In his best-selling book, Earth in the Balance, Vice President A1 Gore speculates that global warming caused by the greenhouse effect will throw "the whole global climate system…out of whack," dramatically reducing rainfall in parts of the world already troubled by drought, melting the polar icecaps, raising ocean levels, and devastating low-lying countries such as Bangladesh, India, Pakistan, Egypt, Indonesia, Thailand, and China. "In the lifetimes of people now living, we may experience a 'year without winter,'" Gore writes. "We are carelessly initiating climate changes that could well last for hundreds or even thousands of years."
Do such predictions have a basis in reality? Central to this question is the greenhouse effect. Contrary to the impression given by the mainstream media, the greenhouse effect is not of recent origin; it has been around for billions of years. But the link between this indisputable phenomenon and Gore's doomsday scenarios is tenuous at best. To understand why, you have to know something about the mechanics of the greenhouse effect.
The sun bathes the earth in sunshine. Some of the sunshine is reflected straight back into space, either by clouds or by the earth's surface. The remainder is absorbed by the earth's surface and thus heats it. Predictions about global warming hinge on the question of where this heat goes. Answering this question requires a brief trip into the world of electromagnetic radiation—light in all its forms, both visible and invisible.
All bodies radiate heat. A simple physical law says that the cooler a body is, the longer the wavelength at which it radiates its heat. The hot plate of a stove, for example, glows an orange-red when it's running at full blast. But turn it down, and as it cools it radiates its heat at longer and longer wavelengths, until it cools to the point where it's radiating exclusively in the infrared. To the eye it now appears to be off, although it is still too hot to touch. Even everyday objects at everyday temperatures are busy radiators of heat in the infrared. A block of ice at melting point, for instance, has a temperature of 273 degrees Kelvin and is a raging furnace compared to a block of ice at absolute zero. (The Kelvin temperature scale is the same as the Celsius scale, except its zero is absolute zero, instead of the freezing point of water.) The earth itself continually radiates heat from both its day-side and its night-side. The balance between sunshine's heating effect and the cooling effect of the radiation the earth pours back into space allows the planet's surface to maintain a roughly constant temperature (apart from diurnal and seasonal variations).
But the earth's surface, with an average temperature of 288 degrees Kelvin, is much cooler than the sun's, with a temperature of 5,800 degrees Kelvin. So while the sun pumps most of its heat into space in the form of user-friendly visible light, the earth returns this heat to space in the form of much-longer–wavelength, invisible, infrared radiation. A greenhouse gas is a gas that is transparent at visible wavelengths but opaque at infrared wavelengths. It thus admits sunshine but blocks the escape of the earth's infrared radiation, thereby warming the planet's surface. As a rule, gases whose molecules have three or more atoms, such as carbon dioxide, are greenhouse gases, while gases whose molecules have only two atoms, such as oxygen, are not.
Among the greenhouse gases, carbon dioxide gets the lion's share of attention because its concentration is increasing, largely due to industrial activity. But it is actually a minor player. If the concentration of carbon dioxide in the atmosphere doubled, the blocking of the earth's infrared radiation would rise from 150 to 154 watts per square meter, an increase of roughly 3 percent. This means that the increasing level of carbon dioxide in the atmosphere is not matched by a corresponding increase in the greenhouse effect.
Over the last 100 years, for example, the level of carbon dioxide has increased by 25 percent, while the greenhouse effect has increased by around 1 percent. (This 1-percent figure assumes that other things have stayed equal in the meantime, but in the real world "other things" are usually not so obliging, so the actual behavior of the greenhouse effect during this time is unknown. Nonetheless, its variation has been much closer to 1 percent than to 25 percent.)
Ordinary water vapor is actually the main contributor to the greenhouse effect. The balance between the natural processes of evaporation, which pumps water vapor into the atmosphere, and condensation into clouds, which squeezes it out, sets the level of water vapor in the atmosphere.
This means that the greenhouse effect and global warming are an integral part of the biosphere. They have been around at least since the formation of the first oceans and must therefore have preceded mankind's appearance by a few billion years. Indeed, if there were no global warming, if the earth's atmosphere were perfectly transparent at infrared wave-lengths, the planet's average surface temperature would be a brisk zero degrees Fahrenheit, instead of the pleasant 59 degrees that we enjoy. Global warming has been an essential ingredient in the evolution of life on the earth.
Yet the illusion that carbon dioxide is the dominant greenhouse gas is extremely widespread. In an impromptu, totally nonscientific survey, I asked 10 of my fellow astronomers, "What is the major greenhouse gas?" Six said carbon dioxide. One added, "But isn't water vapor in there?" Two said water vapor. And one said, "Don't know."
Evidently a surprisingly large number of astronomers think that carbon dioxide is the major greenhouse gas, despite the fact that astronomers need to know how the earth's atmosphere stamps its spectral imprint on the radiation from heavenly bodies and what gases are responsible. In scientific disciplines that do not deal with the earth's atmosphere on a professional basis, the illusion that carbon dioxide is the major greenhouse gas is probably even more prevalent. Among the general public it must be well-nigh universal.
But even if carbon dioxide is a minor greenhouse gas, its level is increasing. Doesn't this mean global warming is increasing? Yes. But the real question is at what level, and is it significant compared to the changes in global warming that take place independent of mankind's activities? Will it cause an 8-degree increase during the next century, as predicted in the most alarming scenarios, or will there be a much more gradual, and mostly beneficial, increase of 1 degree or so?
