While water vapor probably plays a bigger greenhouse role than carbon dioxide, it's not something we can control directly -- the sun evaporates the oceans, and they form clouds, and what are we, mere mortals, to do?

On the other hand, we -- particularly profligate North Americans -- produce astonishing amounts of carbon dioxide by burning forests and fossil fuels. In 1995, the average annual per capita carbon dioxide emission was 19.6 tons for the United States, compared to a world average of 3.9 metric tons (see "Climate Treaty Talks... " in the bibliography).

And since energy use is rising, despite the agreement at the Kyoto Summit to reduce emissions to 5 percent below 1990 levels by 2010, so are carbon dioxide emissions.

The effect of all this combustion is to inexorably raise levels of carbon dioxide, the primary human-made greenhouse gas. What will the constant increase in carbon dioxide mean for our climate? Since nobody can predict with certainty, climatologists must base their forecasts on analyses of past climates and computer models that account for the factors that affect climate: sunlight, clouds, ocean circulation, and chemical content of the atmosphere, to name a few.

Climatologists would love another tool for forecasting climate, but none is available, says Jonathan Foley, a climatologist at the University of Wisconsin-Madison. "Climate science is either observational or theoretical, using computer models," he says. "We can't rebuild Earth in the lab and do stuff to it."

Although computer models cannot simulate the true complexity of climate, they are the basis for a gathering outcry over global warming. The authoritative word comes from the United Nations'sIntergovernmental Panel on Climate Change which offers a "mid-range" estimate that average global temperature will rise at 2 degrees C (nearly 4 degrees F), by year 2100, with an expected range of 1 degree C to 3.5 degrees C.

If 2 degrees C doesn't sound like much, it's roughly one-quarter the rise in temperature since the last ice age -- when kilotons of ice created the moraine where Why Files offices now overlook a lake. As the warming to date seems to indicate, the change could occur quite rapidly.

While computer models have faced their share of criticism, they did correctly forecast the slight cooling trend caused by the explosion of Mt. Pinatubo in 1992; high levels of sulfur dioxide in the upper atmosphere did, as predicted, cool the planet for a couple of years. Since the sulfur dioxide dissipated, the warming trend has renewed.

These truths we hold self-evident 
The greenhouse effect is a good thing. Lacking it, we'd be deep in ice, with an average temperature radically below today's 14 degrees C global average, and life as we know it would just not work. So let's give a half-hearted cheer for the greenhouse effect, and a whole-hearted effort to understand its essence -- in two breezy paragraphs.

Global satellite map. NASA Goddard Space Center.

The greenhouse effect starts with light, and light's characteristics are determined by its wavelength -- the distance between adjacent waves. The wavelength of light emitted by an object is determined, in turn, by its temperature -- hotter objects create shorter wavelength, more energetic light. Thus the sun emits a huge amount of energy in the visible and ultra-violet regions. The Earth absorbs the solar radiation that reaches its surface, then reradiates some of that energy back to space. Because earth is much cooler than the sun, this radiation is relatively long-wavelength infra-red light.

Still with us? We're halfway there... While the sun's incoming radiation can pierce the atmosphere, much of the longer-wavelength infra-red bouncing back toward space is trapped by "greenhouse gases" in the atmosphere, particularly water vapor, methane, and carbon dioxide. They're called greenhouse gases simply because they act like the glass of a greenhouse -- they allow solar energy to enter, but prevent it from escaping.