Many people are familiar with the ozone hole over Antarctica, but what is less well known is that sometimes the protective stratospheric ozone over the Arctic also breaks down, thinning the ozone layer there. The last time this happened was in the spring months of 2020, and before that in the spring of 2011.
Each time the ozone layer thinned, climate scientists subsequently observed weather anomalies throughout the northern hemisphere. In Central and Northern Europe, Russia, and especially in Siberia, those spring seasons were exceptionally warm and dry. However, wet conditions prevailed in other areas, such as the polar regions. These weather anomalies were particularly pronounced in 2020. That spring in Switzerland was also unusually warm and dry.
Whether there is a causal relationship between stratospheric ozone depletion and observed weather anomalies is a matter of debate in climate research. The polar vortex in the stratosphere, which forms in winter and subsides in spring, also plays its role. Scientists who have studied this phenomenon so far have come to conflicting results and different conclusions.
The new findings shed light on the situation thanks to PhD student Marina Friedel and Gabriele Chioda of the Swiss National Science Foundation Ambizione. Both are part of a group led by Thomas Peter, professor of atmospheric chemistry at ETH Zurich, and are collaborating with Princeton University and other institutions.
The simulation shows a correlation
To uncover a possible causal link, the researchers ran simulations that integrated ozone depletion into two different climate models. Most climate models consider only physical factors, not changes in stratospheric ozone levels, in part because that requires much more computing power.
But new calculations make it clear: the cause of the weather anomalies observed in the northern hemisphere in 2011 and 2020 is mainly the destruction of ozone over the Arctic. The simulations the researchers ran with the two models largely matched observations from those two years, as well as eight other similar events used for comparison. However, when the scientists “turned off” ozone depletion in the models, they were unable to reproduce these results.
“From a scientific point of view, what surprised us the most is that, even though the models we used for the simulations are completely different, they produced similar results,” says co-author Gabriele Chiyoda, SNSF Ambizione from the Institute of Atmospheric and Climate Science.
The mechanism is explained
The phenomenon, as researchers are now studying, begins with the destruction of ozone in the stratosphere. For ozone to break down there, the temperature in the Arctic must be very low. “Ozone depletion only occurs when it’s cold enough and the polar vortex is strong in the stratosphere, about 30 to 50 kilometers above the earth,” Friedel points out.
Normally, ozone absorbs UV radiation emitted by the sun, thereby heating the stratosphere and helping to break up the polar vortex in the spring. But when there is less ozone, the stratosphere cools and the vortex becomes stronger. “The strong polar vortex creates the effects seen on the Earth’s surface,” says Chioda. Thus, ozone plays an important role in temperature changes and circulation around the North Pole.
Greater accuracy is possible for long-term forecasts
The new findings could help climate researchers make more accurate seasonal weather and climate predictions in the future. This makes it possible to better predict changes in heat and temperature, “which is important for agriculture,” says Chioda.
Friedel adds, “It will be interesting to observe and model the future evolution of the ozone layer.” This is because ozone depletion continues even though ozone-depleting substances such as chlorofluorocarbons (CFCs) have been banned since 1989. CFCs are very long-lived and remain in the atmosphere for 50 to 100 years; their potential to cause ozone depletion persists for decades after they have been phased out. “However, CFC concentrations are steadily declining, which raises the question of how quickly the ozone layer is recovering and how this will affect the climate system,” she says.