Researchers at ETH Zurich prove that the depletion of the ozone layer over the Arctic in spring causes abnormal weather in the Northern Hemisphere. In many places it is warmer than average and dry — or too wet.
Many people are familiar with the ozone hole over Antarctica. What is less well known is that the protective ozone in the stratosphere is also occasionally destroyed over the Arctic and the ozone layer is thinned. The last time this happened was in the spring months of 2011 and 2020.
After these two events, climate researchers observed weather anomalies across the Northern Hemisphere. In Central and Northern Europe, Russia and especially in Siberia it was extremely hot and dry in those springs. In contrast, wet conditions prevailed in the polar regions. These weather deviations were particularly pronounced in 2020. It was also particularly warm and dry in Switzerland that spring.
Whether there is a causal relationship between the depletion of the ozone layer in the stratosphere and the observed weather anomalies is controversial in climate research. The polar vortex in the stratosphere, which forms in winter and disintegrates in spring, also plays a role. Scientists who have studied the phenomenon so far have come to conflicting results and different conclusions.
Now PhD student Marina Friedel and SNSF Ambizione Fellow Gabriel Chiodo from the group of Thomas Peter, Professor of Atmospheric Chemistry at ETH Zurich, in collaboration with Princeton University and other universities, are shedding light on the matter.
Simulations Reveal Connections
In April 2020, large-scale ozone depletion was detected over the Arctic. (Image: NASA Ozone Watch)
To discover a possible causal relationship, the researchers simulated the phenomenon by integrating ozone depletion into two different climate models. Most climate models only take into account physical factors, not variations in stratospheric ozone levels, partly because this would require much more computing power.
However, the new calculations make it clear that the main cause of the weather anomalies observed in the Northern Hemisphere in 2011 and 2020 is ozone depletion over the Arctic. The simulations the researchers ran with the two models largely matched the observation data from the two years and eight other similar events used for comparison purposes. However, if the ozone depletion was “turned off” in the models, the observations could not be reproduced.
In simulations of ozone depletion, it became much warmer over Russia and Siberia and wetter at the Arctic than in simulations that did not take ozone influence into account. (Images: from Friedel M. et al. Nature, 2022)
“From a scientific standpoint, what surprised us most was that the models we used for the simulations are fundamentally different, but gave a similar result,” said co-author Gabriel Chiodo, SNSF Ambizione Fellow at the Institute of Atmosphere and Climate.
Mechanism clarified
At the beginning of the phenomenon, as the researchers have now investigated, is the depletion of the ozone layer in the stratosphere. To break down ozone there, temperatures in the Arctic have to be very low. “Ozone is only destroyed when it is cold enough and the polar vortex in the stratosphere, about 30 to 50 kilometers above the ground, is strong,” emphasizes Friedel.
Normally, ozone absorbs the UV radiation emitted by the sun, heating the stratosphere. This contributes to the disintegration of the polar vortex in the spring. But when there is less ozone, the stratosphere cools and the vortex becomes stronger. And that affects the Earth’s surface. “A strong polar vortex then produces the observed surface effects,” Chiodo says. Ozone thus makes an important contribution to changing the temperature and circulation around the North Pole.
More accurate long-term forecasts possible
The new findings could help climate researchers make more accurate seasonal weather and climate predictions in the future. This makes it easier to predict heat and temperature changes. “This is important for agriculture,” emphasizes Chiodo.
“It will be interesting to observe and model the future evolution of the ozone layer,” Friedel says. Because the depletion of the ozone layer is still ongoing, although Ozone depletion substances such as chlorofluorocarbons (CFCs) have been banned since 1989. CFCs are very persistent and remain in the atmosphere for 50 to 100 years. They continue to unleash their destructive potential decades after being phased out. “However, the CFC concentration is constantly falling and that raises the question of how quickly the ozone layer will recover and how this will affect the climate system,” says the climate researcher.
Article courtesy of ETH Zurich†
Through Peter Ruegg
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