Abstract
This paper reviews the science behind ozone depletion and recovery, highlighting how robust scientific evidence and multilateral environmental agreements—particularly the Vienna Convention and Montreal Protocol—collectively transformed a global environmental crisis into a rare success story of planetary regeneration.
Introduction
Multilateral Environmental Agreements (MEAs) are legally-binding treaties managed by United Nations (UN) organisations to address environmental challenges. They cover different areas for e.g. biodiversity, climate change, pollution control or wetlands. The process till an MEA gets adopted is not that easy, as it requires strong negotiations and lobbying, given that each country has a specific context and its own stand on the topic in question. After signing, countries are expected to ratify the MEA i.e. they become parties to it and implement its various requirements. Interestingly, the official names of MEAs include names of cities, wherein the sigining ceremony took place. Signed on September 16th 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer, referred to as the Montreal Protocol hereafter in this paper, is among the most successful MEAs till date.
Understanding the Nature of Ozone
Ozone (O3) is a colourless and odourless gas. With its 3 oxygen atoms, it is also molecular just as the conventional oxygen (O2) gas, vital for life on Earth. It exists naturally or may be extracted from air for further industrial purposes such as water treatment due to its oxidising properties. Ozone gas has a dual but opposing role for the environment. When found in the lower atmosphere, also known as the troposphere, ozone behaves as a pollutant. It gets formed by chemical reactions between pollutant gases emitted from vehicles, industries or other combustion sources, and is often detected in photochemical smog. It leads to respiratory diseases and irritations of eyes, nose or throat. The World Health Organization has even set a guideline [1]value for ozone as an air pollutant. Moreover, tropospheric ozone gas contributes to climate change due to its high Global Warming Potential (GWP). On the other hand, the same substance plays a totally different role for the Earth when present in a region of the upper atmosphere known as stratosphere (approx. 15 to 50 km above the surface of the Earth.) As a matter of fact, stratospheric ozone molecules act as a protective shield around our planet within a very thin radius. This is often described as the ozone layer and represents the ‘good ozone’. The ozone layer prevents Ultra-Violet radiation (specifically UV B rays) from the sun to reach the surface of the Earth. It has been found that the UV B rays can cause skin cancers, eye cataracts and affect the immune system of mankind. Moreover, other forms of life are also at risk from the UV B rays, e.g. cultivated crops may get dried up in case of too much sunlight exposure. This can in turn cascade into food security issues and other societal problems. Unfortunately, the use of certain chemical substances is directly linked to the damage caused to the ozone layer.
Understanding Ozone Layer Depletion
Originally considered as safe and non-toxic for the planet, synthetic chemical substances known as ChloroFluoroCarbons (CFCs) as well as halons were massively produced since the 1930s. These molecular substances had various applications, main ones being refrigerants, coolants, propellants or solvents. However, in the second half of the last century, these uses plummeted when it was confirmed that the CFCs and halons were the actual ‘culprits’ behind ozone-layer depletion. Being gaseous, they can easily rise up to reach the ozone layer wherein they release chlorine or bromine free radicals (i.e. excited forms of atoms). Through a series of chemical reactions, the damage caused to the ozone layer was thus inevitable. It was even elucidated that one such free radical could destroy up to 100 000 stratospheric ozone molecules leading to the gradual thinning of the protective layer. At some spots, the extent of depletion was so deep that images of the Earth depicted as if ‘holes’ had been created. The phenomenon was more pronounced at the polar regions as prevailing conditions favoured a greater destruction of ozone resulting in holes as big as Antarctica.
Science has indeed been instrumental in providing explanations about the role of ozone and the damage caused by Ozone Depleting Substances (ODS), yet another term used for CFCs and other similar pollutants. Scientific evidence on this new phenomenon started pouring in late 1960s. The ground-breaking point was in 1974, when Dr. M. Molina and Prof. F. S. Rowland published a research paper in the journal Nature, that depicted the threat to the ozone layer from CFCs. These two US-based chemists together with Prof. P. Crutzen, atmospheric scientist, researching in West Germany, later on, received the 1995 Nobel Prize in Chemistry further to their [2]work. This should be considered as an outstanding recognition since there have not been many Nobel Prizes, all-fields included, that bear direct relevance to the environment. Moreover, other eminent scientists have helped adding new knowledge on stratospheric ozone as a result of their works for instance by conducting surveys in the Antarctic regions (Farman et al, 1985). As a result of technological advances in satellite imaging and computer graphics, ozone science has been better understood and accepted with time.
Responding to the Challenge
In the 1980s, ozone layer depletion cropped up as a new environmental challenge to the world. Under the guidance of the UN Environment Programme (UNEP), policy makers at national level, readily converged on actions to address the matter. Accordingly, an international treaty named the Vienna Convention for the Protection of the Ozone Layer came into existence in March 1985. Originally signed by only 28 countries, this MEA was eventually ratified by 197 states and the European Union thereby achieving unanimous agreement over its framework. In a short lapse of 2 years, it paved the way to the Montreal Protocol which also achieved full ratification. Through these 2 MEAs, CFCs began to experience production and trade bans. Alternative substances with reduced halogen contents, namely HydroChloroFluoroCarbons (HCFCs), were thus introduced, but the UNEP called for their gradual phase-out as well. It must be stated that new technologies, featuring other refrigerant gases such as ammonia, can prove costly or present safety risks. Another group of substances that emerged in the process, namely HydroFluoroCarbons (HFCs), were found to be safe for the ozone layer, but being a greenhouse gas it possess high Global Warming Potential (GWP). In other words, the benefit is overshadowed by the HFCs’ ability to cause climate change. The alternative proposed have fewer chlorine atoms i.e. HCFCs, and no chlorine atoms i.e. HFCs. Also some CFCs/HCFCs may also contain bromine atoms like bromochlorodifluoromethane. Both chlorine and bromine will behave similarly in their chemical reactions with ozone molecules.
