The ozone layer is better, but we must not declare victory too quickly

• Within four decades, the ozone layer, which protects the Earth from dangerous solar radiation, could be replenished, estimated on Monday a group of experts under the aegis of the United Nations.

• The Montreal Protocol and its amendments, gradually banning the use of gases that reduce the ozone layer, have thus borne fruit.

• How did we go from great anxiety in the 1980s to an environmental problem on the way to being resolved?

  20 minutes

  returns, thanks to the lighting of the climatologist Didier Hauglustaine, on this rare good news on the front of the environment.

  However, this progress remains fragile.

The hole in the ozone layer was the great anxiety of the 1980s and 1990s.

The pressure has since dropped significantly.

And for good reason: the situation would be clearly improving, even to the point of hoping for the complete reconstitution of the ozone layer in the next four decades.

This is at least the observation drawn up by the scientific evaluation group of the Montreal Protocol, charged by the UN with monitoring the evolution of this infamous hole.

“If current policies remain in place, the ozone layer is expected to return to 1980 values ​​by approximately 2066 over Antarctica, 2045 over the Arctic, and 2040 in the rest of the world,” thus assure these scientists in a report made public on Monday.

While this is (rare) good news on the environmental front, be careful not to declare victory too quickly.

takes stock in four questions.

What is the ozone layer?

"Ozone is everywhere in the atmosphere", begins by recalling Didier Hauglustaine, climatologist at the Laboratory of Climate and Environmental Sciences (LSCE).

But 90% are still nestled in the stratosphere, this second layer of the Earth's atmosphere, between 12 and 50 km above sea level.

“And in this stratosphere, there is this part, between 20 and 30 km in altitude, where the concentration of ozone is the highest, continues the CNRS researcher.

It is the ozone layer.

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Size precision, if the molecule is the same, the ozone present in the stratosphere is not generated in the same way as surface ozone.

“The latter is produced from nitrogen oxides resulting from human activities and is considered an atmospheric pollutant, explains Didier Hauglustaine.

In the stratosphere, it is much more solar radiation which, by breaking the oxygen molecules present at these altitudes, produces ozone.

It is this process that makes the ozone layer.

“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 cancers and cases of cataracts.

But Didier Hauglustaine invites us to see even further: “The ozone layer is just as important as oxygen for the appearance of life on the terrestrial layers of the Earth.

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How was the great threat averted?

The first alerts date back to the 1960s and 1970s, "when we realized the negative impact of supersonic aircraft like the Concorde [which fly in the stratosphere], on the ozone layer, due to their emissions nitrogen oxide", says Didier Hauglustaine.

But the big slap came in the 1980s. Several observations then noted an unexplained hole in the ozone layer above Antarctica, forming each September and growing for several months before shrinking and then reappearing. the following austral spring.

Very quickly, chlorofluorocarbons (CFCs) find themselves in the crosshairs.

“These fluorinated gases, widely used in the refrigeration industry (refrigerators, air conditioners, etc.) or as solvent agents,

have a long life which gives them this ability to climb into the stratosphere, explains Didier Hauglustaine.

There, they release chlorine and bromine, molecules that will destroy ozone.

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The Montreal Protocol, signed in 1987 and now ratified by 198 countries, then banned these substances, from 1996 for developed countries and since 2010 for developing ones.

Several amendments will follow to extend the bans to CFC substitutes found by manufacturers.

The latest: the Kigali agreement, signed in 2016, which is phasing out hydroflurocarbons (HFCs).

This international cooperation has borne fruit.

The report published on Monday confirms in any case that the concentrations of chlorine and bromine continue to fall, although slowly.

So, is this famous hole behind us?

Not yet.

It still happens that the hole in the ozone layer is particularly large and long in certain years.

This was the case in 2021, above Antarctica, the year during which it reached 23 million km2 (more than 40 times the area of ​​France).

Admittedly, these variations are largely influenced by meteorological conditions and do not call into question the progress in the reconstitution of the ozone layer”, write the authors of the report.

"This shows all the same that we have not yet returned to levels of chlorine in the stratosphere which make it possible to avoid the appearance of a hole each year in the ozone layer", indicates Didier Hauglustaine.

In short, this is not the time to relax the efforts.

And this is the warning of the report published on Monday.

"There are always risks of going back," warns David Fahey, co-chair of the evaluation group.

Between 2012 and 2019, moreover, measurements have shown an upsurge in emissions of CFC-11, theoretically prohibited.

The increase was largely attributed to factories located in eastern China, reports

.

And if, under pressure, Beijing has taken measures to stop it, Didier Hauglustaine makes these circumventions of the legislation a first point of vigilance.

"Another threat is the great return of supersonic or even hypersonic aircraft projects, flying in the stratosphere and which could have an impact on the ozone layer", adds the climatologist.

Can we both fight for the ozone layer and against global warming?

There are possible synergies, if only because the gases that weaken the ozone layer can also contribute to global warming.

This is particularly the case with HFCs, which affect the ozone layer much less than CFCs, but are on the other hand powerful greenhouse gases.

Didier Hauglustaine also recalls that "the increase in greenhouse gases warms the surface and the lower layers of the atmosphere but has the opposite effect of cooling the stratosphere".

"However, the years when the temperatures are low at these altitudes are also those during which the hole in the ozone layer is the most marked and lasts the longest", specifies the climatologist.

our climate dossier

However, "killing two birds with one stone" does not always work.

Sometimes, the fight against climate change leads us to fear the worst for the ozone layer.

Monday's report points in particular, as a point of vigilance, to geo-engineering projects which would consist of dropping aerosols into the stratosphere.

The idea?

Let them act like a parasol by sending light rays back into space, allowing the climate to be cooled.

And Didier Hauglustaine to conclude: “It would also have the effect of destroying the ozone layer.

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A hole only above Antarctica?

"It is in this region of the globe that the conditions are most favorable to the appearance of this hole in the ozone layer", replies Didier Hauglustaine.

However, this destruction of ozone does not only affect Antarctica.

"In 2020, this also concerned the Arctic, even if the conditions are less favorable there," recalls the climatologist.

Anyway, this hole in the ozone layer concerns us all: "when the polar vortex (mass of cold air) disappears, the masses of ozone-poor air above Antarctica then mix with the surrounding air masses, so that the ozone concentration decreases everywhere.

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