Washington

TODAY "An Inconvenient Truth," Al Gore's movie about the greenhouse effect, opens in New York and California. Many who already believe global warming is a menace will flock to the film; many who scoff at the notion will opt for Tom Cruise or Tom Hanks. But has anything happened in recent years that should cause a reasonable person to switch sides in the global-warming debate?

Yes: the science has changed from ambiguous to near-unanimous. As an environmental commentator, I have a long record of opposing alarmism. But based on the data I'm now switching sides regarding global warming, from skeptic to convert.

Once global-warming science was too uncertain to form the basis of policy decisions — and this was hardly just the contention of oil executives. "There is no evidence yet" of dangerous climate change, a National Academy of Sciences report said in 1991. A 1992 survey of the American Geophysical Union and the American Meteorological Society found that only 17 percent of members believed there was sufficient grounds to declare an artificial greenhouse effect in progress. In 1993 Thomas Karl, director of the National Climatic Data Center, said there existed "a great range of uncertainty" regarding whether the world is warming. Clearly, the question called for more research.

That research is now in, and it shows a strong scientific consensus that an artificially warming world is a real phenomenon posing real danger:

The American Geophysical Union and American Meteorological Society in 2003 both declared that signs of global warming had become compelling.