The ozone hole was the major concern of the 1980s and 1990s. The pressure has since dropped significantly. And with good reason: the situation would improve significantly, to the point of hoping for a full restoration of the ozone layer in the next four decades. At least, that is the observation of the Montreal Protocol’s Scientific Review Group, which has been commissioned by the UN to oversee the development of this infamous hole.
“If current guidelines remain in place, the ozone layer is expected to return to 1980 levels by about 2066 over Antarctica, 2045 over the Arctic and 2040 in the rest of the world,” the scientists said in a report released Monday. While this is (rare) good news on the environmental front, be careful not to be too hasty in declaring victory. 20 Minutes takes stock in four questions.
What is the ozone layer?
“Ozone is everywhere in the atmosphere,” recalls Didier Hauglustaine, climatologist at the Laboratory of Climate and Environmental Sciences (LSCE). But 90% is still in the stratosphere, that second layer of Earth’s atmosphere, between 12 and 50 km above sea level. “And in this stratosphere there is this part between 20 and 30 km altitude where the ozone concentration is highest,” continues the CNRS researcher. It’s the ozone layer. »
Size precision: if the molecule is the same, the ozone present in the stratosphere is not produced in the same way as surface ozone. “The latter arises from nitrogen oxides produced by human activities and is considered an air pollutant,” explains Didier Hauglustaine. In the stratosphere, it is much more solar radiation that creates ozone by breaking down the oxygen molecules present at these altitudes. This process makes the ozone layer so valuable. “It absorbs a large part of the ultraviolet radiation before it reaches the surface,” continues Didier Hauglustaine. In particular, ultraviolet B and C, the most harmful to organisms. In short: By preserving the ozone layer, we avoid millions of skin cancer and cataracts. But Didier Hauglustaine invites us to look further: “The ozone layer is just as important for the emergence of life on the terrestrial layers of the earth as oxygen. »
How was the great threat averted?
The first warnings come from the 1960s and 1970s, “when we realized the negative effects of supersonic aircraft like the Concorde [qui volent dans la stratosphère]due to their emissions of nitrogen oxides on the ozone layer,” says Didier Hauglustaine. But the big blow came in the 1980s: Several observations back then identified an inexplicable hole in the ozone layer over Antarctica, forming every September and growing for several months before it shrank and then reappeared the following southern spring. Very quickly, chlorofluorocarbons (CFCs) are in the crosshairs. “Widely used in the refrigeration industry (refrigerators, air conditioners, etc.) or as solvents, these fluorinated gases have long lifetimes that give them Didier Hauglustaine explains that they have the opportunity to rise into the stratosphere, where they release chlorine and bromine, molecules that destroy ozone. »
The Montreal Protocol, signed in 1987 and now ratified by 198 countries, banned these substances in industrialized countries from 1996 and in developing countries since 2010. Several changes will follow to extend the bans identified by manufacturers to CFC substitutes. The latest: the Kigali Agreement signed in 2016, which provides for the phase-out of hydrofluorocarbons (HFCs). This international cooperation has borne fruit. In any case, the report published on Monday confirms that the concentrations of chlorine and bromine continue to fall, albeit slowly.
So is this famous hole behind us?
Not yet. It still happens that the ozone hole is particularly large and long in certain years. This was the case over Antarctica in 2021, the year in which it reached 23 million km2 (more than 40 times the area of France). However, these fluctuations are strongly influenced by meteorological conditions and do not call into question progress in rebuilding the ozone layer,” write the authors of the report. “However, this shows that we have not yet returned to levels of chlorine in the stratosphere that make it possible to avoid the occurrence of a hole in the ozone layer every year,” says Didier Hauglustaine.
In short, this is not the time to slack off. And that is the warning of the report released on Monday. “There’s always a risk that it will go back,” warns David Fahey, co-chair of the assessment group. Between 2012 and 2019, measurements have also shown an increase in theoretically banned CFC-11 emissions. The increase was largely attributed to factories in eastern China, Le Monde reports. And if Beijing, under pressure, has taken action to stop it, Didier Hauglustaine makes this evasion of legislation a first point of vigilance. “Another threat is the big return of supersonic or even hypersonic aircraft projects that fly in the stratosphere and could have an impact on the ozone layer,” adds the climate scientist.
Can we fight both for the ozone layer and against global warming?
Synergies are possible, if only because the gases that weaken the ozone layer can also contribute to global warming. This is especially true for HFCs, which are much less ozone-depleting than CFCs but are potent greenhouse gases. Didier Hauglustaine also recalls that “the increase in greenhouse gases warms the surface and lower layers of the atmosphere, but has the opposite effect of cooling the stratosphere”. “However, the low-temperature years at these altitudes are also those in which the ozone hole is most pronounced and lasts the longest,” the climate researcher specifies.
However, “killing two birds with one stone” does not always work. Sometimes fighting climate change makes us fear the worst for the ozone layer. Monday’s report specifically points to geoengineering projects, which would consist of venting aerosols into the stratosphere, as a measure of vigilance. The idea? Make them act like a parasol, sending rays of light back into space, cooling the climate. And Didier Hauglustaine finally: “It would also have the effect of destroying the ozone layer. »