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The ozone layer in the stratosphere protects life on earth from harmful UV radiation, but it also has a strong influence on the weather. (Image courtesy of NASA.)

Climate Change

Ozone Depletion Over North Pole Causes Weather Anomaly

Researchers at ETH Zurich prove that the depletion of ozone over the Arctic in spring causes abnormal weather throughout the northern hemisphere. In many places it will be 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 O3 layer in the stratosphere is also occasionally destroyed over the Arctic and the ozone layer is thinned out. 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 entire northern hemisphere. In Central and Northern Europe, Russia and above all in Siberia, it was extraordinarily warm and dry in those springs. In contrast, wet conditions prevailed in polar regions. These weather anomalies were particularly pronounced in 2020. In Switzerland, too, it was particularly warm and dry that spring.

Whether there is a causal connection between the ozone depletion 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 contradictory results and different conclusions.

Now doctoral 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 depletion was detected over the North Pole. (Graphic: Nasa Ozone Watch)

In order to uncover a possible causal relationship, the researchers simulated the phenomenon by integrating ozone depletion in two different climate models. Most climate models only take physical factors into account, not variations in stratospheric ozone levels, partly because this would require much more computing power.

However, the new calculations make it clear that the cause of the weather anomalies observed in the northern hemisphere in 2011 and 2020 is mainly ozone destruction over the Arctic. The simulations that the researchers carried out with the two models largely corresponded to the observation data from the two years and eight other such events that were used for comparison purposes. However, if that destruction was “switched off” in the models, the observations could not be reproduced.

In simulations with depletion, it became much warmer over Russia and Siberia and wetter at the North Pole than in simulations that did not take the ozone influence into account. (Graphics: from Friedel M. et al. Nature, 2022)

“From a scientific point of view, what surprised us most was that the models we used for the simulations are fundamentally different, but gave a similar result,” says co-author Gabriel Chiodo, SNSF Ambizione Fellow at the Institute for Atmosphere and Climate.

Mechanism Clarified

At the beginning of the phenomenon, as the researchers have now investigated, is ozone depletion in the stratosphere. In order for ozone to be broken down there, the temperatures in the Arctic must be very low. “The ozone is only destroyed when it is cold enough and the polar vortex in the stratosphere, around 30 to 50 kilometers above the ground, is strong,” emphasizes Friedel.

Normally, ozone absorbs the UV radiation emitted by the sun, thereby warming the stratosphere. This contributes to the disintegration of the polar vortex in spring. But if there is less ozone, the stratosphere cools down and the vortex becomes stronger. And that affects the surface of the earth. “A strong polar vortex then produces the observed surface effects,” says Chiodo. So ozone is a major contributor to changing the temperature and circulation around the North Pole.

More Precise Long-Term Ozone Forecasts Possible

The new findings could help climate researchers to create more accurate seasonal weather and climate forecasts 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 development of the ozone layer,” says Friedel. Because ozone depletion is still going on, although ozone-depleting 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 they have been phased out. “However, the CFC concentration is constantly falling, and this 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.
By Peter Ruegg

 
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