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 Earth’s 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, it’s had a noticeable effect on the Southern Hemisphere’s atmosphere. Since about the year 2000, there’s 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 NOAA’s 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 Hemisphere’s 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.

“What we showed is that after the year 2000, the effects of ozone recovery have actually been balancing the effects of increasing CO 2 , such that the position [of the jet stream] has remained approximately constant,” Banerjee told E&E News. “And that has been referred to previously in the literature as the ‘tug of war’ between those two effects.”

It’s unclear what will happen in the future, as the ozone layer continues to recover and the planet continues to warm.

There’s some indication in the data that the pause is leaning toward a small reversal of the 20th-century trends. This suggests that the ozone recovery is currently a stronger influence on the Southern Hemisphere’s atmosphere than greenhouse gas emissions, said Alexey Karpechko, a scientist at the Finnish Meteorological Institute, in a published comment on the new research.

“This is a crucial contribution to the long-standing debate about the relative role of these two factors in past and future circulation trends,” he wrote.

But that doesn’t mean the effect will last forever. In fact, Banerjee noted it’s likely that greenhouse gas emissions could become the dominant influence within a few decades.

“We expect that by about midcentury ... when the ozone hole is near to fully recovered, the jet stream will start to move poleward again, just because CO 2 is continuing to increase,” she added.

These trends matter for countries in the Southern Hemisphere. Scientists believe the shifting atmosphere has already altered some weather patterns, and future changes may have a significant effect.

In Australia, some researchers have suggested that the poleward-shifting jet stream led to drier winters over time. On the other hand, some parts of South America have experienced more rainfall as the tropics have expanded, Banerjee noted.

Predicting future continuations or reversals in those trends can help communities adapt their agricultural strategies or plan for droughts and other disasters accordingly.

More broadly, the new study adds to a growing number of studies highlighting the significance of the Montreal Protocol.

A January study in Nature Climate Change pointed out that the treaty not only saved the ozone layer but also helped mitigate some global warming (Climatewire, Jan. 21). A December study in Environmental Research Letters came to similar conclusions.

Today, many experts point to the Montreal Protocol as proof that swift, successful international action on a major environmental problem is possible. With enough global cooperation, the same could be achieved on climate change.

“I’d just like to emphasize that the Montreal Protocol is really an international success story,” Banerjee said. “And it shows what we can do if countries come together to tackle global threats.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.