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21 September 2021. Credit: NASA

The ozone hole has reappeared over Antarctica as expected. Current observations show a somewhat more severe ozone hole, but this is because of the below average temperatures in the Antarctic stratospheric. But this above average severity is consistent with the continued decline of ozone depleting substances and the colder meteorological conditions.

Each year, severe depletion of the ozone layer occurs over the Antarctic during the spring season (from August to October), known as the ‘ozone hole’, reaching its maximum between mid-September and mid-October. This phenomenon is not a sign that the Montreal Protocol is not working, as the area, depth and duration of the ozone hole depends strongly on stratospheric weather conditions. The very low winter temperatures in the Antarctic stratosphere lead to the formation of polar stratospheric clouds (PSCs). The particles in these clouds release chlorine into reactive forms which deplete ozone.

The ozone hole area varies significantly from year to year because of the varying weather conditions in the southern hemisphere. For example, in 2019, NOAA and NASA scientists reported that abnormally warm temperatures in the Antarctic stratosphere dramatically limited ozone loss in September and October, resulting in the smallest ozone hole observed since 1982; the third time in 40 years. Similar weather patterns in the Antarctic stratosphere in September 1988 and 2002 also produced atypically small ozone holes. Contrary to the small 2019 ozone hole, the 2020 ozone hole was considered record-breaking as the longest-lasting hole since the ozone layer monitoring began. That hole finally closed at the end of December after an exceptional season of naturally occurring meteorological conditions.

Currently, the 2021 ozone hole growth rate is similar to last year’s and as of 16 September was around 23 million square km, making it a relatively large ozone hole. As ozone depletion normally peaks between mid-September and mid-October, the severity of the hole won’t be fully determined until late-October.

The year-to-year variability of the ozone hole’s area is large, somewhat masking long-term trends. The larger area of the 2021 ozone hole is tied to the below average cold conditions observed in the Antarctic stratosphere in August and September. Even so, the current ozone hole observations are clearly less severe than the extremely severe ozone holes of the 1990-2010 period. Long-term measurements of gases in the stratosphere show that ozone depleting chlorine and bromine are decreasing because of controls on ozone depleting substances, and the ozone layer is healing.