The turning point of 1987

For more than ten years, research on the massive destruction of the ozone layer increased dramatically and alarm bells began to ring. Not only does chlorine destroy our best protection against the sun, but it is mainly of human origin. More precisely, it is a by-product of industry. In 1987, experts and the public authorities of several nations gathered in Quebec and signed the Montreal Protocol. The treaty put in place a schedule for the elimination of CFC gases, and more broadly, industrial products made up of chlorine and bromine gases whose catalytic cycles destroy the ozone layer. The ambition is to re-establish a maximum concentration of ozone.  But the first measures were not sufficient. Up to 2007, the protocol was amended or adjusted several times. Other products were added to the list of prohibited substances.

“Mankind has turned off the tap”, says Emmanuel Mahieu by way of illustration. “As long as we no longer allow the emission of these gases, it is quite logical to expect that there will be a reduction in the presence of HCl in the stratosphere”. And this is exactly what has happened. Among the obligations imposed by the protocol, an assessment of the situation must be made every four years(2). A complete report aimed in equal measure at scientists, decision-makers and a curious public. The international scientific community must task itself therefore with the development of the ozone layer, and has recorded a decrease in HCl of approximately 1% per year since 1995. This may not seem like a lot, but because CFC gases are very stable, they can remain in the atmosphere for several decades and continue to create hydrogen chloride for many years before disappearing completely.

The unexpected increase in HCl in the Northern Hemisphere

It is in this general context that, against all expectations, Emmanuel Mahieu has observed a new increase in HCl in the Northern Hemisphere. “This was quite contradictory”, the chemist explains. “While there was a decrease in the emission of the underlying causes of HCl in the atmosphere, its concentration increased in a particular place. By comparing this data with other data from other observatories, we were able to apply this trend to the Northern Hemisphere alone. In the south, no site had recorded this phenomenon”. The first reaction of the researcher was to contact a network of observers around the globe to see if they had detected an increase in the sources of this chlorine emission. “This was my first theory. Since the Montreal Protocol, all countries have to account for the chlorine gas they are still using.  But it is possible to imagine that there are rogue emissions that the authorities are unaware of, the clandestine use of an unexhausted stock of the gases or even the processing of new compound. We immediately thought of the polluting countries of the Northern Hemisphere”. But the results were unequivocal. Nobody was able to account for an increase in the sources of chlorine gases. Emmanuel Mahieu then concentrated on the second theory. The phenomenon must be linked to a variation in atmospheric circulation.

(2) Scientific assessment of ozone depletion, the last dates back to 2010, and the 2014 assessment is still under embargo.