The ozone hole was healing — until a mystery CFCs spike in 2018

The 1987 Montreal Protocol banned chlorofluorocarbons (CFCs) responsible for destroying stratospheric ozone, and by the 2010s the ozone hole was measurably healing. Then in 2018, scientists detected rising levels of CFC-11 in the atmosphere that couldn't be explained by previously known sources. Investigations traced the emissions to illegal manufacturing in China, reported by The New York Times in 2019. Once the source was identified and Chinese authorities acted, emissions fell back. The episode showed both the fragility of environmental progress and the importance of atmospheric monitoring as a law-enforcement tool.

Source: Montzka et al., Nature (2018)