Guest essay by Eric Worrall

According to a study of the Southern Hemisphere, the warming we should have experienced in the south is being masked by the recovery of the Ozone Layer over Antarctica.

Shrinking Ozone Hole, Climate Change Are Causing Atmospheric “Tug of War”

The Southern Hemisphere jet stream is shifting, bringing more rain to some spots and less to others

By Chelsea Harvey, E&E News on March 26, 2020

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.

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.

“What we showed is that after the year 2000, the effects of ozone recovery have actually been balancing the effects of increasing CO2, 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.

Read more: https://www.scientificamerican.com/article/shrinking-ozone-hole-climate-change-are-causing-atmospheric-tug-of-war/

The abstract of the study;

A pause in Southern Hemisphere circulation trends due to the Montreal Protocol

Antara Banerjee, John C. Fyfe, Lorenzo M. Polvani, Darryn Waugh & Kai-Lan Chang 

Observations show robust near-surface trends in Southern Hemisphere tropospheric circulation towards the end of the twentieth century, including a poleward shift in the mid-latitude jet1,2, a positive trend in the Southern Annular Mode1,3,4,5,6 and an expansion of the Hadley cell7,8. It has been established that these trends were driven by ozone depletion in the Antarctic stratosphere due to emissions of ozone-depleting substances9,10,11. Here we show that these widely reported circulation trends paused, or slightly reversed, around the year 2000. Using a pattern-based detection and attribution analysis of atmospheric zonal wind, we show that the pause in circulation trends is forced by human activities, and has not occurred owing only to internal or natural variability of the climate system. Furthermore, we demonstrate that stratospheric ozone recovery, resulting from the Montreal Protocol, is the key driver of the pause. Because pre-2000 circulation trends have affected precipitation12,13,14, and potentially ocean circulation and salinity15,16,17, we anticipate that a pause in these trends will have wider impacts on the Earth system. Signatures of the effects of the Montreal Protocol and the associated stratospheric ozone recovery might therefore manifest, or have already manifested, in other aspects of the Earth system.

Read more (paywalled): https://www.nature.com/articles/s41586-020-2120-4

Sadly the full study is paywalled, but I think we get the idea.

Whether we accept the study or not, either way this is bad news for climate science.

If we accept the study, climate models calibrated against pre-1980 temperatures are running way too hot, because in the pre-1980s period the anthropogenic global warming signature was being augmented by the deterioration of the ozone layer.

I’m not talking about a small calibration error. if the ozone layer recovery is strong enough to stop southern warming in its tracks, given the vast amount of CO2 we have dumped into the atmosphere in the last 20 years, then the deterioration of the ozone layer in the years leading up to the 1980s must have contributed substantially to the pre-1980s observed warming.

On the other hand, accepting the study means accepting that two powerful opposing forcings can almost perfectly balance each other for two decades when they move into opposition to each other. It is not impossible that two independent forcings have the same magnitude, but it is not terribly likely either. The easiest way to explain two powerful independent opposed forcings which just happen to perfectly balance each other, without the uncomfortable coincidence of perfect balance, is to assume neither forcing actually exists.


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