The evidence about global warming has become so clear and ominous that it's hard to believe anyone could question it. Yet many politicians and businessmen still argue that global warming is an unproved theory that can be explained away by statistics. In An Inconvenient Truth, former Vice President Al Gore addresses the issue in a straightforward, authoritative manner. Those on the left will appreciate the facts presented logically and forcefully; those on the right will accuse the film of distortions and evasions. Unfortunately, the people who need to see the film the most-those who haven't bothered to form an opinion on the subject yet-aren't likely to seek out what is the equivalent of a filmed lecture.

Gore is a polished speaker, and by his count has given his global warming speech over a thousand times. His familiarity with the material shows in his dry, dispassionate delivery. He raises his voice only once, while exhorting his audience to take political action; otherwise, his calm style can lull viewers into missing important points.

The speech's content can seem overly defensive. Gore spends most of the movie debunking junk science and defusing anticipated attacks from the right. Much of this material will be good ammunition for dinner-party debates, but Gore could have made better use of his time than preaching so relentlessly to the converted.

Gore and the filmmakers do make some attempts to personalize the material. He speaks about how his older sister, a lifelong smoker, died of lung cancer, and about how an accident to his six-year-old son changed his priorities as a politician. It's a welcome move that adds some of the sincerity and sensitivity that politicians often seem to lack.

Director Davis Guggenheim has tried hard to spice up the film visually, but it is difficult to disguise the fact that, no matter how gussied-up, this is basically a speech with slides (programmed in Keynote, Apple's version of PowerPoint). Occasional film footage of ecological disasters, a stray clip or two from the TV cartoon "Futurama," and candid shots of Gore traveling or at work fill out the visual side of the film.

An Inconvenient Truth delivers a depressing message. In fact, the film succeeds in presenting a case so damning that despair seems the only response. What can the average person do with the knowledge that glaciers in Nepal that supply drinking water to some 40 percent of the world's population are disappearing? The record-breaking temperatures, spread of communicable diseases, worsening storms, shrinking lakes, combined with political inaction or outright deception, can leave viewers feeling helpless. Gore resists offering any easy or obvious solutions, apart from what individuals can do to lessen their own contributions to environmental pollution.

Still, if you believe, as I do, that global warming is a critical problem, then An Inconvenient Truth could not arrive at a more important time. If it can help persuade even one person to take steps to effect a change, then Gore will have accomplished more good than his presidential opponent apparently ever will.

-Daniel Eagan