The night before the day that would make him famous, James E. Hansen listened to a baseball game on the radio. But his mind kept wandering: What would he say to Congress the next day to convey that humans were endangering the planet?

He had long been trying to raise the alarm without success, and so had other scientists. But then, on June 23, 1988 — 30 years ago Saturday — a Colorado senator named Tim Wirth convened yet another hearing on the topic. Dr. Hansen was one of several scientists on the witness list.

Few people had ever heard of him, nor of the obscure NASA unit that he headed. He and a small group of colleagues studied the Earth’s climate, working in a suite of offices above the Manhattan diner that “Seinfeld” would later make famous.

He had conducted rigorous studies of historical temperatures, concluding that the planet was warming sharply. He had helped to pioneer computer modeling of the climate, and the results predicted further warming if people kept pouring greenhouse gases into the atmosphere.