Seats are still available on locally chartered buses to what’s billed as the largest climate rally in history.

Thousands from across the country are expected to descend on Washington, D.C., this Sunday, to call on President Obama to back up his strong language from his inaugural address, when he said, “We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations.”

The Forward on Climate Rally comes at a crucial time: Obama is expected to decide soon whether to permit the Keystone XL pipeline, a conduit for Canada’s dirty tar-sands oil. One climate expert has said the pipeline would mean “game over” for the climate.

The rally is organized by the national Sierra Club, climate activist group 350.org and the Hip-Hop Caucus.

On Wednesday, 48 people including veteran civil-rights leader Julian Bond and actress Darryl Hannah were arrested in front of the White House in a peaceful protest against the Keystone XL pipeline. Among them were Sierra Club executive director Michael Brune and Sierra Club President Allison Chin; it was the first civil disobedience in the group’s 120-year history.

Recognizing that Congress is unlikely to curb greenhouse gasses, environmentalists are also calling on Obama to use executive authority (though the EPA and Clean Air Act) to reduce carbon emissions.

Scientists point to rising incidence of droughts, flooding, wildfires and other extreme weather as evidence of the havoc rising concentrations of such emissions already have wrought. And that’s after less than two degrees Fahrenheit of global warming.

“I just think climate change is the No. 1 issue on the planet,” says Allegheny Sierra Club President Barb Grover. “If we don’t do something about this, our planet is going to end up ininhabitable.”

Grover is among more than 100 Pittsburghers already booked to hit D.C. for the rally. It’s her first environmental protest there. At age 72, she says she’s doing it out of concern for the world her children and grandchildren will inhabit.

The Allegheny Sierra Club has two buses booked for D.C. About 15 seats remain. The bus leaves at 7 a.m. Sunday from Edgewood Town Center and returns that night, after the march and rally. Seats are $40 ($25 for students). Register here.

Ed Perry, a State College-based climate-change campaigner for the National Wildlife Federation, tells CP that buses are booked from towns statewide, carrying some 1,000 protestors from Pennsylvania to the rally. The Allegheny Sierra Club’s Peter Wray says other buses locally are leaving from Chatham, Pitt and Carnegie Mellon.