I don't know whether "An Inconvenient Truth" is actually a movie. Yes, it's called a documentary, and of course it's not fiction, but it is also not a movie in any traditional sense. What it is is an illustrated lecture on global warming, given by an attractive, articulate teacher who knows how to use Powerpoint. He's interspersed the charts with filmed footage of crumbling ice floes, powerful hurricanes, melting glaciers and dying coral reefs - the canaries, as he says, in the coal mine of earth.

He's Al Gore, of course, once upon a time the next president of the United States, as he points out with understated wit, and he has given this lecture more than a thousand times, in communities all over the United States and around the world. The question is, whether his lecture is having any effect on America's national behavior. So far, it looks like the answer is no, which makes "An Inconvenient Truth" one of the most depressing experiences you're likely to have this year.

Does that mean you shouldn't watch it? Of course not; everyone should watch it, because if you know the facts and still continue to act as though you don't, you'll have to accept personal responsibility for your share of what will be the permanent degradation and even death of much of the earth. Americans are fortunate in still being able to exert great power over government policies; "An Inconvenient Truth" is the kind of factual support we need to do it. Over the years, Mr. Gore has refined his presentation and dealt with any possible ambiguities or misstatements that opponents might have pointed out. We can accept his film in its entirety because every statement and claim in it has been carefully checked, reviewed and approved by those in the field. And inconvenient or not, it is true from beginning to end.

Having said all that, this is not an easy piece of work to review as a film critic. It was directed by Davis Guggenheim, who's made any number of television shows, and he's brought in multiple cameras and allowed them to move around as Mr. Gore speaks. It's an infinitely better approach than simply filming the lecture, and it smooths out the long recitations of facts, figures and historic trends that comprise so much of the story. The graphs are well done and well-projected onscreen for us in the audience. And Mr. Gore, while totally without anything resembling charisma, is soft-spoken, unpedantic, and even self-deprecating when it's called for.

Nevertheless, the message is the focus of the film, and neither Mr. Gore nor the filmmaker let us forget it. We may walk out dazed by the numbers, but we've learned what he's trying to tell us, and that's the important thing. The rest is up to us.