The ozone layer in the earth’s atmosphere has continued to recover in recent years. The annual ozone hole over Antarctica continues to occur but appears to be gradually shrinking. But a current study now calls this into question. Accordingly, the ozone density in the core of the Antarctic spring ozone hole has decreased by 26 percent since 2004. In addition, the phase of this greatly reduced ozone density in the past three years apparently lasted longer than before. However, how these results should be assessed is controversial among climate scientists.
In the so-called Montreal Protocol, starting in 1987, the United Nations banned certain chemical substances used in spray cans and refrigerators that have been shown to damage the earth’s ozone layer, particularly long-lived chlorofluorocarbons (CFCs). Since then, measurement data has shown that the stratospheric ozone layer is slowly recovering. The ozone depletion that occurs over Antarctica, especially in spring, has eased since 2001, especially at the beginning of spring; the long-term trend is for the ozone hole to become smaller and smaller. However, fluctuations in the local climate and atmospheric currents also influence how large the Antarctic ozone hole becomes each year.
But no recovery of the ozone layer in Antarctica?
A research team led by Hannah Kessenich from the University of Otago in New Zealand has now examined the development of Antarctic ozone in more detail. To do this, they compared the daily and monthly increases or decreases in ozone density between 2001 and 2022. For the spring months of September to November, the researchers also compared the amount of ozone in different layers of the stratosphere.
Their analysis found that Antarctic ozone appears to have not recovered as well since 2001 as previously reported and predicted. On the contrary: Since 2004, the amount of ozone in the middle stratosphere over Antarctica has continuously decreased, by a total of 26 percent in the middle of the ozone hole, as Kessenich and her colleagues report. In the past three years in particular, the stratospheric ozone depletion over Antarctica has reached alarming proportions. Every spring month of October, a large and particularly long-lasting ozone hole appeared. “For most of the spring, the hole is not only larger in terms of area, but also deeper,” reports Kessenich. The researchers also observed that the ozone hole appeared later and closed later than in previous years. And preliminary data for 2023 also shows a particularly large ozone hole over Antarctica, even “larger than in the past three years”.
In order to classify these observations, the researchers also evaluated satellite images. Based on this, they suspect that the long-lived ozone holes of recent years were caused not only by CFCs, but also by complex changes in the overlying atmospheric layer, the mesosphere. Accordingly, a changed composition of the falling air contributes at least to the younger, persistent and large ozone holes in Antarctica. “Most major publications about the ozone layer in recent years have given the public the impression that the ‘ozone problem’ has been solved,” says Kessenich. But although the CFC ban has helped enormously, the extent and causes of ozone loss may be more complex than expected, the researchers report.
Do climate and the ozone layer influence each other?
The Antarctic ozone layer must therefore be continuously monitored and evaluated, also because the climate on Earth is changing more and more, warn Kessenich and her colleagues. “Although the ozone hole is independent of the effects of greenhouse gases on the climate, it does interact with the delicate balance in the atmosphere,” says Kessenich. This could lead to heat build-up and local climate changes around the polar region.
Climate researcher Peter von der Gathen from the Alfred Wegener Institute, who was not involved in the study, considers the CFC ban in the Montreal Protocol to be a success. However, the work of Kessenich and her colleagues shows “that in addition to these substances, other processes – such as climate change – also determine the type and timing of the recovery of the ozone layer,” says von der Gathen. “These additional processes have been increasingly becoming the focus of science for some time now. They complicate clear evidence and prediction of the pace of future ozone recovery.”
Studies in recent years also indicate that human emissions of the short-lived chemical dichloromethane and other ozone-destroying halogen compounds, which are not listed in the Montreal Protocol, have increased and are slowing the recovery of the ozone layer. And aerosols from volcanoes and large-scale forest fires, for example in Australia, could also have influenced the Antarctic ozone hole.
Doubts about the study
However, the years 2002 and 2019, in which the Antarctic ozone hole was unusually weak, do not fit into the picture of a long-term decline in ozone drawn by the current study. Kessenich and her colleagues attribute this to short-term warming of the stratosphere and deliberately excluded this data from their calculations. But Martin Dameris from the German Aerospace Center and other climate scientists criticize this and doubt the results. “In 2019 there was the smallest ozone hole in the past 35 years. If the year were included in all the analyzes, completely different values would certainly come out,” says Dameris. There may not be a long-term downward trend in Antarctic ozone, but just a few exceptional years. Therefore, further measurement data and research will be necessary in the coming years to test the study’s statements and to understand the role of climate change and air masses from the mesosphere.
On average, the ozone hole will become smaller in the coming years thanks to the Montreal Protocol, predict both the authors and the critics of the study. “Nevertheless, there can still be years with particularly large or small ozone holes – such as 2020, 2021 and 2022 or 2019 – if the corresponding meteorological conditions exist,” says Dameris.
Source: Hannah Kessenich (University of Otago) et al., Nature Communications, doi: 10.1038/s41467-023-42637-0; Science Media Center