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AS the Clinton Administration prepares to announce in the next few weeks a plan for controlling waste industrial gases that trap heat in the atmosphere, conservatives and industry groups have mounted a renewed assault on the idea that global warming is a serious and possibly catastrophic threat.
In a drum roll of criticism over the last few months they have characterized the thesis of global warming as a "flash in the pan," "hysteria," "scare talk" and a ploy by socialists to justify controls on the economy.
The rhetoric is the mirror image of some that was heard five years ago, at the height of the 1988 North American heat wave, when some environmentalists and politicians warned of climatic apocalypse on the basis of assertions by a minority of scientists that global warming was already under way.
The evidence that warming or harm from warming will occur in the foreseeable future is "ludicrously small," argues a 1993 book published by the Cato Institute, a free-market research organization in Washington. Reining in global warming "will require a degree of bureaucratic control over economic affairs previously unknown in the West," wrote Ben W. Bolch and Robert D. McCallum, professors of economics and chemistry at Rhodes College in Memphis, in the book, "Apocalypse Not: Science, Economics and Environmentalism."
Dr. Dixy Lee Ray, the former Governor of Washington and a former chairwoman of the Atomic Energy Commission, called carbon dioxide "an unlikely candidate for causing any significant worldwide temperature changes" in another 1993 book, "Environmental Overkill: Whatever Happened to Common Sense?"
Continue reading the main storyIn recent months, a number of newspapers, including The Washington Times and The Wall Street Journal, have published articles debunking global warming. In the July issue of Commentary, Jeffrey Salmon, executive director of the George C. Marshall Institute in Washington, stated categorically that there is "no solid scientific evidence to support the theory that the earth is warming because of man-made greenhouse gases."
In the midst of this revisionist onslaught, whom is the public to believe?
Global warming is not a cut and dried issue, and scientific experts are still debating most of its aspects. But few scientists are to be found at either of the extremes that have characterized the political debate. A substantial number of highly regarded climate researchers have long believed that global warming set off by industrial and automotive emissions is a real possibility that could have serious consequences sooner or later. But they cannot say exactly how severe the effects of the warming will be or when it will come. And very few climatologists are ready to declare that global warming has begun. Most believe that no clear sign of human-induced change has yet made itself apparent amid the wide natural fluctuations of the earth's climate.
There are two undisputed facts about global warming: first, carbon dioxide, the waste gas produced by burning coal, oil and wood, has been accumulating in the earth's atmosphere over the last century; and second, the gas traps heat that is produced when the sun's energy is absorbed by the earth and then re-radiated.
Given those physical facts, the practical question of interest is how much the earth's climate will heat up after injection of a given amount of carbon dioxide. Since no experiment can answer that question, other than the global one now in progress, scientists have turned to their next best method, which is to simulate the earth's climate system in a series of equations that are run on a supercomputer. This exercise, known as computer modeling, is somewhat contentious because the models are far from perfect and represent a simpler, stripped-down version of the earth's real climate.
After examining results from the best computer models, a scientific advisory committee to the United Nations concluded in 1990, and again in 1992, that a doubling of atmospheric carbon dioxide -- expected to occur by the year 2100 without remedial action -- will raise the average global temperature by 3 to 8 degrees Fahrenheit. In comparison, the earth has warmed by only 5 to 9 degrees since the last ice age.
The United Nations panel and various committees of the National Academy of Sciences, made up of leading experts in the field, have consistently found merit in the global warming theory. Still, the issue is far from settled and critics of global warming continue to attack the models' predictions on various grounds.
One, Dr. Richard Lindzen, a meteorologist at the Massachusetts Institute of Technology, argues that the computer models fail to account accurately for the role of water vapor, a heat-trapping gas which by its sheer quantity exerts a far stronger greenhouse effect than carbon dioxide.
Most climatologists believe carbon dioxide is pivotal because the warming it induces, though quite small in itself, causes more water to evaporate from land and sea. The water vapor in turn traps more heat, thus amplifying the effect of the carbon dioxide.
Dr. Lindzen denies that this amplification takes place, and if he is right, then a doubling of carbon dioxide would indeed cause little or no greenhouse problem. But other climate researchers dismiss his argument as speculation, whereas the amplifying role of water vapor is well documented, in their view.
Critics of global warming say that the computer models cannot be trusted because they fail to reproduce the exact pattern of warming actually experienced over the last century. The models predict that in response to a 25 percent rise in atmospheric carbon dioxide over the last century, average global temperature should have risen by 2 degrees. In fact the climate has warmed by 1 degree.
Defenders of the models counter that with an extra refinement -- taking into account the fact that industrially emitted sulfate aerosols reflect sunlight -- the models' forecasts match the historical record much better.
