The ozone hole is a region of depleted layers of ozone above the Antarctic region, whose creation is linked to increased cases of skin cancer.

Atmospheric scientists have seen signs of the mending of the ozone hole above the Antarctic. Susan Solomon and co-workers report, in an article published online in Science on 30 June, that this healing is a direct result of the curb on the release of chlorofluorocarbons following from the Montreal protocol of 1987.

The ozone hole is a region of depleted layers of ozone above the Antarctic region, whose creation is linked to increased cases of skin cancer. Depletion of ozone is due to many factors, the most dominant of which is the release of chlorine from CFCs (Chlorofluorocarbons) which destroys the ozone. CFCs are released by products such as hairsprays, old refrigerators etc, and the decision taken by all countries in the Montreal protocol to ban products that release CFCs has been effective.

There are three stages in the ozone recovery process, as the researchers write in the Science paper: (a) reduced rate of decline (b) levelling off of the depletion and last (c) ozone increase linked to reduction of the levels of CFCs. And the scientists had observed the third stage of recovery.

Typically, depletion of ozone becomes significant in September and peaks in October. So the measure of the “ozone hole” in September is a marker of the extent of ozone depletion. After observing the layer since 2000, the researchers observe “fingerprints of September healing” though the decreases in the areal extent of the ozone hole and increase in the ozone column amounts.

The team has found that the ozone hole has shrunk by more than four million square kilometres since 2000. This is the year when ozone depletion was at its peak. They also determine by comparing with a simulation that this healing is due to the reduction of chloroflourocarbons in the atmosphere.

In addition, and throwing doubts in the theory of healing, there is an observed peak in the size of the ozone hole in 2015. The scientists attribute to a volcanic eruption that took place that year.

This result is remarkable in more ways than one. Of course, the shrinking of the ozone hole and its role the incidence of skin cancer is the most striking inference. Also remarkable is the way all nations cooperated in reducing their CFC emissions.

This has however been a fairly slow process of healing, slower than expected. “The Montreal protocol to ban the use of CFCs was implemented more than 37 years ago. Many people expected the ozone hole to heal quickly. This has not happened because of the long residence time of CFCs in the atmosphere and the role of natural processes such as El Nino and volcanic eruptions,” says J Srinivasan, professor at Centre for Atmospheric and Ocean Sciences and head of Divecha Centre for Climate Change, Bengaluru, in an email to this correspondent.

This comment is significant because it draws to attention to the other, much-debated issue — that of climate change and its primary causative agent, excessive emission of carbon dioxide. Like the CFCs, carbon dioxide is also an inert gas which has a long residence time. As in the case of climate change, the ozone depletion process and controlling CFCs was also debated vigorously. However, unlike the climate change debate, global leaders agreed to cut their manufacture and use of CFC releasing products, which now has had a positive effect on the ozone hole.

“The Montreal protocol showed us how the leaders of the world can work together to prevent an environmental disaster. We all hope that a similar spirit of cooperation will be seen to reduce carbon dioxide emissions in order to prevent global warming,” adds Dr Srinivasan.