WEATHER CHANGE: POORER HARVESTS

In rich fields of India's Rejasthan state, where the monsoon rains usually sweep in faithfully each summer, the rice crop has been devastated by the first drought in years. Eastward on the Indian subcontinent, great floods have ruined the Bangladesh harvest. Far off in Africa's Sahel region, six years of drought have only recently been interrupted by rain. In the U.S., both the corn and soybean crops will fall far below expectations this year because of a freakish succession of excess spring rains, summer drought and early fall frost.

Is this roster of natural disasters an omen of worse weather to come? The forecasters can only guess. Even the most skilled meteorologists admit that theirs is one of the least exact sciences. But as they ponder the earth's current erratic weather and study their steadily increasing store of knowledge about past climate, more and more scientists are raising storm warnings for the future. At the very least, they foresee troublesome changes in global temperature and rainfall patterns that could seriously jeopardize the earth's ability to feed itself.

Despite such regional woes as America's Dust Bowl of the 1930s, the world's major agricultural areas have enjoyed an unparalleled record of beneficent weather for the past half-century. It has been "the most abnormal period in at least a thousand years," says Reid A. Bryson, director of the University of Wisconsin's Institute for Environmental Studies. Temperatures were surprisingly high, and the warmth fostered plant growth in normally well-watered areas, while some deserts shrank under the influence of regular rainfall.

Now that era may well be ending. From his studies of weather history, British Climatologist Hubert H. Lamb concludes that climate runs in roughly 200-year-long cycles, and that the earth is now entering one of its chilly phases. Perhaps the gloomiest of the weather prophets, Bryson speculates that the earth may be reverting to a frigid interlude comparable to what some scientists call the "little ice age" that cooled Europe from the 16th through 19th centuries. During those years Greenland's once lush fields vanished, England's productive vineyards withered, and agricultural disasters like Ireland's great potato famine came to be accepted as a natural feature of life.

Since the 1940s, the mean global surface temperature has fallen only about 1DEG 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.

Despite concern about the harm that could be done by nuclear explosions in the atmosphere, scientists have yet to find any significant effect on climate. At present, many weather researchers are far more interested in the effects of sunspots, the fierce magnetic storms on the solar surface, which are often accompanied by the eruption of great flares of immensely hot gases. The streams of particles shot off during these episodes are already known to disturb the earth's magnetic field and disrupt communications. Astrophysicist Walter Orr Roberts, former director of the National Center for Atmospheric Research, thinks that they also may influence weather, at least temporarily. Among other evidence, he cites an apparent link between periods of minimum activity in the sunspot cycle and recurring droughts on the east side of the Rockies.

Still other climatologists suspect that global temperature changes could stem from wobbles in the earth's rotation that alter the amount of sunlight striking the surface. Some think that there may even be subtle changes in the earth's orbit, which would increase or decrease the distance from the sun.

However they explain the recent cooling, there are many observers who are not convinced that it is part of a long-term trend. Two Government experts, Donald Oilman and J. Murray Mitchell Jr., argue that it may only be a random fluctuation, rather than part of any fixed cycle. In fact until a few years ago, many scientists suspected that the earth would heat up, largely because of mankind's increasing output of carbon dioxide. A byproduct of fossil-fuel burning, the gas lets sunlight pass down through the atmosphere but prevents the escape of infra-red heat waves that are radiated from the earth's surface. Thus the gas adds to the planet's heat store.

To ease the adverse effects of changing climate, some people talk of "weather modification." Cloud seeding, for example, has been tried to release rains over parched fields. But the technique is still primitive and it raises serious political and ecological questions: if rain makers manage to bring water to one region will they be depriving another--perhaps in a neighboring country? The skilled plant breeders who created the Green Revolution can breed tougher grains to meet changes of climate. But these will take time to perfect, and their use will be limited unless science makes greater progress in long-range weather forecasting. Otherwise, farmers will not know which of the new seeds to plant. Unfortunately, even the best long-range weather predictions are notoriously inaccurate. Fluctuations in climate depend on so many variables--winds, temperature, rates of evaporation, etc.--that meteorologists have yet to formulate accurate mathematical models to show how all such factors affect one another.

Some new approaches promise help. As part of a multinational scientific program called NORPAX (North Pacific Experiment), Meteorologist Jerome Namias of Scripps Institution of Oceanography has been investigating unexplained links between ocean temperatures and weather. He has found, for instance, that the formation of hot and cold patches in specific parts of the Pacific appears to be followed by colder-than-normal winters in the Eastern U.S. and warmer-than-normal winters in the West. If enough patterns of that kind can be found, says Namias, they could tip off long-range trends.

More such work is surely needed to brighten the future of prognostication. For the only thing that weather experts generally agree on is that the world's prolonged streak of exceptionally good climate has probably come to an end--meaning that mankind will find it harder to grow food.

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