The notorious Antarctic ozone hole sparked worldwide concern after its discovery in the 1980s, and for good reason declining ozone allows harmful ultraviolet radiation to reach the Earths surface, a major threat to public health.
But the ozone hole had another effect on the planet: It caused major atmospheric changes in the Southern Hemisphere.
With less ozone trapping solar radiation higher in the atmosphere, the stratosphere began to cool. The jet stream shifted toward the South Pole. The warm, wet tropics expanded, and the dry zone below the tropics shifted southward, as well. Weather patterns in certain parts of the Southern Hemisphere began to change.
Thanks to the Montreal Protocol, an international agreement to phase out ozone-depleting chlorofluorocarbons, the ozone hole is now in recovery. The treaty has been regarded as one of the most successful cooperative environmental efforts in history.
As it turns out, its had a noticeable effect on the Southern Hemispheres atmosphere. Since about the year 2000, theres been a pause in the shifting of the jet stream and the other changes caused by the declining ozone.
These are the findings in a study published yesterday in Nature.
Led by Antara Banerjee of NOAAs Earth System Research Laboratory, the researchers analyzed historical data from the Southern Hemisphere and revealed that past trends in the shifting atmosphere had been on hold for two decades. Then they used climate models to test whether the pause is the result of the recovering ozone layer or some other factor, like greenhouse gas emissions or natural climate cycles.
The study indicates that changes in the ozone layer are the primary driver.
At the same time, the models do suggest that greenhouse gas emissions and declining ozone have similar effects on the Southern Hemispheres atmosphere. The exact mechanisms are a little different, but climate change seems to encourage a poleward shift of the jet stream and an expansion of the tropics.
Before the Montreal Protocol kicked in, it seems likely that greenhouse gas emissions were slightly reinforcing the effects of the ozone hole.
Greenhouse gases may also be part of the reason the trends have only paused since 2000, instead of being fully reversed. The effects of human-caused climate change are now somewhat at odds with the effects of the recovering ozone hole.
scientificamerican.com, 26 March 2020