After struggling with heart disease for decades, the renowned climate scientist Wallace Smith Broecker had made it clear he was acutely aware of his own mortality. So when he sat down in front of a video camera to record a final message to his fellow scientists in mid-February, the 87-year-old researcher knew his days were few.

The man who popularized the term “global warming” and first described the critical role oceans play on climate had an urgent message for 40 of the world’s top climate scientists. Humanity is not moving quickly enough to slow the production of carbon dioxide that is warming the Earth, Broecker said Feb. 11, his livestreaming image projected onto a big…