A coordinated international effort managed to curb the depletion of the ozone layer within just a few decades. What were the risks involved? How was this accomplished? And what are the similarities and differences between the ozone layer’s restoration and the ongoing battle against global warming?

Of the many natural phenomena that have facilitated the evolution of life on Earth, the ozone layer is undeniably one of the most vital. This atmospheric layer, situated at great distance from the Earth’s surface, plays a crucial role in absorbing the majority of the Sun’s dangerous ultraviolet radiation before it reaches the ground. Without the ozone layer, we wouldn’t be able to walk around safely in the sunlight.  Many animals that have adapted to life on land might have remained in the oceans instead, where water filters out the ultraviolet radiation, providing protection.

But despite the ozone layer’s great importance to our existence, we were on the verge of completely destroying it just thirty years ago. The process of dealing with this self-inflicted environmental crisis represents a significant milestone in human history and can provide us with valuable lessons for dealing with similar challenges in the present day. So what exactly is ozone? How did we create a hole in it? And why is it no longer a prominent issue in public discourse?

The ozone layer absorbs nearly all of the most intense ultraviolet radiation (illustrated by arrows on the right) and enables life on Earth as we know it | Illustration: INNA BIGUN / SCIENCE PHOTO LIBRARY


All About Ozone

Ozone gas is continuously created in the atmosphere when ultraviolet radiation interacts with oxygen gas molecules. The radiation splits the oxygen molecules, each of which is made of two oxygen atoms (O2), into individual oxygen atoms. These single atoms then immediately react with other oxygen molecules to form molecules comprising three oxygen atoms (O3), known as ozone. Ozone molecules themselves are also sensitive to ultraviolet radiation and when it interacts with them, the ozone breaks down again into an oxygen molecule and a single oxygen atom, which once again bonds with an oxygen molecule, and so on, again and again, in a repetitive and infinite process known as the oxygen–ozone cycle. This cyclical reaction, in which radiation transforms oxygen molecules into ozone and vice versa, is in equilibrium: the rate of ozone creation is equal to the rate of its destruction, thus the concentration of ozone in the atmosphere remains steady over time. This absorption of radiation during this process is highly efficient; the oxygen-ozone cycle is responsible for the absorption of between 97 to 99 percent of all the dangerous ultraviolet radiation reaching the Earth.

The term “the ozone layer” does not refer to a discrete layer within the atmosphere. Rather, it describes a region of the atmosphere in which the concentration of ozone is higher compared to other atmospheric layers, albeit still very low: only about 8 parts per million (ppm) - only about 8 molecules out of a million in the atmosphere. The ozone layer is generally located at a height of 15–35 km above sea level, and its thickness varies across geographical regions and seasons of the year. Ozone gas is primarily found in the upper atmosphere, due to the high efficiency of the oxygen-ozone cycle, which absorbs most of the ultraviolet radiation penetrating the atmosphere. The small amount of ultraviolet radiation that reaches the lower layers of the atmosphere isn’t sufficient to produce ozone.

Generated by ultraviolet radiation from space, ozone serves as a protective shield for Earth from the harmful rays of that same radiation. An ozone gas molecule consists of three oxygen atoms | Illustration: RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY


Refrigerant Gasses vs. Ozone Depletion

Among the various new substances introduce into the atmosphere by humans since the dawn of the Industrial Revolution, the ones that had the most severe impact on the ozone layer were halogenated hydrocarbons (halocarbons): these are carbon compounds containing atoms of elements from the halogen family, found in the seventh column of the periodic table. Such hydrocarbons, particularly those containing the elements chlorine and fluorine, referred to as chlorofluorocarbons (CFCs), were adopted as refrigerant gasses in refrigerators and air conditioners back in the 1930s. Refrigerant gasses possess high heat capacity that are employed to transfer energy between different parts of an appliance. CFC gasses,  favored for their low toxicity, stability, and non-flammability compared to the refrigerant gasses used previously, gained widespread popularity. They were soon put to use for a variety of other purposes, for example, fire extinguishers and cosmetic aerosols, such as deodorants, where they served as packaging gasses to create pressure inside the canisters. It is estimated that during the latter decades of the 20th century, over a million tons of CFC gasses were produced annually.

