 2.2 degrees Fahrenheit since pre-industrial times, and the planet is poised to race past the hoped-for limit of 1.5 degrees Celsius of warming. But fewer people know that burning fossil fuels doesn't just cause global warming, it also causes global cooling. It is one of the great ironies of climate change that air pollution, which has killed tens of millions, has also curbed some of the worst effects of a warming planet.