Besides the imperfections of computer models, critics point out an apparent anomaly in global warming theory: that over the last century the chief rise in global temperature occurred before 1940, though most of the carbon dioxide has been spewed out since then. Defenders respond that the climate is an erratic system and that a neat, constant relationship between gas input and temperature rise should not be expected.
The critics go on to say that the small temperature spike seen so far could easily be nothing but another jiggle in the zigzag course of the climate's normal variability. Climatologists almost universally agree that this is indeed possible. The United Nations panel said it was equally possible, however, that a natural cooling trend could be masking an even larger greenhouse warming than what has been observed. Estimates of Global Warming
Conservative columnists and industry groups have jumped on another apparent discrepancy, that the models predict greater warming in the arctic than seems to be the case. Defenders say the measurements were not made in places where the warming would be most manifest, and that many observations have borne out the greater warming predicted for northern latitudes.
The models appeared to gain some important empirical support last December when Martin I. Hoffert of New York University and Curt Covey of Lawrence Livermore National Laboratory reconstructed two ancient climates, one much warmer than today's and one much colder. Since they were able to estimate temperature changes and prevailing carbon dioxide levels from evidence in the geological record, they were then in a position to answer the critical question -- at least for those two periods -- of how much warming is produced by a given rise in carbon dioxide levels.
Their answer was that when carbon dioxide levels double, the temperature of the atmosphere rises by about 4 degrees. This figure agrees well with the best estimate from the computer models which, in the view of the United Nations panel, is that a doubling of carbon dioxide induces a 4.5-degree rise in average global temperature. Some scientists believe the Hoffert-Covey findings rule out a warming in the lower end of the panel's broader range of 3 to 8 degrees. A climate system that responds so sluggishly to external forces could not have produced the great temperature swings of the past, they say.
Dr. Lindzen, on the other hand, questions the Hoffert-Covey study's utility in predicting future climate change on the ground that the climate may have behaved differently in the remote past.
Critics note that evidence for the observed global warming trend ofthe 1980's depends on ground-based measurements of temperature, which amount to only a sampling of the whole earth. These measurements are also suspect, in the critics' view, because many of the measuring sites are near cities, which are warmer than the surrounding countryside. Satellite observations that started in 1979 failed to see any warming trend. Defenders say the satellite measurements could be distorted by atmospheric factors, that their record is too short to support any generalization and that the ground-based temperature measurements have been correctly calibrated to discount warmth from cities.
Some critics contend that even if the atmosphere does heat up, the warming will be benign. Dr. Patrick J. Michaels, a meteorologist at the University of Virginia, points out that the warming seen so far has made itself evident mostly at night and in winter. If that continues, he argues, global warming will be a boon, not a catastrophe. Growing seasons will be longer, winters less harsh and plants invigorated by rising carbon dioxide will grow faster.
But Thomas R. Karl, chief of the global climate laboratory at the National Climatic Data Center in Asheville, N.C., responds that the warmer nights may have other causes, possibly natural, quite apart from greenhouse gases. James E. Hansen, a climatologist at the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York, notes that warmer winters, far from being welcome, could disrupt the chilling that many plant seeds must experience to germinate properly in the spring. Botanists have found that carbon dioxide does spur plant growth, but also that the plants can then outgrow their nutrient supply, making them less nutritious as food.
As for other effects of the predicted warming, the United Nations panel estimated that an average global warming of 4.5 degrees would cause sea level to rise by two feet by the year 2100 and would make heat waves more frequent and cold spells rarer. Model simulations have suggested that although average global rainfall would increase, the interiors of continents would become drier, droughts more common and tropical storms more violent. Some climatologists also believe that climatic zones would shift and regional weather patterns would be dislocated, disrupting agricultural production and natural ecosystems. But these effects are regarded by climatologists as being uncertain.
Where does all this leave the debate?
Issues in science are not decided by taking votes, but polls of climate researchers show that most believe there is a better than even chance that the climate will warm by at least 3.5 degrees over the next century.
Climatologists also know that once in the atmosphere, carbon dioxide stays there for centuries; whatever climatic effect it has will not be reversed in several human lifetimes. This may partly explain why the weight of opinion among climate scientists, as measured by polls, is that lack of certainty should not stand in the way of prudent steps to control greenhouse gas emissions.
Indeed, a panel of the National Research Council -- the Government's chief source of scientific advice -- concluded in 1991 that "despite the great uncertainties, greenhouse warming is a potential threat sufficient to justify action now."
How much action, what kind and how soon is an economic and political issue of great consequence in a world that runs on the burning of fossil fuels. Proponents of the greenhouse theory therefore need not be surprised at the intense fire now being rained down on their ideas.
An article in Science Times on Tuesday about global warming misidentified an author of the book "Apocalypse Not: Science, Economics and Environmentalism." He is Harold Lyons.