A synergy between several observers

To verify his hypothesis, Emmanuel Mahieu called upon a network of researchers who enthusiastically joined the research programme. “We needed to verify this data with other instruments. After comparing them with the results from the network of sites on the ground, we wanted to use space instruments. By chance, three satellite missions were responsible for measuring these parameters. These were; HALOE which stands for HALogen Occulation Experiment, MLS for Microwave Limb Sounder and ACE for Atmospheric Chemistry Experiment. The first two are American and the third is Canadian, but with a Belgian involvment.  The data from the three instruments had just been calibrated and compared at the Jet Propulsion Laboratory of the California Institute of Technology. I contacted them and asked them if they wanted to join us. They accepted and sent us their data. The data confirmed the trends we had observed from the ground with the same level of precision. There was therefore no instrument-based bias”.

The final stage to be completed was modelling. Once again, Emmanuel Mahieu knew whom he could depend on and he contacted Martyn Chipperfield at the University of Leeds and Thomas Reddmann of the Karlsruhe Institute of Technology in Germany. Both developed a 3D model representing the circulation and behavior of the atmosphere. These models take account of the gases, the interactions between the component parts of the air and solar radiation; they include meteorological data, wind temperatures and atmospheric pressure etc. Both at Leeds and at Karlsruhe, by taking account of the observed scenarios, the researchers were able to simultaneously reproduce the increase in HCl in the stratosphere and its decrease in the troposphere. 

Other meteorological conditions included in the models

In order to better understand this phenomenon and to try to isolate the causes, Martyn Chipperfield had the idea of carrying out a particular experiment. Now that he had modelled all the factors leading to the phenomenon observed by Emmanuel Mahieu, he could explain the different ways, to observe what could change and therefore determine the causes of the two climatic phenomena.  He therefore modelled the developments of HCl for each year between 2000 and 2011 by imposing on each of the years in question the meteorological conditions of the year 2000.  This initiative should make it possible to see if under other atmospheric conditions the level of HCl continued to drop as opposed to increasing in 2007. “And indeed, by doing this, we found that there was no longer an increase in HCl, but rather a monotonous decrease similar to the one we had expected. By isolating the dynamic factor, Martyn Chipperfield showed that it was indeed due to a change in atmospheric circulation that was responsible for the increase in hydrogen chloride”. This conclusion made it possible to confirm that the Montreal Protocol was a success. Not only that, in the Southern Hemisphere, HCl continued to decrease while in the Northern hemisphere it was to be expected that the recent increase would be cancelled out. On a global scale, HCl would continue to show a decrease in the long-term.