Not so long ago, people talked about global warming in apocalyptic terms--imagining the Statue of Liberty up to its chin in water or an onslaught of tropical diseases in Oslo. Recently, however, advances in our understanding of climate have moved global warming from a subject for a summer disaster movie to a serious but manageable scientific and policy issue.
Here's what we know. Since sunlight is always falling on the earth, the laws of physics decree that the planet has to radiate the same amount of energy back into space to keep the books balanced. The earth does this by sending infrared radiation out through the atmosphere, where an array of molecules (the best known is carbon dioxide) form a kind of blanket, holding outgoing radiation for a while and warming the surface. The molecules are similar to the glass in a greenhouse, which is why the warming process is called the greenhouse effect.
The greenhouse effect is nothing new; it has been operating ever since the earth formed. Without it, the surface of the globe would be a frigid -20[degrees]C (-4[degrees]F), the oceans would have frozen, and no life would have developed. So the issue we face in the next millennium is not whether there will be a greenhouse effect, but whether humans, by burning fossil fuels, are adding enough carbon dioxide to the atmosphere to change it (and our climate) in significant ways.
You might think that, knowing what causes greenhouse warming, it would be an easy matter to predict how hot the world will be in the next century. Unfortunately, things aren't that simple. The world is a complex place, and reducing it to the climatologist's tool of choice--the computer model--isn't easy. Around almost every statement in the greenhouse debate is a penumbra of uncertainty that results from our current inability to capture the full complexity of the planet in our models.
There is one fact, though, that everyone agrees on: the amount of carbon dioxide in the atmosphere is increasing steadily. It is near 360 parts per million today, vs. 315 p.p.m. in 1958 (when modern measurements started) and 270 p.p.m. in preindustrial times (as measured by air bubbles trapped in the Greenland ice sheet).
An analysis of admittedly spotty temperature records indicates that the world's average temperature has gone up about 0.5[degree]C (1[degree]F) in the past century, with the '90s being the hottest decade in recent history. This fact is quoted widely in the scientific community, although there are nagging doubts even among researchers. Recent satellite records, using different kinds of instrumentation, fail to show a warming trend.