It has been more than 30 years since the world banned the chemicals that dropped the earth’s protective ozone layer and at the same time triggered some worrying changes in atmospheric circulation in the southern hemisphere.
Now new research was published this week in Nature notes that these changes have paused and perhaps even reversed because of the Montreal Protocol – an international treaty that successfully discontinued the use of ozone-depleting chemicals.
“This study contributes to growing evidence demonstrating the profound effectiveness of the Montreal Protocol. Not only has the Treaty stimulated ozone layer healing, but it is also driving the latest changes in air circulation patterns in the Southern Hemisphere,”
The ozone hole, which was discovered in 1985, has formed every spring in the atmosphere high above Antarctica. Ozone depletion cools the air, strengthens the winds in the polar vortex and affects the winds all the way down to the lowest layer of the Earth’s atmosphere. In the end, the ozone depletion has shifted the semi-jet and the dry regions at the tropical edge to the south pole.
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Previous studies have linked these circulation trends to changes in the southern hemisphere, especially precipitation over South America, East Africa and Australia, and to changes in ocean currents and salinity.
The Montreal Protocol of 1987 discontinued the production of ozone-depleting substances such as hydrochlorofluorocarbons (CFCs). Starting in 2000, concentrations of chemicals in the stratosphere began to decline and the ozone hole began to recover. In this study, Banerjee and her co-authors have shown that around the year 2000, circulation in the Southern Hemisphere stopped expanding polewards – a break or a slight reversal of the past trends.
“The challenge in this study was to prove our hypothesis that ozone recovery actually drives these atmospheric circulation changes and it’s not just a coincidence,” Banerjee said.
To do so, the researchers used a two-stage statistical technique called detection and attribution: to detect if certain patterns of observed wind changes are probably not due to natural variation and, if so, whether the changes are attributable to human-caused factors, such as the release of ozone-depleting chemicals and CO2.
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Using computer simulations, the researchers first determined that the observed break in the circulation trends could not be explained by natural wind changes alone. Then they isolated the effects of ozone and greenhouse gases separately.
They showed that while rising carbon dioxide emissions have continued to increase circulation near the surface (including jet stream), only the ozone changes can explain the break in circulation trends. Before 2000, both ozone depletion and rising CO2 levels drove the circulation pole near the surface. Since 2000, carbon dioxide has continued to drive this circulation powder and balance the opposite effect of ozone recovery.
“Identifying the ozone-driven break in circulation trends in observations in reality confirms for the first time what the scientific ozone community has long predicted from theory,” said John Fyfe, a researcher at Environment and Climate Change Canada and one of the paper’s co-authors.
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With ozone recovering and CO2 levels continuing to climb, the future is less certain, including for the southern hemisphere regions whose weather is affected by the jet stream and on the outskirts of the arid regions.
“We call this a ‘break’ because poleward circulation trends can resume, remain flat or reverse,” Banerjee said. “It is the tug of war between the opposite effects of ozone recovery and rising greenhouse gases that will determine future trends.”
However, a 2018 UN report says that the notorious hole in the ozone layer could be completely healed during the 2060s – and in some areas of the world it could be as soon as 2030.
Reprinted from the Cooperative Institute for Research in Environmental Sciences
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