IDEAS & TRENDS
IDEAS & TRENDS; Was That a Greenhouse Effect? It Depends on Your Theory
By MALCOLM W. BROWNE
Published: September 4, 1988
THE memorably uncomfortable summer of 1988 has left many Americans with a suspicion that nature is at last getting even for mankind's wanton pollution of the atmosphere.
From California to the Carolinas, the summer's heat wave and drought took a sobering toll. Electric power faltered, vast forests went up in flames, river navigation was throttled, crops failed. The ''greenhouse effect'' - the trapping of solar heat by pollutant gases in the atmosphere - became a household phrase. Some climatologists warned that unless we quickly mend our ways, the world's grain belts will turn to dust bowls, coastal regions will be flooded, forests will die and countless species will become extinct.
But are we really experiencing the start of a change in climate that could trigger a global catastrophe? The prospects are far from clear.
On June 23, Dr. James E. Hansen of the National Aeronautics and Space Administration caught the nation's attention when he told a Senate committee that the warming trend almost certainly stems from the greenhouse effect. A crisis, he warned, may not be long in coming.
Carbon dioxide, the main cause of atmospheric greenhouse warming, is produced by the combustion of fossil fuels (largely coal and oil), and its level in air has roughly doubled since the beginning of the Industrial Revolution. By the middle of the next century this could result in an increase in global temperature amounting to as much as 9 degrees Fahrenheit -enough to drastically alter the planet's climate.
But forecasting climate has never been as straightforward as scientists could wish. They are not even sure that this summer's weather was really symptomatic of any trend at all.
North America had a fairly hot summer, but according to the Climate Analysis Center of the National Weather Service, July of this year, when the heat wave was at its peak, was only the 11th hottest July in 58 years of record-keeping.
The United States also had a dry summer, but it was by no means the driest on record. The record was posted in July 1934, when 62 percent of America's land was afflicted by drought; this July, the figure was 42 percent. Serious droughts also occurred in 1936, 1953 and 1956.
The United States was not alone in its bad weather this summer. China suffered a heat wave and a drought. On the other hand, Ireland and parts of Western Europe were unusually chilly and wet. As winter ends in the Southern Hemisphere, the Gran Chaco region of Paraguay, Bolivia and northern Argentina, normally warm year-round, just had one of its coldest seasons on record.
A. James Wagner, an analyst at the Weather Service, acknowledges that during this decade the world has seen the four warmest years of the past century - 1980, 1983, 1986 and 1987. ''But I don't feel that the evidence is overpowering that this is anything more than a normal fluctuation,'' he said.
Climatologists constantly invent and refine mathematical models in an attempt to understand fluctuations. One such model, which seems to mimic the real climate quite realistically, was devised by Dr. Edward Lorenz of the Massachusetts Institute of Technology. This model, which does not take carbon dioxide into account but does reckon on the interactions of the atmosphere with the ocean, exhibits large variations.
''The Lorenz model was run backward on a computer for the equivalent of about 400 years,'' Mr. Wagner said, ''and the large fluctuations it sometimes produced, which were not entirely random but which were not cyclical either, were quite startling.''
The swings, he said, were as much as plus or minus 3.6 degrees Fahrenheit in global temperature from one year to the next. The model sometimes produced clusters, in which several years close together were unusually hot - a pattern imitating the real climate of the 1980's.
Frederick J. Gadomsky, a member of a team of climate experts at Pennsylvania State University, believes that the summer now ending, warm though it may have been in North America, did not bear out theoretical predictions based on the greenhouse effect.
''From a global greenhouse warming,'' he said, ''one would expect regional warming to show up first in the polar regions rather than, say, in North Dakota. But this has not been the case.''
There is a possibility, experts say, that the earth has self-corrective mechanisms that tend to stabilize the climate, even when carbon dioxide and other greenhouse gases, such as methane, are added. Much of the infrared radiation from the sun is ordinarily reflected back into space by clouds, the oceans and the ground, but greenhouse gases absorb some of that energy, preventing its escape. The greenhouse molecules, thereby heated, are dispersed throughout the atmosphere and warm it.
''A bewildering mixture of positive-feedback and negative-feedback loops affect climate, and it is very difficult to gauge how they interact,'' Mr. Wagner said. The negative-feedback effects would tend to bring conditions back to normal.
''For example,'' he said, ''a global increase in temperature would increase evaporation from the oceans, thereby producing more clouds and rainfall. Rain falling on land might cool the land and subsequently cool the air over the land. The increased cloud cover, moreover, would reflect more sunlight away from the earth than usual, and this would also cool the atmosphere. It's conceivable that global temperatures would end up lower than normal as a result of this compensation.''
Among the factors presumed to affect global climate to some degree are shifts in ocean currents, large wildfires, automobile exhaust, fluctuations in the earth's orbit, variations in solar radiation (which seem to be related to the sun's 11-year sunspot cycle), volcanic and meteoric activity and deforestation. Even the methane gas produced in the digestive tracts of termites is thought to play a role.
Sorting out these and many other potential factors is a gargantuan task, still beyond the reach of the best mathematical models and supercomputers. One problem is the relative scarcity of weather records. Human beings have recorded more or less reliable measurements of weather conditions only in the last century, a period far too short to use as a gauge of long-term changes. Past climatic conditions can be estimated to some extent from historical records; 17th-century European writers, for example, chronicled a ''little ice age.'' Indications are also found in tree-ring patterns and core samples extracted from glaciers and ice caps. But such information is sketchy at best.
That large fluctuations in global temperature do occur independently of man's activities is a matter of geological record; 70 million years ago, heat-loving dinosaurs apparently thrived along Alaska's North Slope, and 15,000 years ago Manhattan lay under a polar ice sheet.
Sooner or later the greenhouse effect is very likely to alter conditions on earth for the better, or, more likely, for the worse.
''The effects of carbon dioxide pollution are probably irreversible and therefore we do have to worry about them,'' said Paul G. Knight, a Penn State climatologist. But he added: ''In the short term, I don't see any major climate shift in the offing, and I don't feel we should be packing our bags to move to Manitoba just yet.''