The UNEP further underpinsned its own commitment to address this challenge by establishing a dedicated legal department and a secretariat for ozone matters. Ministries or departments responsible for the environment within the parties to the Montreal Protocol are thus called upon to collaborate with the latter. It may be asserted that parties have the obligation to submit respective inventories of CFCs produced and consumed. Alike to legislations, MEAs are also subject to changes with time. In this context, the parties agreed to adopt the Kigali Amendment in October 2016, which came in force three years later. A number of laudable measures are proposed under the latter for e.g. the phase-down of HFCs via baseline targets and provision of further assistance to developing countries. Interestingly, the Kigali Amendment calls for greater stakeholder engagements by national ozone units. Such stakeholders include customs and border control departments and other enforcing agencies with regards to trade of refrigerant gases and related equipment; the private sector for the trade, labelling, installations, repairs and maintenance of various cooling and air conditioning equipment employing such gases, and the training and academic institutions providing technical courses in these areas.
Growing Popularity and Good News
From 1990s onwards, ozone layer depletion got more popular in the media resulting in greater awareness to the layman. With the 2 MEAs already in place, ozone layer depletion also featured in the discussions of the 1992 UN Conference on Environment and Development (UNCED), which was a [3]landmark gathering for policy makers, NGOs and other groups. In December 1994, the UN General Assembly adopted a resolution proclaiming that every 16th September be observed as the International Day for the Preservation of the Ozone Layer, popularly termed as ‘Ozone Day’. The resolution was highly symbolic as the Montreal Protocol was signed on that date. Accordingly, the year 2025 marks the 30th anniversary of the Ozone Day. Moreover, with regulatory bans popping up, the popularity grew further as commercial products like aerosols, deodorant sprays and refrigerators started displaying ‘CFC-free’ or ‘Ozone-friendly’ labels and signs. However, such types of environmental claims by manufacturing industries and distributors have also contributed to the ‘greenwashing’ phenomenon in some cases. On a different, but highly important note, it did not take much time for academic institutions to incorporate ozone science in their programmes, especially in the chemistry syllabus, thus ensuring that new generations become familiar about this environmental aspect. Latest scientific evidences show that the holes in the ozone layer are gradually being fixed. It is estimated that the ozone layer can regain its original surface area around the 2060s. In other words, this signifies good news for the good ozone. It will be indeed a major victory not just for the UN or the scientific community but for humanity at large. In many cases, environmental damage tends to be irreversible, while for ozone layer depletion, history is in the making. With more advanced technologies and techniques, compared to those of the 80s / 90s era, it is easier to get real-time data and follow the evolution of the healing process. Such a situation has been made possible by collective actions undertaken since 40 years ago with the advent of the Vienna Convention backed up by scientific evidences. However, there should not be room for complacency, for instance any previous stocks of ODS should be prevented from re-entering the market illegally. Also, further research is needed in alternative technologies for cooling and refrigeration processes to be affordable and sustainable especially for developing countries.
Conclusion
It can be stated that protecting the ozone layer has witnessed a worldwide mechanism since past few decades. The chosen theme by the UNEP for the 2025 Ozone Day namely ‘‘From science to global action’’ is highly justified. From their respective messages for the occasion, His Excellency, Mr. A. Guterres, UN Secretary General, and Dr. I. Andersen, UNEP Executive Director, both expressed appreciation of the role of multilateralism for the healing the ozone layer. As a matter of fact, 2025 was a triple anniversary year for the following reasons: the signing the Vienna Convention (40 years), observing the first Ozone Day on September 16th (30 years) and the award of the Nobel Prize in Chemistry (30 years). Besides the celebratory aspects, further progress is expected for the complete reconstitution of the ozone layer in decades to come. Better news is thus awaited with new milestones to come. Above all, the status of the ozone layer demonstrates the endless contribution of science to mankind’s survival on Earth.
In conclusion to this paper, I could not resist in citing these meaningful lines from the December 1995 Nobel Prize lecture of Prof. P. Crutzen: “To the generation of Jamie Paul (his grandson) and our future grandchildren, who will know so much more and who will celebrate the disappearance of the ozone hole. I hope you will not be disappointed by us.”
References:
- United Nations Environment Programme
- World Health Organization
- Environmental Protection Agency (US)
- National Oceanic and Atmospheric Administration (US)
- The Nobel Foundation
- British Antarctic Survey
- Report of the United Nations Conference on Environment and Development
- OzonAction, UNEP
- Kigali Amendment Factsheet
- Ozone Secretariat, UNEP
- Paul J. Crtuzen, Nobel Prize Lecture
[1] The latest value is 100 μg/m3 over 8-hours period (WHO, 2021)
[2]Due to their “work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone”, the Royal Swedish Academy of Sciences jointly awarded these three former scientists the 1995 Nobel Prize in Chemistry.
[3] The Conference was popularly called the Earth Summit or the Rio Conference and laid foundations for international environmental legislations.
