CONCORD, N.H.—Democratic presidential candidate Sen. Bernie Sanders was endorsed by Friends of the Earth, a progressive political action committee fighting climate change, during a campaign stop in New Hampshire today.

“I am very honored to receive the endorsement of one of the great environmental organizations not only in America, but in the world,” Sanders, I-Vt., said during the event.

“Climate change is the greatest threat facing the planet,” Sanders said.

He added that “the debate is over.”

“The scientific community is virtually unanimous: climate change is real,” Sanders said. “Climate change is caused by human activity.”

He said he is opposed to a so-called “all of the above” energy strategy, and the nation must transition to use only renewable sources of energy.

“We need to transform our energy system away from fossil fuel and into sustainable energy,” Sanders said.

He said that climate change is not just an environmental issue, but an economic and national security one as well.

“Our task is clear,” Sanders said. “We must dramatically reduce greenhouse gas emissions.”

He called for programs to allow more American families to be able to afford to put solar panels on their houses, and to help low-income people “retrofit” their homes to make them more energy-efficient.

He also called for a carbon tax.

Sanders said he remains “vigorously opposed” to Keystone XL, a proposed oil pipeline that would carry oil from Canada to Nebraska, where it would meet with existing pipelines that would carry it to refineries on the Gulf Coast.

“If you’re concerned about climate change, say no to the Keystone XL pipeline,” Sanders said.

Sanders called himself “perhaps the most progressive member of Congress” on the issue of climate change.