Al Gore will now find out whether his movie fans are a renewable resource.

The critically acclaimed if controversial documentary "An Inconvenient Truth" that Gore narrates opened May 24 to a stunning $70,332-per-screen average in four theaters. But last weekend, in 404 theaters, that average was down to a strong but not as passionate $4,732 per screen. Distributor Paramount Classics is testing how far "Truth" can go.

On Friday, the movie arrived in 100 more theaters in 40 cities. So far, it has taken in $7.3 million. That's small by "Da Vinci Code" standards, but it's huge for the art-house circuit it has played in up until now.

Gitesh Pandya of BoxOffice Guru.com calls the film a sleeper hit that he predicts will continue strongly as it expands into theaters. Pandya says the audience for the movie skews older, and adult ticket-buyers don't always rush out to a film on opening weekend.

Brandon Gray of BoxOffice Mojo.com says "Truth" could "double what it has" by the end of summer and cross into the top five documentaries of all time, surpassing "Super Size Me," "Mad Hot Ballroom" and "Winged Migration."

"It's by no means a smash," Gray says of "Truth's" numbers now. "But it's a solidly performing documentary."

Feature documentaries are a notoriously hard sell — none harder than one starring a former politician discussing a doomsday topic. Only two have become smash hits: Michael Moore's George W. Bush-skewering "Fahrenheit 9/11," which took in $119.2 million in 2004, and last summer's "March of the Penguins" ($77 million).

Few documentaries make more than a couple of million dollars in theaters before moving on to DVD and television. "Truth" already is the 10th-highest-grossing documentary of all time.

"These movies sometimes preach to the choir, but this film seems to be reaching beyond that because it's not divisive," says Tori Baker, executive director of Salt Lake Film Society."