When the Montreal Protocol phased out the use of chlorofluorocarbons starting in 1989, scientists expected to see improvements in the Antarctic ozone hole that was opening each spring. They weren’t anticipating anything immediate—instead, improvement was expected within 20 to 30 years of the protocol’s implementation. Ozone recovery occurs through distinct stages, including a reduction in the rate of ozone decline and a leveling off of the depletion. Only once these are done should we expect to see an ozone increase, termed “ozone healing.”

Previous studies of the Antarctic ozone have provided evidence that the early phases of ozone recovery have been occurring. A new paper published in Science presents the first signs ozone healing may be occurring as well. The data shows an overall increase in the thickness and abundance of the Antarctic ozone layer in the last 15 years.

The Antarctic ozone hole fluctuates in size over the course of a year. It is largest during the spring, which occurs from September to November in the southern hemisphere. Historically, the majority of each year’s ozone hole growth occurs in late August and early September, building to a peak in October.