evidence: The story of changing temperatures in the atmosphere at all levels is largely the story of CO2. We know all too well that our emissions of more than 40 billion tons of the gas annually are warming the troposphere. This happens because the gas absorbs and re-emits solar radiation, heating other molecules in the dense air and raising temperatures overall. [...] One big concern is the already fragile state of the ozone layer in the lower stratosphere, which protects us from harmful solar radiation that causes skin cancers. For much of the 20th century, the ozone layer thinned under assault from industrial emissions of ozone-eating chemicals such as chlorofluorocarbons (CFCs). Outright ozone holes formed each spring over Antarctica. [...] In the spring of 2020, the Arctic had its first full-blown ozone hole with more than half the ozone layer lost in places, which von der Gathen blames on rising CO2 concentrations. It could be the first of many. In a recent paper in Nature Communications, he warned that the continued cooling means current expectations that the ozone layer should be fully healed by mid-century are almost certainly overly optimistic. On current trends, he said, "conditions favorable for large seasonal loss of Arctic column ozone could persist or even worsen until the end of this century … much longer than is commonly appreciated."