The ozone layer is showing its first sign of recovery. Stratospheric ozone, which shields the earth’s surface from much of the sun’s ultraviolet radiation, has been depleted by man-made compounds used in a variety of products, such as refrigerators and insulation. A recent UN report credits the 1987 Montreal Protocol, which banned or phased out these ozone-depleting chemicals, for this turnaround. Under full compliance, the ozone layer is projected to recover to 1980 benchmark conditions by mid-century in most places, albeit somewhat later in Antarctica. The Protocol has yielded significant environmental and public health benefits: reducing greenhouse gas emissions from ozone-depleting substances by over 90 percent, preventing skin cancer by a projected two million avoided cases per year by 2030, and averting damage to both ecosystems and human eyes and immune systems. The hydroflourocarbons (HFCs) that frequently replace ozone-depleting substances, however, are a potent greenhouse gas, and their growing use threatens to undermine the climate gains. The ozone experience underscores the promise of, and need for, coordinated global action on climate change as the Climate Summit in New York and negotiations in Paris approach.