(7 of 9)
Such changes may already be under way. Climatologists have noted an increase in mean global temperature of about 1 degrees F since the turn of the century -- within the range predicted if the greenhouse effect is on the rise. But, warns Roger Revelle, of the University of California at San Diego, "climate is a complicated thing, and the changes seen so far may be due to some other cause we don't yet understand." The absence of a clear-cut signal, however, does not disprove the theory. Scientists expect any excess greenhouse warming to be masked for quite some time by the enormous heat-absorbing capacity of the world's oceans, which have more than 40 times the absorptive capacity of the entire atmosphere.
"Right now," declares University of Chicago Atmospheric Scientist V. Ramanathan, "we've committed ourselves to a climatic warming of between one and three degrees Celsius ((1.8 degrees F to 5.4 degrees F)), but we haven't seen the effect." This extra heat, now trapped in the oceans, he says, should be released over the next 30 to 50 years -- unless, of course, an event like a big volcanic eruption counteracts it. Notes Ramanathan: "By the time we know our theory is correct, it will be too late to stop the heating that has already occurred." Schneider sees no need to wait. Says he: "The greenhouse effect is the least controversial theory in atmospheric science."
Maybe. But climate is governed by an array of forces that interact in dizzyingly complex ways. The atmosphere and oceans are only two major pieces of the puzzle. Also involved: changes in the earth's movements as it orbits the sun, polar ice caps, and the presence or absence of vegetable and animal life. "The feedbacks are enormously complicated," says Michael MacCracken, of the Lawrence Livermore National Laboratory in California. "It's like a Rube Goldberg machine in the sense of the number of things that interact in order to tip the world into fire or ice."
One of the most fundamental elements of the Rube Goldberg machine is the three astronomical cycles first described by Serbian Scientist Milutin Milankovitch in the 1920s. The swings, which involve long-term variations in the wobbling of the earth's axis, its tilt and the shape of its orbit around the sun, occur every 22,000, 41,000 and 100,000 years, respectively. Together they determine how much solar energy the earth receives and probably cause the earth's periodic major ice ages every 100,000 years or so, as well as shorter- term cold spells.
But Milankovitch cycles only scratch the surface of climatic change. Volcanoes, for example, send up veils of dust that reflect sunlight and act to cool the planet. Deserts, with their near white sands, also reflect sunlight, as do the polar ice caps. Tropical rain forests, however, have the opposite effect: their dark green foliage, like the dark blue of the ocean, absorbs solar radiation; both tend to warm the planet.