What is the current state of the ozone layer?

Depletion of stratospheric ozone occurs over both hemispheres of the Earth. However, this phenomenon is more pronounced in the Southern Hemisphere (Antarctica) than in the Northern Hemisphere (Arctic). This is the case because the formation of the ozone hole is directly linked to the stratosphere's temperature. Once temperatures drop below -78°C, polar stratospheric clouds tend to form, which exacerbate ozone depletion. In the Antarctic, long presence of low temperatures in the stratosphere is stimulating their formation, whereas the Arctic is characterised by larger year-to-year meteorological variability.

Dobson Units (DU) measure how much ozone is in the air above us. On a global scale, the average total ozone concentration is typically around 300 DU. Ozone levels tend to be higher near the poles and lower at the equator. Generally, the ozone hole is defined as the area for which ozone column values amount to 220 Dobson Units (DU, marked by the thick contour line in Figure 1) or less (represented in blue colours in Figure 1). This is only apparent in the southern hemisphere. Here, the largest historical extent of the ozone hole — 28.4 million square kilometres (Figure 1) — occurred in September 2000. This area is equivalent to almost seven times the territory of the EU.

This year's ozone hole over the Southern Hemisphere had a maximum area of 21.9 million km² at the end of September (Figure 2), making it the smallest ozone hole since 2020. Data from the Copernicus Atmosphere Monitoring Service (CAMS) already indicated an unusually large and persistent ozone hole over the Antarctic in the period from 2020 to 2022 for which the drivers are currently still subject to research. While UNEP's scientific assessment report projects that global stratospheric ozone will return to 1980 levels around 2040, the behaviour of the southern ozone layer contrasts with observations in the past 40 years. Nonetheless, CAMS researchers expect to see a full recovery of the ozone hole by mid-century.

When it comes to the identification of drivers of ozone depletion, the complex interaction of chemical and meteorological factors makes it difficult to attribute ozone hole evolution to a single individual component. One aspect is the strength of the polar vortex. For instance, the strength of the polar vortex in 2021 exceeded all other years which resulted in a large ozone hole. In late 2023, the ozone hole was unusually persistent during November, remaining over 14.2m2, roughly the area of Antarctica, until early December, closing on 20 December 2023 being the third longest-lived ozone hole, after the years 1999 and 2020 closing 27 December. In 2024, a highly disrupted southern polar vortex during austral winter contributed to late depletion of ozone starting in late August. Under normal meteorological conditions the hole begins to form in mid to late August and closes towards the end of November. Overall, the evolution of the 2024 ozone hole appears to be like the one in 2022.

Another facet which acts as a driver to the ozone hole’s size is stratospheric temperature, with warmer temperatures leading to a smaller ozone hole, such as in 2019 and 2024 (CAMS, 2024 - see the list of scientific references). Two episodes of sudden stratospheric warming in July 2024 added to the above-mentioned phenomenon. When the polar vortex is weak, with higher temperatures and slower winds than usual in the stratosphere, ozone depletion process is weaker, leading to ozone columns above 220 Dobson units (DU).

Increasing concentrations of greenhouse gases (GHGs) cannot directly be attributed to a larger ozone hole, as they exert a dual effect. While GHGs are thought to lead to warmer temperatures, they tend to have a cooling effect in the middle and upper stratosphere which reduces the temperature exchange between the different layers of Earth's atmosphere. This stratospheric cooling effect is generally positively associated with ozone recovery, except for the polar regions. Here, very low temperatures can lead to an increase in the formation of polar stratospheric clouds, which facilitate ozone depletion as explained above.

The ozone hole can also be periodically influenced by volcanic eruptions and forest wildfires, perturbing chemical and dynamic processes which in return affect stratospheric ozone amounts. Researchers suggest the eruption of the Hunga Tonga - Hunga Ha’apai volcano in early 2022, that injected huge amounts of water vapour into the stratosphere, might also have influenced the extent and intensity of ozone depletion in 2023.

Finally, smoke-charged vortex (SCV) resulting from wildfires transport aerosols into the stratosphere, and this leads to both depleting and increasing the ozone layer stemming from different chemical reactions at different atmospheric layers, depletion being the bigger part (Max Planck Institute for Chemistry, 2024 - see the list of scientific references). With the increasing frequency and intensity of wildfires driven by global warming, the formation of SCVs and their impact on the stratosphere could become more common, posing a threat to the ozone layer.

The new findings by researchers of the Max Planck Institute highlight that natural events, exacerbated by climate change, pose additional risks to this fragile stratospheric layer. Since current observations show that the size and persistence of the ozone hole are largely dynamically driven, the urgence of continuing global efforts under the Montreal Protocol to ensure a swift recovery of the ozone layer and mitigating climate change remain key.

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