We know two things about global warming. One: in the last 100 years the average temperature has risen between two-thirds and one degree Fahrenheit. Two: in the same time the amount of carbon dioxide and other heat-trapping gases in the atmosphere has risen about 40 percent.

Everything else you hear about the greenhouse effect contains a good deal of hot air. Nobody has yet proven that facts one and two are connected–much less whether they foreshadow a disastrous “runaway greenhouse” in the 21st century complete with scorching summers, flooded coastlines, parched farms, monster hurricanes, and numerous extinctions.

Many people go ahead and take the leap of faith anyhow. Author Andrew Revkin does so, in his gorgeously illustrated new book Global Warming: Understanding the Forecast, cosponsored by the Environmental Defense Fund and New York’s American Museum of Natural History. At first Revkin acknowledges that “any signal of global warming is still largely hidden in the statistical ‘noise’ produced by normal fluctuations in weather.” But a few pages later, he writes that future scientists may call the present geologic era the “Anthrocene,” because of the effect human CO2 emissions are having on the climate, and urges us not to leave “a legacy of irresponsibility and neglect.”

How does Revkin know which way to leap, given that most of this century’s warming took place prior to 1940, before the recent run-up in CO2? He’s following the global climate models constructed by more than a dozen different groups of climatologists. These models–actually systems of equations programmed into supercomputers–predict that when the amount of carbon dioxide in the air doubles (by about 2040), it will trap enough extra solar heat to raise average world temperatures three to eight degrees Fahrenheit.

That may not seem like a lot–Halsted is often that much warmer than the lakefront–but remember it’s an average, and the average temperature during the most recent ice age was only nine degrees cooler than today. The difference, global-warming activists point out, is that living things had thousands of years to adjust to that nine-degree warm-up following the ice age; we may have less than a century to adapt if the models’ high-end predictions are right.

But are the models correct? It is annoyingly difficult for an outsider to get a clear answer. Revkin says there is a scientific consensus that they are, citing a 1991 National Academy of Sciences study calling for “action now.” So does climate modeler Stephen Schneider of the National Center for Atmospheric Research, whose books and articles express dismay at governmental inaction on global warming. Science magazine reporter Richard A. Kerr concluded in May that “the consensus is stronger than ever–greenhouse warming does pose a serious threat for the planet’s future.”

On the other hand, veteran climatologist Stanley Changnon, former chief of the Illinois State Water Survey and now head of its Global Climate Change Program, says that although the models would probably win a majority vote of knowledgeable atmospheric scientists, that falls short of consensus. “There is total agreement–99.9 percent–that trace gases are increasing and that they can and do produce an enhanced greenhouse effect. That does not say that an enhanced greenhouse effect will produce higher temperatures at the surface of the earth. Most scientists would say so, but there is a small honest group of dissenters who say other forces may intervene.” Extra carbon dioxide will trap more heat, for instance, and that increases cloud cover, which in turn could cancel out the extra heating. “You’ve got the [greenhouse] vacuum cleaner running, but somebody may be spilling dirt faster than it’s running.” Changnon’s program is prudently named “global climate change” just in case the climate cools instead of heating up.

Changnon hasn’t written a book, but climatologist Robert Balling Jr. of Arizona State University has. In The Heated Debate: Greenhouse Predictions Versus Climate Reality, published by the libertarian/conservative Pacific Research Institute, he explains lucidly the weaknesses of the models and then offers his own best prediction:

“Our future will see a rise in temperature of approximately 1.0 degrees C (1.8 degrees F), with most of that warming occurring at night, in higher latitudes, and in the winter season. Extreme high temperatures probably will not increase [when atmospheric CO has doubled] and the earth will probably be wetter, cloudier, with substantial increases in soil moisture; droughts may diminish in frequency, duration, and intensity. These predictions for the future are largely consistent with the models and largely consistent with how the climate system has already responded to a 40 percent increase in equivalent CO over the past century. From a climate perspective, we have had a far more moderate greenhouse effect than what the public has been led to believe.”

