“Kamala Harris is a courageous advocate for the people and the planet," said Ben Jealous, Sierra Club's executive director.
"She has worked for decades to combat the climate crisis and protect our health and future," he added.
Manish Bapna, president of NRDC Action Fund, agreed that the vice president was well-equipped to step into the top role and deal with the climate crisis.
"Harris grasps the urgency and scale of the challenge," Bapna said. "She'll advance the climate progress we've made at home and internationally. She'll raise climate ambition to make sure we confront the climate crisis in a way that makes the country more inclusive, more economically competitive, and more energy secure."
The joint statement followed a wave of endorsements from leading Democrats in the day and a half after Biden dropped out of the race and backed Harris. Evergreen Action, a climate advocacy group, also endorsed Harris.
The Sunrise Movement thanked Biden for stepping aside, after pushing him to do so. The group hasn't endorsed Harris but has, on social media, touted Harris' earlier climate proposals and initiatives, encouraging her to be as bold as she was on the issue in 2019 while running for president. That year, as a senator from California, she co-sponsored a Green New Deal bill pushed by Sen. Ed Markey (D-Mass.) and Rep. Alexandria Ocasio-Cortez (D-N.Y.), saying that climate change posed an "existential threat to our nation."
As a presidential candidate, Harris ran to the left of Biden on climate issues, calling for $10 trillion in climate investment, a carbon tax, and a ban on fracking and on new oil leases on federal lands. She even said that she would support eliminating the Senate filibuster rule in order to pass a Green New Deal.
And during a 2019 town hall on climate, Harris identified an underlying climate problem more squarely than many corporate Democrats are willing to do.
"On this issue, guys, as far as I'm concerned, it's not a question of debating the science," Harris said at the time. "It's a question of taking on powerful interests, taking on the polluters, understanding that they have a profit motive to pollute."
Yet that Harris candidacy, wedged awkwardly between corporate Democrats such as Biden and progressives such as Sen. Bernie Sanders (I-Vt.), flopped and she dropped out of the race in December 2019.
As vice president, Harris cast the tie-breaking vote for the Inflation Reduction Act, which Biden, in his momentous letter on Sunday, called "the most significant climate legislation in the history of the world." She also represented the U.S. at the COP28 climate change summit in Dubai last year, speaking in strong terms about the need for action.
"The urgency of this moment is clear," she said in Dubai. "The clock is no longer just ticking, it is banging. And we must make up for lost time."
"Around the world, there are those who seek to slow or stop our progress, leaders who deny climate science, delay climate action, and spread misinformation," she added. "In the face of their resistance and in the context of this moment, we must do more."
Longtime Harris observers in California commend her environmental record there. As district attorney of San Francisco, she established one of the nation's first environmental justice departments. She later pushed environmental measures as state attorney general and U.S. senator representing California: electrifying school buses, replacing lead water pipes, and strengthening vehicle emissions standards, for example.
As attorney general, she sued oil companies including Chevron, BP, and ConocoPhillips over pollution issues and took legal action against the Obama administration over fracking. Later, in the town hall event, she she said was proud to be a "fighter" who "took on the Big Oil companies—great, powerful interests."
Bloombergreported Sunday that Harris is "seen as [a] tougher oil industry opponent than Biden."
Though Harris no longer calls for a Green New Deal and has moderated her rhetoric as part of the Biden administration, she still offers a stark contrast to Republican nominee Donald Trump, whose administration rolled back over 100 climate policies from 2017 to 2021. The new Republican platform doesn't mention climate change and vows to "drill, baby, drill"—in all caps.