Gaylord Nelson, then a Wisconsin U.S. senator harnessed mounting public concern to spearhead that first Earth Day. Alice Thompson was in 9th grade when she found out Earth Day was on the horizon.

BUFFALO, N.Y. — The first Earth Day in 1970 was a “youthquake.” It wasn’t one event. It was at least 12,000, and most were organized by teenagers and twenty-somethings, says Adam Rome, a University at ...