I asked Changnon about Balling’s credentials, since his book, like Revkin’s and Schneider’s before him, is published by an interested party. “Bob Balling is doing science,” he said flatly. “Schneider is selling books.”

Given the endless complexities of weather and climate, the models are both astonishingly successful and still not good enough to tell us what we really want to know. They correctly reproduce the cycle of the seasons, for instance. (That’s not as dumb as it sounds. Temperatures change a lot more between winter and summer than they would between now and any imaginable greenhouse earth–so a model that failed to predict seasonal changes would be of little use in subtler investigations.)

On the other hand, the models predict that the amount of carbon dioxide now in the earth’s atmosphere should have caused about twice as much warming as has actually occurred. And, as Balling explains, other global changes appear to have contributed to that warming–in particular, desertification (whereby the ground absorbs more heat) and reduced volcanic dust in the stratosphere (which lets in more sunlight).

The models are much better at predicting world averages than regional changes. And some critical questions are still largely unanswered. Will the oceans slow down global warming or speed it up? If the earth warms and we get more clouds, will they be the high wispy cirrus that trap more heat than they reflect (accelerating the greenhouse effect), or the low thick stratus that reflect sunlight away from the earth in the first place (decelerating the greenhouse effect or even cooling the earth)?

“The models are all tested on today’s weather,” cautions climatologist Changnon. “The public needs to understand the difficulty of determining how many of these factors–volcanic activity, clouds, atmospheric radiation, ocean storage–are going to stay the same.” Some model builders, he says, worry that environmentalists have gotten too alarmist with what are still fairly crude simulations of the real thing.

Whatever the degree of consensus on significant warming, there is even less agreement on its effects. Carbon dioxide is plant food, after all; more of it might enhance vegetative growth if a fast-changing climate doesn’t force species to move hundreds of miles on short notice. Drought may or may not occur: most greenhouse models actually predict more precipitation in most places, but higher temperatures might cancel that out by increasing evaporation and plant transpiration, thereby drying out the ground. But the historical record (and, to a lesser extent, the models) suggests that the actual temperatures do not rise equally at all times of day–nighttime lows are going up, while daytime highs stay about the same. If this proves to be the pattern of greenhouse warming, it won’t cause a drought in most places.

Revkin writes, “Many atmospheric scientists say we are literally taking a global gamble by modifying the atmosphere so significantly, and so quickly.” Balling on the other hand concludes, “We are already watching the greenhouse effect unfold before our eyes, and the observational evidence is not pointing to disaster.”

Those of us who don’t read equations over breakfast hear this scientific story through the media, and that’s a problem. Journalism is more the child of politics than of science. Politicians try to conceal disagreements within their own camp and emphasize disagreements with the other side, so any public fracas is likely to have at least two plausible sides. Scientists, on the other hand, do not waste time reiterating everything they agree on. They get right to the frontier subjects where disagreement is rampant and sometimes raucous. In these ways science is almost the exact opposite of politics.

Some believe that journalism’s traditional preoccupation with politics ill equips journalists to cover the subject of global warming. Journalists insist on looking for “both sides,” inadvertently focusing on details being debated, polarizing issues, and obscuring what may be a broad agreement on basics. Revkin, a journalist himself, objects that this evenhandedness “confuses and paralyzes the reader.”

But it’s not all the media’s fault. For one thing, public knowledge of even basic atmospheric science is abysmal; and most of us exaggerate or misperceive the significance of the occasional extreme season. In a recent survey, 95 of 100 Alabama professionals said that they believed their state has been getting warmer because of the greenhouse effect. In fact the state has been cooling significantly since 1935. Worse yet, believers and skeptics alike serve their own agendas by driving the politics of global warming to extremes, or “abortionizing” the debate. For environmentalists, the specter of a disastrous climate change is a powerful tool with which to persuade reluctant Americans to drive less and conserve more. Libertarians and conservatives portray the global warming scare as a Trojan horse being used to sneak in more counterproductive government controls on industry–and perhaps even to surreptitiously promote redistribution of wealth rather than economic growth. Some of them green-bait the environmentalists as they used to red-bait liberals, back in the good old days when more reds were available.

