one, blames the earth's cooling on an increase of dust in the atmosphere. Acting like tiny mirrors, dust particles reflect some of the sunlight striking the earth's atmosphere, depriving the surface of solar heat. Bryson believes that the excess dust comes in part from volcanic eruptions, which seem to have increased in recent years. Still other atmosphere polluters could be: 1) extensive land clearing and deforestation by slash-and-burn techniques, and 2) the increased use of fossil fuels, which release soot into the air.