The stratospheric ozone layer is a protective shield that enables life on earth by reflecting UV radiation from the sun. In the early 1980s, scientists began to observe an abrupt drop in the thickness of the ozone layer due to the use of refrigerating chemicals called chlorofluorocarbons that were depleting the ozone layer and creating risks for human health. In the mid-1980s, the world banned the use of chlorofluorocarbons with the Montreal Protocol, an example of international cooperation to move toward a safe operating space. There is still an ozone hole over Antarctica. Professor Rockström mentions the choice between bromine and chlorine to refrigerate, and the catastrophic effects of bromine. This problem is still not resolved because we still use other types of dangerous refrigerants; methane also poses a threat to the ozone layer.