California Gov. Jerry Brown had a song stuck in his head, and he wanted to hear it.
Standing outside a conference center shortly after giving a speech on climate change, he asked for someone to play it on their phone. And when the refrain arrived, he sang along to the Barry McGuire tune from 1965 -- "We're on the eve of destruction."
"We have a theme song!" said Nancy McFadden, the governor's top aide.
It was a fitting anthem for Brown's trip to the U.N. summit on climate change, a trip that revolved around the governor's favorite topic -- dire threats to life as we know it.
Brown's swing through Paris reflected not only the environmentalism that could be his political legacy, but also his preoccupation with sources of potential catastrophe.
In conversations about climate change, Brown has vividly described a hellish landscape with powerful wildfires, tropical diseases, forests devastated by beetle infestations and mass migrations of people displaced by rising temperatures.
Occasionally he would veer into warnings about nuclear weapons and the possibility that terrorists could "decapitate any government."
It's an unusual focus not just for a state leader, but for any American politician.
"Normal politics is much more focused on satisfying people's immediate needs," said Jon Christensen, a UCLA senior researcher who moderated a panel on "the nuclear menace" with the governor last year.
Brown, however, feels it's "the job of political leaders to make us think about these threats," Christensen said.
It's a point the governor made himself at his final public appearance, when he spoke to graduate students at a French university, before leaving Paris on Thursday.
"We have to be able to imagine the horrors that might unfold, and then take steps to prevent it," he said.
In Brown's telling, the enormity of the problem is matched only by the immensity of the changes necessary to deal with it.
The governor has pushed for slashing oil consumption and boosting renewable energy in California. But he went further in his comments in Paris, calling for nothing less than a radical transformation of life on Earth.
"Instead of being a burden, it's really an opportunity to live lighter on the planet," he said during one event. "Friendship, beauty, art has to take the place of this heavy commodification of our entire existence."
Dealing with climate change, Brown said, requires confronting modernity itself.
"Modernity is two things: Individualism and oil," he said. "That's who we are."
For the world to reduce its carbon output, he continued, "we're talking about a different kind of life, a life not based on oil, and a life not based on so much emphasis on the individual as opposed to the common good."
It's the type of conversation that Brown has been drawn to for decades. Before leaving for Paris, in a speech in Chicago, the governor said he is "kind of oppressed by the deeply superficial nature of most of my existence."
"Whenever I find something really important, existential ... I get very engrossed in that," he said. "That is the reason why I've become so interested in the environment."
Brown's focus on climate change is noteworthy for a 77-year-old politician with three years left in his final term, and no plans to run for another office.
"I think it's fair to say as you become more mature ... your ability to influence what comes after you is limited," said Mary Nichols, chair of the California Air Resources Board who also worked in Brown's first administration in the 1970s. "You want to make sure that everything you do is impactful."
The governor's interest in grave threats to humanity extends beyond the political and into the philosophical. He sometimes discusses the issues with Jean-Pierre Dupuy, a French philosopher he first met decades ago.
Dupuy recalled being invited to Brown's home after sending the governor one of his books, which Publishers Weekly said "warns of catastrophe if a re-examination of Western cultural traditions does not take place." By the time he arrived, Brown had highlighted passages and was ready to talk all day.
"Whenever I meet with Jerry Brown, he can talk for seven hours without feeling any need for food and drink," Dupuy said.
Dupuy helped arrange Brown's lecture on Wednesday night with the French graduate students.
While most of the governor's remarks were focused on California environmental policies, he didn't limit himself to climate change. The world, he said, should also be concerned about the proliferation of nuclear weapons.
With a small amount of nuclear material, Brown said, "terrorists could decapitate any government, or create a horror that would be a climate change, but a lot quicker."
The discussion recalled a remark Brown made in 1980 while running for president for the second time. Appearing on a radio show, journalist Joe Klein wrote in New York magazine, Brown told a caller that "if you asked me if I'd be surprised by a nuclear holocaust, the answer is no."
After his lecture Wednesday night, Brown sat for a discussion with Dupuy, as well as a physicist, a climate scientist and an economist.
The governor took notes as they addressed issues such as nuclear fusion and whether it's immoral to give people hope about a potentially hopeless situation.
The collection of academics reflected Brown's feeling that politicians can only do so much on their own to alter the world's trajectory on climate change.
"We're faced with a technical challenge. We're faced with a challenge in our beliefs, and a challenge in how we live," he said. "Politics is not in the business of changing things at that fundamental level."
Despite the heavy subject matter, the governor was in good spirits while driving to the airport Thursday.
"I've never seen so many people walking around talking about climate change," Brown said of the U.N. conference.
But out the window was further evidence of the steep challenges ahead -- bumper-to-bumper traffic of gas-burning cars heading into Paris.
"What we're doing relative to what needs to be done leaves open a lot of things to be figured out," Brown said.