I was persuaded of the reality of man-made global warming back in the mid-1970s, at much the same time and for much the same reasons that the idea first recommended itself to Al Gore. The notion of the artificial "greenhouse effect" is one of those intuitive, important-if-true ideas that appeal to science buffs. The hypothesis was not new, but in those days the first data were showing up to suggest a secular warming trend. I even remember realizing, or at least hearing, that the most noticeable effect would not be a general rise in surface temperatures, but changes in the mechanical behavior of the atmosphere. Weather patterns would be different. In some regions, global warming could even cause local cooling.

The most alarming prospects that global warming suggested to my liberal-arts-major mind turned out to be phantoms. For instance, there had been some early speculation that a runaway greenhouse effect might occur on Earth, as it had on Venus. The image of an oceanless Earth with a novel atmosphere seemed to chime with Revelation 21:1, 2 (as well as with Arthur C. Clarke's Against the Fall of Night, one of the first books I ever read). I have later learned, though, that Earth is just not close enough to the sun for that to happen. If Earth had a predominantly carbon dioxide atmosphere, as Venus does, the surface temperature would be around 130 degrees Fahrenheit, rather than its current 55 degrees or so. Earth would be a nasty place, but the oceans would not evaporate. (Incidentally, if all the ice on the surface of the Earth melted, the oceans would rise just 220 feet. The film Waterworld was not jut a flop; it was a badly researched flop.)

The real story about Earth's greenhouse effect is much more interesting. Earth's distance from the sun suggests a surface temperature well below freezing; the oceans should be glaciated, not boiling. The natural greenhouse effect stops that from happening. The remarkable point, though, is that the work the greenhouse effect has to do changes over time. The sun has been getting hotter over the course of the solar system's history, contrary to what you might expect. Nonetheless, the surface temperature of the planet has been remarkably stable.

The best-known explanation for this stability is the famous Gaia Hypothesis, proposed by James Lovelock and Lynn Margulis. This idea has occasioned much mystical nonsense, but the basic notion is probably true: the atmosphere interacts with the biosphere to moderate the greenhouse effect. Thus, the percentage of gases in the atmosphere that are connected with biology tends to vary over time. Methane, a very potent greenhouse gas, was once a major component, but today there is just a trace. Carbon dioxide is not the only greenhouse gas today, but it seems to be the chief regulator of the greenhouse effect. This means that, despite the occasional uptick, the amount of carbon dioxide in the atmosphere decreases in the long run, thereby balancing the increase in energy from the sun.

The problem with this, of course, is that plants breathe carbon dioxide. By the time mankind appeared 40,000 years ago, which is still "now" in geological terms, the carbon dioxide atmosphere had almost reached the minimum necessary to support higher plant life. The point where higher plant life becomes extinct is just a moment later: on the order of ten million years, or a bit over one-fifth of one percent of the age of the 4.5 billion-year-old planet. This is not to say that all life becomes extinct thereafter, or even all plant life. The grasses can do with much less carbon dioxide than can, say, oak trees. It does mean that the biosphere, at least on land, is about to become dramatically thinner.

The human race appeared at almost the last moment it could have done so. For some time, cosmologists have been talking about what are known as "anthropic coincidences," details in the laws of physics or the structure of the solar system that seem highly unlikely but without which the human race could not exist. There have been elaborate attempts to explain these things statistically, but I find them unpersuasive. The appearance of man at the sunset of biological history has something to do with the inner life of the world. There is something extraordinary about now.