In 1970, headbands were in, Simon & Garfunkel topped the charts, and Denis Hayes, a graduate student at Harvard, read about a fledgling environmental movement. Determined to volunteer, he flew to Washington, DC, with the intention of organizing Earth Day at Harvard. Instead, he became the organizer for Earth Day across the entire United States.

Inspired by the role of teach-ins in anti-war and civil rights protests, US Senator Gaylord Nelson started the Earth Day movement as a “national teach-in on the environment.” The movement swiftly gained popularity, and today Earth Day is celebrated in more than 190 countries.

We sat down with Denis Hayes to discuss the first 49 years of Earth Day, the evolution of the movement, and how energy—and efficiency—play a huge role in climate action and a sustainable future.

What was it like to be the national coordinator of the first Earth Day?

It was like riding a Tesla with the pedal to the floor. The acceleration was from zero to 1,000 in a matter of months. Early community meetings often had fewer than a dozen people. But April 22, we had 20 million participants, with more than one million in Manhattan alone!

My hope—in some large measure realized—was that Americans would come to see that they had a “right” to a clean, healthy environment, and that the law would guarantee that. The first Earth Day was passionate and intense, and it led directly to a wave of change. Clean Air Act. Clean Water Act. Endangered Species Act. Marine Mammal Protection Act. EPA and NOAA. Superfund. On and on.

What role have energy and energy efficiency played in Earth Day over the past 49 years?

Energy has been at the core of much of what Earth Day has sought to achieve over the decades. (The Santa Barbara oil spill of 1969 was one of the fuses that set off Earth Day.) By seeking to internalize the real costs of acid rain in the price of electricity and steel, through CAFE (Corporate Average Fuel Economy) regulations of automobile fuel efficiency, by promotion of LED lights and super-efficient refrigerators and high-efficiency heat pumps, etc., we’ve tried to envision a comfortable life requiring vastly less energy. We have made clear that a healthy environment does not involve living in caves and reading by candle light.