CFCs have largely been phased out globally, but new research in the journal Nature Geoscience shows five of the ozone-depleting chemicals reached record levels in 2020.

 In 1987, the Montreal Protocol set a timetable for phasing out chlorofluorocarbons; in 2010, it added hydrochlorofluorocarbons.

But CFCs could still be used as raw materials, or “feedstocks,” for making other compounds, or be released as by-products of such processes.

The study found five CFCs (CFC-113, CFC-112a, CFC-113a, CFC-114a, CFC-115) with few or no known uses rose rapidly between 2010 and 2020, causing a warming effect roughly equal to 150% of London’s CO2 emissions in 2018.

The authors say the ozone impact was small but, left unchecked, could undo some of the progress of the past 36 years.