Ozone levels above Antarctica may not be recovering after all.

The ozone hole over Antarctica usually emerges around August and stays open until the end of November, and it had been thought that total ozone levels were slowly recovering after ozone-depleting substances were banned. 

However, a new NZ-led study has found that, over the last 20 years, there has actually been a 26 per cent reduction in the core of the Antarctic ozone layer during the middle of this period, around October. 

The researchers also found that when they added the most recent satellite data into the observations, the trends towards recovery of total ozone also disappeared. 

The researchers found that these changes may be driven by changes in the atmospheric layer above the ozone layer.

To assess recent changes in the Antarctic ozone hole, including during the 2022 season, Annika Seppälä, Hannah Kessenich and colleagues analysed monthly and daily ozone changes between 2001–2022. 

Data from 2002 and 2019 were excluded as sudden stratospheric warming broke up the ozone hole anomalously early in these years. 

They looked at different stratospheric layers throughout the key austral springtime months of September to November. 

When satellite data from 2022 is considered, they found that previously reported recovery trends in Antarctic spring total column ozone (the total ozone above a given point on Earth across all atmospheric layers) from 2001 onward disappear. 

The middle stratosphere has been dominated by continued, significant ozone reduction since 2004, amounting to 26 per cent loss in the core of the ozone hole. 

This reduction is potentially driven by dynamical changes in the mesosphere (the atmospheric layer above the stratosphere and the ozone layer).

The findings suggest that changes in the Southern Hemisphere atmosphere are contributing to a persistent Antarctic ozone hole.

However, some experts are not convinced of the results of the study. 

The results rely heavily on the large ozone holes we have seen in 2020-2022. However, existing literature has already found reasons for these large ozone holes, according to Dr Martin Jucker - a lecturer at the University of New South Wales and Associate Investigator at the ARC Centre of Excellence for Climate Extremes

“Smoke from the 2019 bushfires and a volcanic eruption (La Soufriere), as well as a general relationship between the polar stratosphere and El Niño Southern Oscillation”, he explains, 

“We know that during la Niña years, the polar vortex in the stratosphere tends to be stronger and colder than usual, which means that ozone concentrations will also be lower during those years. 

“The years 2020-22 have seen a rare triple La Niña, but this relationship is never mentioned in the study.”

In addition, the study removes two years in the record, 2002 and 2019, on the basis of exceptional events happening during those years; so-called Stratospheric Sudden Warmings. 

“Those events have been shown to have strongly decreased the ozone hole size, so including those events would probably have nullified any long-term negative trend in ozone concentrations,” Dr Jucker said. 

He questions how the authors can remove 2002 and 2019 from the record but not 2020-22, given that all of these years have been shown to be dominated by very special and rare events.

“It is important to note that the ozone hole is extremely variable from year to year, meaning that it can be large one year and small the other year,” Dr Jucker added. 

“It is only over longer terms that a trend can be identified. Using only 22 years and then removing two inconvenient years does not help make this study convincing.”

The full study is accessible here.