For more than 30 years, nations have been working to protect Earth’s stratospheric ozone layer by banning the new production and trade of ozone-depleting substances. Part of that effort has included tracking the atmospheric concentration of such chemicals. In the 1970 and 80s, scientists discovered that chemicals widely used in refrigerants and insulating foams were rising into the stratosphere. Even after production ceased, scientists still expected chemicals like CFC-11 to continue leaking from existing products for years, but at a gradually declining rate. In 2018, NOAA first reported that atmospheric CFC-11 had declined less than expected, hinting that something had changed.