Fortunately for the global-warmers, journalists have another occupational bias that offsets their reflexive two-sidedness. They love disaster. Mayor Daley was not far off: media people didn’t really want the Great Chicago Flood to kill anyone, but with a catastrophe at hand they definitely had more interesting work to do (and more to moralize about). When Revkin visited local radio talk shows in May to promote his book, you could hear the relish in Roy Leonard’s verdict on Bush’s go-slow policy–“we ought to be ashamed of ourselves.” And Revkin himself, who scrupulously avoids scare tactics, had to pull Sondra Gair of WBEZ back from the brink when she asked if “the Industrial Revolution, which changed the whole world, is now responsible for the destruction of the world.” In short, for every journalist who conscientiously and ignorantly interviews a crackpot in order to get “both sides,” there is at least one other journalist happy to board the latest bandwagon and crack the whip on the horses.

Even without the media’s distorting glasses, the science of global warming is so complex it makes you want to change the subject. The highbrow way to do so is to liken global warming to another nasty-looking environmental crisis. “The natural world is full of surprises,” warns Revkin, citing the unexpected ozone hole over Antarctica as “a classic example of an unexpected, unpleasant twist in the dynamics of the atmosphere. . . . The lesson of CFCs [chlorofluorocarbons, which deplete the protective ozone layer] can be applied directly to the looming problem of greenhouse warming.”

This line of argument, however, is also open to criticism from those who remember other looming problems that quit looming once scientists took a close look. In his book Revkin maintains a discreet silence on acid rain, which many once believed would make thousands of lakes ten times more acidic in just ten years. Balling fills in the blank: “After 10 years of study, the involvement of over 2,000 scientists, and an expenditure of over $500 million, the [federally funded] NAPAP [National Acid Precipitation Assessment Program] conclusion was that an acid rain problem exists, but it is not as serious or urgent as many had feared.” Acidification is proceeding at most only one-fourth as fast as feared, and in fewer lakes; NAPAP’s director told the New York Times early in 1990 that acid rain was no longer a top-priority environmental issue.

So analogies just make a murky situation murkier. The more popular way to change the subject is just to go straight to the question of what to do. Environmentalists who foresee a global-warming disaster tend to propose their favorite policies as ways to slow it down: conserve resources, use less fossil fuel and more renewables, increase energy efficiency, build more mass transit, plant trees, and help less-developed countries develop along more efficient lines than just pouring on more coal and oil. Why should nonscientists break their brains trying to decode the degree of scientific consensus, since these all seem like good ideas anyway?

Unfortunately, this comfortable argument is bogus. None of these policies is an absolute good: they’re only good if their benefits exceed their costs. (You wouldn’t drive your gas-guzzler 20 miles to drop off one pop can at a recycling center.) Either global warming adds another reason to recycle, take the bus, etc, or it doesn’t. If it does, how much? How certain is it and how bad could it be? The same hard-to-answer questions are back.

Revkin complains about people who still think of energy conservation as a cost–“it’s an investment.” But that’s just the point. Every investment you make in x is one that, for the time being at least, you cannot make in y.

The point is not academic: not everything sold under the name of “energy conservation,” for instance, really is. Higher gas-mileage standards for cars, unlike higher gas taxes, may have the counterproductive effect of encouraging new-car owners to drive more than they would otherwise. And even though the U.S. is the largest single source of carbon dioxide, the cheapest place to cut CO emissions is in the developing third world–by improving the efficiency of, say, the Chinese economy so that it can grow without relying on inefficient coal combustion as much as the West did. Revkin and Balling actually agree on this, but Balling puts it more provocatively. He graphs carbon-dioxide emissions per $1,000 of gross national product rather than per capita. According to this standard, the least efficient nations by far are China and Poland; the greedy old U.S., while well behind France and Japan, comes in a bit better than the global average. “One way to reduce CO emissions substantially,” Balling concludes, “is to make much of the world as ‘efficient’ as the United States.”

