Mr. Hayes explained his principles of ecology, of rejecting unbridled growth that strips away the world’s resources, causes pollution and harms people. “The ecological freak is not questioning his share of the pie so much as he is questioning how we’re getting our flour,” he said. “The problem isn’t technological; the problem is a matter of values.”

These days, Mr. Hayes doesn’t use phrases like “ecological freak.” But the fire is still there.

“This was not an anti-litter campaign,” he recalled. “This was talking about fundamental changes in the nature of the American economy.” The cause that drew 20 million people into the streets was, he said, “in some ways much more profoundly radical” than the anti-Vietnam War movement.

Michael B. Gerrard, a Columbia University law professor and director of the school’s Sabin Center for Climate Change Law, said that “Earth Day was, to the environmental movement, like a caterpillar transforming into a butterfly and taking flight.” It changed lives, including his: He was a student at Columbia and covered the first Earth Day for the campus newspaper. He calls that day “one of the steps along the path that made me decide to make a career as an environmental lawyer.”

In the years after the first Earth Day, Mr. Hayes served as a senior fellow at the Worldwatch Institute in Washington, D.C., and President Jimmy Carter made him director of the Federal Solar Energy Research Institute in Colorado, where he promoted solar power from an institution with nearly 1,000 employees and a $130 million budget.

That was when he first heard about climate change, in discussions with scientists at the National Center for Atmospheric Research in Boulder. “I was asking, ‘How certain are you of all this?’ The answer was, ‘You’re never certain of anything,’ but that their level of certainty was at the 98 percent level.”

He began dovetailing concerns about climate change with his promotion of renewable energy. In January 1980, he delivered an address to the American Association for the Advancement of Science and said that the continued use of fossil fuels would lead to warming of Earth’s atmosphere. That would have “many adverse consequences for life as it now exists,” he warned, such as sea-level rise, inundated coastal farmland and communities, disrupted weather patterns and food production, among others.