The ozone hole over Antarctica shrank to its smallest peak since 1988, NASA said Thursday.
The huge hole in Earth's protective ozone layer reached its maximum this year in September, and this year NASA said it was 7.6 million square miles (19.6 million square kilometers). The hole size shrinks after mid-September.
This year's maximum hole is more than twice as big as the United States, but it's 1.3 million square miles smaller than last year and 3.3 million square miles smaller than 2015.
Paul Newman, chief Earth scientist at NASA's Goddard Space Flight Center, said stormy conditions in the upper atmosphere warmed the air and kept the chemicals chlorine and bromine from eating ozone. He said scientists haven't quite figured out why some years are stormier — and have smaller ozone holes — than others.
"It's really small this year. That's a good thing,'' Newman said.
Newman said this year's drop is mostly natural but is on top of a trend of smaller steady improvements likely from the banning of ozone-eating chemicals in a 1987 international treaty. The ozone hole hit its highest in 2000 at 11.5 million square miles (29.86 million square kilometers).
Ozone is a colorless combination of three oxygen atoms. High in the atmosphere, about 7 to 25 miles (11 to 40 kilometers) above the Earth, ozone shields Earth from ultraviolet rays that cause skin cancer, crop damage and other problems.
Scientists at the United Nations a few years ago determined that without the 1987 treaty, by 2030 there would have been an extra 2 million skin cancer cases. They said that overall, the ozone layer is beginning to recover because of the phase-out of chemicals used in refrigerants and aerosol cans.