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Research  shows that there continues to be a decrease in stratospheric ozone (O₃), despite a detectable increase in the ozone levels in the upper stratosphere and at the poles.

Of most concern is that ozone, which protects animals and plants from the DNA-scrambling effects of the Sun’s UV radiation, is specifically declining in areas where most of the world’s human population lives.

It was recognised back in the 1970s that CFCs—chemicals typically used in refrigeration and aerosols—were destroying the ozone in the stratosphere (10–50 km above the Earth). The most significant damage resulted in an ozone ‘hole’ over Antarctica, and significant thinning over areas including Australia and New Zealand.

In response, the was established, outlining a timeline for the restriction and elimination of ozone-depleting chemicals across the globe. The protocol has been ratified by 196 countries and is widely considered to be the world’s most successful environment protection agreement.

The research paper, along with previous research, confirms the Montreal Protocol has led to a successful increase in ozone in the upper stratosphere in the regions near the poles. There is no doubt there, so what is causing this unexpected decrease between the poles?

Possible explanations include underestimated effects from volcanic eruptions or greenhouse gases, unexpected chemical reactions in the atmosphere, or the warming of the troposphere (up to 10 km above the Earth) which may speed up atmospheric circulation. This circulation redistributes ozone to the poles and above, meaning that ozone may be transported out of the lower stratosphere faster than previously. This would contribute in part to the recovery of the damage of the ozone layer at the higher stratosphere near the poles, but result in a thinning in the lower stratosphere nearer the equator.

However, the current atmosphere circulation models (that include climate changes) do not completely explain this depletion, so the research suggests another from ‘very short-lived substances’ (VSLSs). These chlorine- and bromine-containing chemicals are used in solvents, paint strippers, degreasers and even an ‘ozone-friendly’ replacement for CFCs. It was presumed that VSLSs did not persist long enough to reach the stratosphere and affect ozone, so they were not included in the Montreal Protocol. More research is needed.

The Montreal Protocol is definitely stalling further damage to the ozone layer, but perhaps its efficacy in promoting repair is not quite as good as thought. Changes in atmospheric temperatures might be leading to changes in atmospheric circulation, and so the positive results we’ve seen at the poles and above may actually be less impressive than we’d hoped. There is more work to do to properly identify the cause of the lower stratosphere ozone depletion and address the issue.

There still is a good news story here. This research shows that the global effort has made a positive impact on the ozone layer in the regions that were of most concern back when the protocol was established. It is hope-inspiring that with solid global policy informed by science, we can continue to work together for the benefit for all.