|Fig. 1: A Satellite image of the recovering ozone hole covering Antarctica in 2007 obtained from a Solar Backscatter Ultraviolet instrument on an NOAA-18 weather satellite, dated 13 Sep 07. (Source: Wikimedia Commons )|
Ozone is a gas that exists in the atmosphere through a multistep, photon-driven chemical reaction in the stratosphere between the oxygen molecule O2 and UV light from the sun.  This process is critical to the health of humans because ozone gas in the stratosphere is able to absorb UV light and acts as a UV radiation shield for plants and animals. From the chemical reactions 1-3, it is shown that the creation of ozone is a reversible process, and ozone is able to react with UV light from the sun.  In the late 1980s, it was discovered that the ozone layer in the atmosphere was deteriorating, and the cause was due to halogen-containing compounds like Chlorfluorocarbons (CFCs) found in some refrigerants, aerosols, and foams.  When CFCs degrade, they release chlorine atoms into the atmosphere, which then interact with ozone according to the reactions
|O2 + hν||→||O + O|
|O + O2||→||O3|
|O3 + hν||→||O2 + O|
|Cl + O3||→||ClO + O2|
|O + ClO||→||Cl + O2|
The last two of these show the catalytic destruction of ozone by chlorine: The free chlorine atom continuously reacts with more and more ozone without itself becoming used up.  This cycle is the root cause of the ozone depletion.
In 1985, scientists found a massive hole in the ozone layer above Antarctica. This is shown in Fig. 1. While ozone concentrations vary a lot between seasons, this discovery was frightening for the future because ozone was depleting faster than it could regenerate, meaning a potential increase in the number of UV related diseases. In 1987, many countries around the world got together and discussed the consequences of the ozone depletion, and made an effort to counteract it. Together, they ratified the Montreal Protocol, which was the first treaty signed by all the countries in the world.  The goal of the treaty was to limit the use of ozone depleting substances (ODS). As new substances were found to have an effect in depleting the ozone layer, they were slowly phased out. At first, this treaty created a political problem for the United States during the Reagan Administration because a lot of the industry producing these ozone depleting substances were against the Montreal Protocol due to the implications the treaty could have on their business. However, President Reagan backed the treaty despite lobbying from the ODS producing industry. As a result, worldwide production of ODS including halogen producing chemicals like CFCs began to stop.  There have been several amendments to the treaty as more ODS have been discovered. Currently, 99% of ODS have been phased out by 197 countries. 
According to the 2014 Scientific Assessment Report by UNEP, all the ODS controlled by the Montreal Protocol had decreased levels in the troposphere by 10% since 1994.  The article by National Geographic projected that without the Montreal Protocol, the ozone layer would have collapsed by 2050, resulting in increased skin cancer occurrences and deaths in the world.  Coincidentally, ODS have been shown to cause global warming because they are also potent greenhouse gases.  Thus, not only has the Montreal Protocol directly helped slow ozone layer depletion, the treaty has also indirectly helped slow global warming by decreasing greenhouse gases that doubled as ODS. While the Montreal Protocol has been effective statistically according to the Scientific Assessment Report, there are some other harmful chemicals that haven't been phased out yet. For the Montreal Protocol to continue its success in saving the ozone layer, scientists must recognize new ODS and the treaty must be constantly be amended to ban any new ODS.
© Michelle Xiao. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 S. F. Rowland, "Stratospheric Ozone Depletion," Phil. Trans. R. Soc. 361, 769 (2006).
 J. Gillis, "Montreal Protocol, a Little Treaty That Could," New York Times, 9 Dec 13.
 S. Leahy, "Without the Ozone Treaty You'd Get Sunburned in 5 Minutes," National Geographic, 25 Sep 17.
 "Scientific Assessment of Ozone Depletion: 2014," World Meteorological Organization, Global Ozone Research and Monitoring Project, Report No. 55, December 2014.