Temperatures in Nearly All Major U.S. Cities Have Warmed Since First Earth Day

After more than half a century of Earth Days, one planetary challenge—climate change—threatens our planet more than ever. In 1970, the year Sen. Gaylord Nelson, D-Wisc., organized the first Earth Day events, the annual average concentration of carbon dioxide in the atmosphere was 326 parts per million. In 2025, it was 31% higher, at 427 parts per million.