The hole in the ozone above Antarctica will appear earlier than usual this year.

The World Meteorological Organization said in a report that an unstable vortex has been observed above the South Pole since July, a harbinger of an earlier ozone hole phenomenon. But the U.N. agency expects the scale of the ozone reduction to be smaller.

The phenomenon refers to a drastic thinning of the ozone over the polar regions largely due to emissions of chlorofluorocarbons and other ozone-depleting substances. It has been observed during the Antarctic spring between September and November since the early 1980s.