Black carbon casts a deadly shadow worldwide, from the sprawl of Los Angeles, to the slums of Mumbai, to the Arctic ice that sustains polar bears and other wildlife. But quick action to cut black carbon can slow Arctic melting, fight global warming and save lives.

The chief culprit in global warming is carbon dioxide. But recent studies show that black carbon—microscopic airborne particles commonly known as soot—is also a big factor. According to a recent study, black carbon may account for as much as half of Arctic warming. Along with deep cuts in carbon dioxide, curbing black carbon is crucial for slowing Arctic and global warming, and for averting catastrophic tipping points such as the melting of sea ice and the Greenland ice sheet.