Figuring out what to do about global warming is not really a matter of either science or politics: it’s insurance. Most of us buy health insurance, but we don’t buy a special policy against being struck by a meteorite. We want to know, how large is the risk? How bad would it be if it happened? The nice thing about insurance is that it doesn’t require certainty, which we don’t have and won’t get in time. But it does require some knowledge, not just dogmatic pronouncements: the greater the likelihood and the greater the severity, the more insurance. An insurance agent who tries to sell you a policy “just in case,” and ducks these basic questions, is looking after his or her commission, not your best interest.

Such an insurance agent is Greenpeace’s Jeremy Leggett, writing in a recent issue of the Bulletin of the Atomic Scientists. Since there is a worst-case global-warming scenario, he says, we must act on it. “In evaluating military threats throughout history, policy response has been predicated on a worst-case analysis. The standard military yardstick must also apply to environmental security.” But you can’t provide for the worst case of everything; you have to establish priorities, which means asking those same old questions, how bad? and how likely?

However, Leggett is getting at a fact that makes conservatives and libertarians very uncomfortable–namely that natural processes do not always have the same elasticity as human creations. We could rebuild the Wright brothers’ original biplane; we can’t rebuild the passenger pigeon. If there were reason to think that a delay in acting on global warming would set in motion an irreversible series of atmospheric events leading to a runaway greenhouse approaching that on Venus, then it would be dopey for us to sit around weighing costs and benefits–like Jack Benny’s inapt response to the mugger who demanded, “Your money or your life!” (Very long pause, then: “I’m thinking, I’m thinking.”)

So is this a reasonable fear? How soon should we buy our insurance policy? George Bush’s dishonest waffling on the environment, and corporate special pleading, have given proposals destined for “more study” a bad name. But there are three good reasons to think that the case for immediate action against global warming has not been made.

(1) The September 1991 report of a National Academy of Sciences committee concluded that the U.S. can adapt to predicted climate change without hurrying into drastic measures: “Like generals building a Maginot Line in the wrong place, we might bankrupt ourselves building dikes against floods that never come.” (Environmentalists, who lean heavily on NAS authority in other circumstances, rarely mention this report.)

(2) More persuasively, the models themselves do not convey great urgency. For purposes of argument Balling cites the “consensus” forecasts of the Intergovernmental Panel on Climate Change (which are, of course, subject to all the uncertainties of the models on which they are based). If we do little about global warming–what IPCC calls the “business-as-usual scenario”–then the earth’s average temperature is predicted to rise about 4.1 degrees Fahrenheit by 2040. If instead the world moves away from coal, makes large improvements in energy efficiency, reverses deforestation, and strictly controls carbon monoxide and CFCs–“scenario B”–then the temperature in 2040 will still go up about 3.4 degrees. That’s a savings of just seven-tenths of a degree. Since climate naturally varies more than that, writes Balling, “we may never be able to detect the temperature response of our policies.”

(3) Along the same lines, last year University of Illinois climate modelers Michael Schlesinger and Xingjian Jiang described one of their model runs in Nature. It showed that even after a ten-year delay in action, we could still reduce projected temperatures as of 2100 by 95 percent as much as we could by starting now. They concluded:

“To us this small penalty does not indicate that we should ‘wait and see’ and do nothing during this decade–quite the contrary. The study of the greenhouse effect, both theoretically and observationally, should be accelerated into a “crash programme’ so that we do not squander the time that nature has given us to obtain a realistic understanding of the climate response to increasing concentrations of greenhouse gases.”

Global Warming: Understanding the Forecast by Andrew Revkin, Abbeville, $29.95.

The Heated Debate: Greenhouse Predictions Versus Climate Reality by Robert C. Balling Jr., Pacific Research Institute, $21.95.

Art accompanying story in printed newspaper (not available in this archive): illustration/Lisa Michniuk.