SLASHING SULFUR DIOXIDE pollution in the United States and Europe over the past decades counts as one of the biggest environmental success stories. Air pollution kills. The US Clean Air Act will add more than a year of life to the average American. But decreasing this pollution also comes with an important, unintended, and unexpected consequence: a warming planet. Aerosols, the fine particles in air pollution, reflect a small portion of sunlight back into space and, thus, actually help keep global warming in check. That’s true despite the carbon dioxide emissions coming from the same smokestacks that warm the planet. Global warming doubters now like to point to fears of global cooling in the 1970s as proof that climate scientists can’t get their story straight. That assertion is entirely wrong. Concerns in the 1970s—and predictions around global cooling—were correctly based on the cooling power of aerosol pollution.