After more than thirty years of observation, scientists have proof that the hole in the ozone layer is shrinking thanks to the gradual elimination of the use of certain chemicals decided since the end of the 1980s. In 1974, two chemists from the University of California at Irvine, Mario Molina and Sherwood Rowland, detailed the threats posed to the ozone layer by chlorofluorocarbon gases, CFCs, as they accumulate in the atmosphere, which have only a limited capacity to absorb chlorine atoms in the stratosphere.
Cathy Clerbaux, director of research at the CNRS, specifies that the ozone layer protects from ultraviolet radiation C, B and A emitted by the Sun by filtering this radiation and that there would not have been the emergence of life without it. The ozone layer is a stratum of gas several kilometers thick that appeared 3.5 billion years ago and owes its origin to the oxygen released into the atmosphere by the very first photosynthetic organisms.
This constant supply of oxygen allowed the creation of a protective ozone layer, the transition to an oxidizing atmosphere and the appearance of oxygen breathing life forms. The fight against the hole in the ozone layer that appeared at the South Pole in the mid-1980s led to the Montreal agreements in 1985, which enacted a progressive ban on the use of CFCs.
Invented in the 1920s by Thomas Midgley Jr. CFCs are harmless to health, colorless, odorless and do not catch fire, but their great stability allows them to pass through the clouds and into the stratosphere where they are decomposed by the sun, the chlorine molecules thus separated react with the ozone. The hole was formed over Antarctica due to particular climatic conditions.
The return of UV radiation in September, at the end of the polar night, changes the chemical conditions of the stratosphere. During the polar night, the clouds of ice and nitrate that form trap chlorine, which will be reactivated by UV rays with the return of the sun and will destroy the ozone. Cathy Clerbaux reminds us that CFCs do not disappear, they remain in the atmosphere for several decades, but satellite measurements now show that the ozone layer is no longer growing and that the concentration of the gas is decreasing every year.
The damage caused by the depletion of the ozone layer remains confined to Antarctica and is clearly decreasing, as local biodiversity also seems to have been spared by the chronic exposure to UV radiation. The regeneration of the ozone layer comes from the balance between the different forms of oxygen present in the atmosphere, on the one hand, oxygen formed of two oxygen atoms and present at low altitude and, on the other hand, ozone formed of three oxygen atoms and present in the stratosphere, the solar radiation allowing the passage between these two forms. Before 1985, the presence of CFCs in the atmosphere no longer allowed oxygen atoms to be integrated into the transformation cycle and caused a decrease in ozone reserves.
Cathy Clerbaux indicates that the observations carried out since several years show that nothing can prevent the recovery of the ozone except when certain countries do not follow correctly the protocol of Montreal like China in 2018 whose production of CFC had been able to be found thanks to the study of the air currents. The forecast for the recovery of the ozone layer has thus passed from the year 2060 in former reports to 2070 from now on. It also remains to recover the CFCs in the old household appliances still present and not to release them into the atmosphere.