“The Antarctic ozone hole was exceptionally weak this year,” said Paul Newman of NASA’s Goddard Space Flight Center. “This is what we would expect to see given the weather conditions in the Antarctic stratosphere.”

The extent of the hole peaked at 7.6 million square miles on September 11. The amount of ozone depletion during the peak was also the least since 1988. The average area of the daily ozone hole maxima seen since 1991 is around 10 million square miles.

“In the past, we’ve always seen ozone at some stratospheric altitudes go to zero by the end of September,” said Bryan Johnson of NOAA. “This year our balloon measurements showed the ozone loss rate stalled by the middle of September and ozone levels never reached zero.”

The smaller ozone hole is due to natural variability rather than being a signal of rapid healing, according to scientists. The Antarctic ozone hole is expected to recover back to 1980 levels around the year 2070, following introduction of the Montreal Protocol in the late 1980s to limit the use of ozone-depleting substances such as chlorofluorocarbons.