Nearly 50 years ago, three chemists named Mario Molina, Sherwood Rowland and Paul Crutzen found evidence that chlorofluorocarbons, chemicals known as CFCs and released from aerosol sprays, were weakening the ozone layer that functions as the earth’s natural sunscreen protecting humans, animals and plants from harmful radiation.

The discovery made big news and rattled the public. Aerosol sales dropped dramatically, and, despite pushback from the chemical companies that made CFCs, Congress in 1977 added protecting the ozone layer to the Environmental Protection Agency’s duties under the Clean Air Act. Not long afterward, the agency determined that the compounds, then widely used in refrigerators, air-conditioners and some industrial processes, posed an even graver threat to the atmosphere than first thought. Soon after, pressure began to build for a phaseout of CFCs in the United States as well as for an international treaty to find alternatives.

The case for global action became ever more urgent in 1985 when a British team discovered a hole in the ozone layer above Antarctica, followed by confirmation by NASA scientists of a connection between the hole and CFCs. With the rest of the world and even industry on board, the result was the 1987 Montreal Protocol, a landmark agreement banning chlorofluorocarbons and other ozone-depleting chemicals. End of story? Not quite. As it happened, the ozone-friendly replacements for the CFCs, known as hydrofluorocarbons, turned out to be distinctly unfriendly to the climate. So in 2016, the Montreal signatories reconvened in Kigali, Rwanda, and agreed to amend the original protocol to phase out HFCs and find substitutes more friendly to the atmosphere.

The bottom line is that the world, confronted with two dire threats to the earth’s fragile atmosphere, found two planetary responses with positive outcomes. The ozone layer is healing. That’s worth remembering as we struggle, often despairingly, to find common ground in the battle against climate change. Compared with the manifold complexities of global warming, dealing with ozone depletion was, in fact, relatively simple. But the key point is that it happened, and it’s worth asking why the world has not responded with similar resolve in dealing with the main global warming gases like carbon dioxide, about which we have known a lot for a long time.