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.

The authors of the paper built three models to simulate the drivers of polar ozone changes in the years since 2000. These models each account for different aspects of the available data on the Antarctic ozone layer and ozone hole. They also include things like balloon-collected ozone column measurements, satellite-based mapping of the horizontal area of the hole, data on aerosol influences, and data on changes in temperature and wind patterns.

Volcanic eruptions also have to be considered when analyzing the Antarctic ozone hole. Their chemical impacts can cause the hole to expand, and they contribute to our uncertainty about the overall behavior of the ozone hole. For example, in 2011 and 2015, the ozone hole appeared to have grown in diameter due to recent volcanic eruptions despite the Montreal Protocol.

As the ozone layer heals, it is expected that seasonal growth of the ozone hole will occur later in the season. Observations of the 2015 ozone hole were in line with these expectations. In 2015, the ozone hole was considerably smaller during the months of August and September than it was during previous years. Although it reached record size during October of that year, the authors’ climate and atmospheric models indicate that this was an anomaly related to volcanic eruptions.

Positive trends in terms of ozone recovery aren’t yet significant during October, but they are in other months, when the ozone hole is not at its maximum extent. The researchers report considerable ozone healing in the months adjacent to October, with September showing chemical and ozone column healing that stands out despite highly variable weather and temperature influences during this month.

While we’d like to understand the future of the ozone hole, the authors’ models don’t tell us how dynamic seasonal and temperature variations contribute to this phenomenon. The authors write that this underscores the continued need for ozone monitoring over the Antarctic, which will eventually allow a more complete understanding of the forces that contribute to ozone fluctuations.

The good news, however, is that the future monitoring data should continue to show healing.

Science, 2016. DOI: 10.1126/science.aae0061 (About DOIs).