The extensive use of CFCs also had a destructive side. Though designed to remain contained within closed cooling systems and appliances, these gasses often escaped into the atmosphere due to leaks, especially after appliances were no longer in use and were discarded. Once released into the atmosphere, these stable CFC gasses can remain there for over a hundred years, a long enough time for them to reach the upper atmosphere and the ozone layer. In this upper layer, the CFC gasses are exposed to ultraviolet radiation that breaks them down and releases the chlorine atoms trapped within them. The chlorine then breaks down ozone molecules with alarming efficiency: we now know that a single chlorine atom reaching the ozone layer can cause the breakdown of over 100,000 (!) ozone molecules before forming more stable compounds, thereby reducing the quantity of ozone at a much more rapid pace than its natural rate of regeneration.

Only in 1974, nearly 40 years after the widespread adoption of CFC gasses, did a study raise the alarm about their potential negative impact on the ozone layer and, subsequently, on the environment. Conducted by Mario Molina of the Massachusetts Institute of Technology (MIT) and Frank Rowland of the University of California, this research earned them the Nobel Prize in Chemistry in 1995. This study, along with subsequent studies, hypothesized that the damage to the global ozone layer could lead to a 2–4% decrease in ozone levels by the end of the 21st century. This concern prompted initial international efforts to address this issue. However, these efforts were relatively limited, primarily focusing on the establishment of systems to monitor the ozone layer and offering general guidelines to curtail CFC gas usage, without establishing universally binding protocols that all nations would be required to adopt and implement.

Eventually the gas makes its way into the atmosphere. Piles of refrigerators and freezers await recycling at a garbage disposal site in England. | Photo: JERRY MASON / SCIENCE PHOTO LIBRARY

An Unexpectedly Strong Impact

Another ten years would pass before the true extent of the damage was revealed and permeated global consciousness. The first evidence of a severe, unexpected impact on the ozone layer was revealed in 1985, when British researchers - Joseph Farman, Brian Gardiner, and Jonathan Shanklin - discovered that the deterioration of the ozone layer above Antarctica - a regular phenomenon, which normally occurs when the South Pole transitions from the dark winter to the sunny spring - had reached unprecedented levels compared to previous decades. According to their measurements, the concentration of ozone above the research stations on the southern continent decreased by approximately 40% at that time.

The majority of atmospheric ozone is produced near the Equator, a region where the sun’s potent radiation persists year-round. This ozone then spreads to higher latitudes eventually reaching the poles via air currents in the upper atmosphere. However, during the Antarctic winter a cold, “closed” climatic system develops over the South Pole. This system prevents air from other latitudes from penetrating, causing the supply of ozone to the ozone layer to cease until spring.

The pattern of air currents in the Southern Hemisphere did not change from the 1950s to the 1980s in a way that could explain the dramatic decrease in ozone concentration over the South Pole. The main change that did take place was the atmospheric composition, particularly the concentration of CFC gasses that reached the altitudes where the ozone layer resides. The researchers concluded that the increased presence of these gasses within the South Pole’s winter climate system had disrupted the fragile equilibrium of the oxygen-ozone cycle in the region, and that this occurred at a much faster rate than predicted by theoretical studies that were conducted a few years prior. These alarming findings raised the concern that the area of the “ozone hole”—the colloquial term for this phenomenon that describes the highly depleted region of the ozone layer—would continue to expand year after year. This expansion could endanger the populations of Australia, New Zealand, and southern South America, and, in the more distant future, extend its threat to residents of more northern latitudes.

Unprecedented levels. A simulation of the concentration of ozone above Antarctica by NASA’s TOMS satellite in 1983. The dark regions represent low ozone levels | Source, NASA, The Commons


An Effective Battle

The startling discovery in Antarctica’s skies in 1985 spurred greater investment in ozone research and led to the signing of the Montreal Protocol by all UN member nations in 1987. The Protocol set rules for a gradual cessation of the production and use of CFC gasses and other halogenic substances that has been identified as hazardous to the ozone layer. Developed countries were to completely halt the use of CFC gasses by 1996, with the cessation of other halogenic gasses to follow gradually. Furthermore, within the framework of this agreement an international fund was established to provide economical and technical support to developing countries, to enable them to meet similar goals. In subsequent years the agreement was amended and refined and stricter measures were introduced.