We cannot answer this question by means of mere calculation, because our theoretical understanding of the biosphere is too incomplete. The immediate result of increased carbon dioxide is, indeed, an increase of global warming. The slightly higher average temperature leads to increased evaporation from the oceans, which leads to a further increase in global warming because water vapor is also a greenhouse gas. But this is far from the end of the story.
More water vapor in the atmosphere leads to increased cloudiness over the earth as a whole. This means more sunshine is reflected straight back into space and so never reaches the earth's surface. This, in turn, means less heating of the earth's surface and hence lower temperatures.
We don't know which one of these opposing mechanisms wins, so we don't know if the increase in the greenhouse effect is amplified or dampened by the time it feeds through to global warming. The availability of faster computers promises that during the next decade we will be able to get a better grip on these factors and many other, more complicated ones that are currently neglected.
In the meantime, the observational evidence that global warming is actually increasing is very shaky. Some researchers claim to see an increase in global warming of about 0.8 degree since 1860. Although average temperatures since then have increased by this much, it's doubtful that the rise reflects carbon-dioxide–induced global warming.
The large year-to-year fluctuations of average temperature—which are in the neighborhood of 1 degree—mean that the behavior of average global temperature is somewhat like that of a stock market index. Spotting a "real" temperature trend is like deciding if one is in a bull or a bear market. It's not impossible, but no bell rings when one trend ends and a new one begins.
Moreover, natural causes, rather than the increase in carbon dioxide, are a more likely explanation of the temperature increase. Most of the rise took place prior to 1940, before the main increase in the carbon-dioxide level.
Some climatologists interpret this pre-1940 temperature increase as an after-effect of the so-called Little Ice Age, a period of unusual worldwide cold that prevailed from 1600 to 1850. If we look at the 50 years or so from 1940 to the present, which have seen the major part of the increase in carbon-dioxide concentration, the increase in average global temperatures has been only 0.2 degree. This small increment is within the noise of natural variation. Although the level of carbon dioxide in the atmosphere is increasing, it does not seem to be affecting global temperatures much.
In examining the historical record, we also have to consider the urban heat island effect. The buildings and pavement of a city give it a microclimate slightly warmer than that of the surrounding countryside. As cities have grown, their heat islands have grown with them, so their weather stations, which tend to be in downtown locations, have been more and more prejudiced in favor of higher temperatures. Phoenix is a dramatic example. Between 1960 and 1990, as its population grew from 650,000 to 2.1 million, its mean annual temperature heated up by 5 degrees, almost in lockstep with the population increase.
Climatologists who have tried to quantify the urban heat island's influence on the global temperature record estimate that it accounts for somewhere in the neighborhood of 0.2 degree of the 0.8-degree increase seen since 1860. And when Kirby Hanson, Thomas Karl, and George Maul of the National Oceanic and Atmospheric Administration conducted a study of the U.S. temperature record that took into account the urban heat island effect, they found no long-term warming. They confirmed the temperature rise prior to 1940 but found that temperatures have fallen since then.
Another consideration is that most of the earth's surface is covered with water. Temperature data over the oceans is extremely sparse, so the record of the earth's historic average temperature over both land and water is much vaguer than the land record. The recent advent of satellites with the capability to measure global temperatures accurately over both land and sea may solve the problem, but so far their time base is limited to a few years. However, global measurements by satellite of atmospheric (as distinct from surface) temperatures over the last decade show no sign of increasing temperatures.
Far more than historical evidence, the hullabaloo about global warming is based on predictions by computer models. The global climate modeler gives his computer some basic facts, plus a program that recognizes the relevant physical processes and principles insofar as we know them and insofar as they can be calculated. With luck, the computer arrives at a climate not unlike that of the earth. Then the model gets a retroactive "tuning," so that its average global temperature is right.
A calculation of the greenhouse effect and associated global warming is one step in the procedure. Assuming a doubling of atmospheric carbon dioxide, these models predict an increase in global warming of somewhere between 3 degrees and 8 degrees during the next century.
When they are judged by their verifiable accomplishments, however, these computer models are not very impressive. They predict that the temperatures at the poles are lower than those at the equator, and they predict that it's hotter in summer than in winter. But they are weak on specifics. One model predicts an annual rainfall in the central Sahara that is the same as Ireland's.
Clearly, these global climate models are still in a primitive stage of development. They neglect many important factors—both known, such as the poleward transport of heat from the equatorial regions by ocean currents and the atmosphere, and unknown. When it comes to telling us things we don't know already, such as trends in global warming during the next century, they are not far removed from the crystal-ball school of climatology.
Alarmists such as the vice president are impatient with people who point this out. Gore writes: "If, when the remaining unknowns about the environmental challenge enter the public debate, they are presented as signs that the crisis may not be real after all, it undermines the effort to build a solid base of support for the difficult actions we must soon take….The insistence on complete certainty about the full details of global warming—the most serious threat that we have ever faced—is actually an effort to avoid facing the awful, uncomfortable truth: that we must act boldly, decisively, comprehensively, and quickly, even before we know every last detail of the crisis."
But one of the "details" we still don't know is whether we are in fact facing a crisis requiring drastic action. The burden of proof is on the alarmists. They have failed to meet it.
Jocelyn Tomkin is an astronomer at the University of Texas, Austin.