Models suggest that the concentration of chlorine and other ozone-depleting substances in the stratosphere will not return to pre-1980 levels until the middle decades of the 21st century. Scientists have already seen the first definitive proof of ozone recovery, observing a 20 percent decrease in ozone depletion during the winter months from 2005 to 2016. In 2019, abnormal weather patterns in the upper atmosphere over Antarctica dramatically limited ozone depletion, leading to the smallest hole since 1982. Models predict that the Antarctic ozone layer will mostly recover by 2040.