It is now a century since Syante Arrhenius published the idea: As human activity puts ever more carbon dioxide into the atmosphere, global warming becomes ever more likely. (See figure 1 and the box on page 36.) His paper attracted notice, and one might suppose that knowledge of the so‐called “greenhouse effect” has grown steadily ever since. But that is not in fact how the science proceeded. During more than half a century after 1896 almost nothing of value was learned about global warming. Only in the late 1950s did scientists at last begin to regard it as a serious possibility, indeed a potential danger.