Following the success of the Montreal Protocol, the use of halons and chlorofluorocarbons (CFCs) has been reported as discontinued, although their levels in the atmosphere continue to be monitored. Because of their long lifetime, these compounds will remain in the atmosphere for many decades. Even if there were no new emissions, there is still more than enough chlorine and bromine present to cause the complete destruction of ozone over Antarctica from August to December.

As a result, the formation of the Antarctic ozone hole – an area of low ozone concentration – continues to be an annual spring event, with the year-to-year variation in its size and depth governed to a large degree by meteorological conditions. The 2021 Antarctic ozone hole developed relatively early and continued growing, resulting in a large and deep ozone hole. It expanded to 24 million km² on 24 September and remained close to this value until mid-October 2021. The development of the hole, and its extent and severity, were close to that for the 2020 and 2018 seasons.

The ozone hole reached its maximum area of 24.8 million km2 on 7 October 2021, similar to the areas in 2020 and 2018, and close to the highest values observed in earlier years, such as 28.2 million km² in 2015 and 29.6 million km² in 2006, according to an analysis from the National Aeronautics and Space Administration (NASA) (Figure 1).

Figure 1

In terms of the total ozone column, NASA reported a minimum ozone of 92 DU (Dobson Units) on 7 October 2021, which was the lowest value for the 2021 season and for the past 17 years (Figure 2). After September 2021, the concentration of stratospheric ozone was persistently reduced to near-zero values between 15 and 20 km altitude over Antarctica. Together with the 2020 season, these are some of the lowest ozone values ever measured via sondes at the Antarctic stations, as reported by the National Oceanic and Atmospheric Administration (NOAA).

Figure 2

The 2021 hole was larger and deeper than 70% of the ozone holes since 1979 and remained as such until the closing of the hole in the second half of December. It ranked as the thirteenth largest ozone hole by area and the sixth deepest ozone hole in terms of minimum ozone. The unusually deep and large ozone hole in 2021 was driven by a strong and stable polar vortex and colder-than-average conditions in 2021 in the stratosphere.