Police detectives aren’t the only people who look for fingerprints. Climatologists do, too: they’ve been looking for the collective fingerprint of humanity on Earth’s climate. Most of them suspect that the 6 billion tons of carbon we pump into the atmosphere each year, in the form of carbon dioxide, could warm the planet through the greenhouse effect. In the coming century the warming could be dramatic; but is it detectable already? This past year two teams of climate modelers said yes: man-made global warming is happening--almost certainly, anyway, and it’s getting more certain every year.
Certainty would be easier if it were just a matter of looking at the thermometer. We know that Earth has warmed by roughly a degree Fahrenheit in the past century, says Benjamin Santer, an atmospheric scientist at Lawrence Livermore National Laboratory in California, but you could have many different combinations of factors--volcanoes, the sun, carbon ...
