Boxer was flanked by other leading California Democrats, including Sen. Dianne Feinstein, Rep. Nancy Pelosi of San Francisco and Rep. Henry Waxman of Los Angeles during the 75-minute event at the Dirksen Senate office building in Washington.
Boxer identified the worsening droughts and fires in California and storms and floods in other parts of the country as evidence of a crisis.
"Climate change is happening all around us and we need to take action now," she said.
"The first thing we need to do is stop the denial."
But underscoring the partisan divide over the issue, no Republicans were present. Democrats from states that produce coal, oil and natural gas were also absent.
Other California Democrats who spoke at the rally included Reps. Lois Capps of Santa Barbara, Alan Lowenthal of Long Beach, Scott Peters of San Diego, Barbara Lee of Oakland, Mike Honda of San Jose and Jerry McNerney of Antioch.
Boxer repeated her call for a tax on carbon emissions. Virtually all climate scientists agree that carbon dioxide emissions from burning fossil fuels is contributing to the warming of the planet.
"The scientists tell us it's unequivocal that climate change is taking place," said Waxman, who's retiring from Congress this year after four decades.
But most Republicans and some Democrats oppose taxes or caps on carbon emissions because they worry about the economic impacts. Others have gone further, calling climate change a "hoax."
Waxman accused skeptics of putting their heads in the sand. "They have nothing to propose, because they don't want to deal with this issue," he said.
Feinstein said that failure to limit global warming would be "catastrophic" for future generations.
"It's going to come much sooner than we think," she said.
Pelosi called climate change a threat to national security and the economy, but also noted that leading evangelical Christians have taken up the cause.
"It is a moral issue," she said.
PHOTO: U.S. Sen. Barbara Boxer, D-Calif., addresses reporters during a news conference on Capitol Hill in Washington on Sept. 14, 2012. Associated Press Photo/ Manuel Balce Ceneta.