The concerted international effort bore fruit. By the early 2000s, CFC gas production and use had nearly come to a halt globally, and by 2009 the use of most other gasses included in the agreement also ceased. The positive effects on the composition of the atmosphere were soon observed. By the late 1990s, the increase in  halogenic gas concentrations over the South Pole had been curbed, and in the last decade a decrease was detected. As a result, the previously unrelenting decrease in the concentration of ozone above the South Pole, which characterized much of the latter decades of the 20th century, has halted, and according to certain studies the trend has even reversed and the concentrations have now begun to gradually rise.

But the path to complete recovery of the ozone layer remains extensive, filled with ups and downs due to the climate system’s inherently high variability. In 2005 the circumference of the ‘ozone hole’ was the greatest ever measured, reaching about 27 million square km, more extensive than the area of Russia. Then again, in 2015, the hole’s circumference expanded to over 28 million square km. But despite these isolated incidents, the consensus within much of the scientific community is that the ozone hole is shrinking, and adhering to the Montreal Protocol should eliminate human-induced damage to the ozone layer by 2065.

The Montreal Protocol is widely regarded as the most successful international agreement in history, and uniquely, the only one so far to have been signed by every UN member nation. It is widely credited with preserving not only human life as we know it, but also with the survival of many fragile ecosystems. It is estimated that without protective measures to protect the ozone layer, and if the release of harmful gasses had continued unabated, the ozone hole would have spread to encompass the entire Earth by 2040. In such a scenario, prolonged outdoor exposure to daylight would have dramatically impacted life expectancy.

A Landmark Accord. The number of nations signing the Montreal Protocol by geographic region and year of joining the UN | Source: Our World in Data, Wikipedia


From Crisis to Crisis

Parallels abound between the ozone hole crisis and the ongoing global warming crisis. Both have been instigated by human activity that leads to uncontrolled release of gasses to the atmosphere, threatening existing living conditions on Earth and potentially causing  widespread human suffering. However, the manner in which humanity has addressed the ozone hole crisis and its success in managing it stands in stark contrast to the current situation with global warming.

The primary reason for the difference in the way humanity responds to the two crises relates to the measures necessary to undertake in order to turn the wheel back. Ceasing the use of CFC compounds was relatively easy. There was a range of potential alternatives available, and the transition to using these alternatives did not significantly impact any nation’s economy. In contrast, solving the global warming crisis demands much more radical steps, primarily related to reducing fossil fuel usage, the nearly exclusive energy source for most of the world’s countries. The complex nature of this requirement has no simple solutions, as existing solutions demand sacrifices that many nations, industries, and individuals are reluctant to make.

Another complicating factor in dealing with the climate crisis, compared to the ozone hole crisis, is related to the nature of contemporary interactive communication. A significant proportion of today’s society relies heavily on social media platforms for  a significant part of their information, platforms not necessarily geared towards providing reliable information or presenting broad, diverse and comprehensive viewpoints. Frequently these platforms encourage exposure to extreme perspectives, as such views generate heightened engagement on social media.

The prevalence of social media in the lives of humans today is seen as one of the reasons for the growing polarization over the past decade between opinions advocating for climate protection, often associated with the socio-economic left, and those arguing against curbing global economic growth, generally more aligned with the socio-economic right. In such a polarized landscape, opposing camps entrench themselves against one another, rather than seeking common ground and cooperative solutions to a matter crucial for human existence on Earth. The influence of social media, and the associated social dynamic processes, was virtually non-existent when the Montreal Protocol was established over 30 years ago, prompting us to ponder how this discourse would have unfolded in the context of the present day. 

The ozone hole crisis imparts lessons on the surprising ease with which humans can influence natural processes on Earth, even those that at first glance appear insurmountable and beyond the scope of human impact. It also underscores the essential role of science and the scientific community in shaping decision-making processes as well as human relationship with the environment. The increasing politicization of science, coupled with the outright dismissal of scientific evidence by entire political factions, poses collective risks and could potentially lead to situations in which ongoing and future climate crises are not treated with the same responsible management and rigorous attention that enabled the effective and successful resolution of the ozone hole crisis.