Since the Montreal Protocol banning chlorofluorocarbon (CFC) production and consumption was finalized in 1987 and began to take effect in 1989, the world breathed a sigh of relief. Humans had saved the ozone layer. But did they?
CFC substitutes, not as destructive as the original chlorofluorocarbons, and similar chemicals known as halons continue to wreak stratospheric havoc even today, more than thirty years after the Montreal Protocol. Meanwhile, the amount of yet another significant ozone layer depleter, nitrous oxide, is increasing rapidly in the skies.
In this article, find out everything you need to know about the top 3 causes and effects of ozone layer depletion. Fortunately, solutions exist to the problem of stratospheric ozone destruction. Learn how you can help rebuild the life-sustaining ozone layer and keep it intact.
What is the ozone layer?
Just two pennies thick (0.12 in.) and encircling the Earth at approximately 10-12 miles above it, the ozone layer, also called the ozonosphere, straddles the boundary between the troposphere and the stratosphere in Earth’s atmosphere. It acts as a dynamic filter for the ultraviolet (UV) radiation streaming from the sun.
Approximately half of less harmful, longer-wavelength UV-A rays get a pass. But almost all harmful, shorter-wavelength UV-B and UV-C rays get blocked.
Here’s a diagram that shows where the ozone layer is in the atmosphere in relation to the Earth:
The ozone layer makes life on Earth possible. As science historian Dorothy Fisk said in 1934, the ozone layer is “all that stands between us and speedy death.” The ozone layer is normally thinner at the equator and thicker at the North and South Poles.
When ozone exists in the stratosphere, it offers protection to all life forms on Earth. By contrast, ground-level ozone in smog, spewn by cars, trucks, factories, and power plants, obstructs normal breathing and is associated with higher risk for developing respiratory conditions including asthma.
Named for one of the chemicals that comprises it, the ozone layer contains ozone, a highly reactive (unstable) molecule composed of three oxygen atoms. The chemical formula for ozone is O3.
First completely described by the British geophysicist Sydney Chapman in 1931, stratospheric ozone exists in a steady state of equilibrium with its diatomic cousin, O2, containing only two oxygen atoms. This is the form of oxygen that humans breathe.
According to Chapman, the O3-O2 conversion constantly makes and remakes the ozone layer, in a dynamic exchange. Originally on Earth, billions of years before humans appeared, microscopic plants in the ocean produced oxygen (O2) during photosynthesis.
Eventually, the oxygen traveled out into the atmosphere. Once in the stratosphere, short-wavelength UV radiation split the molecules, releasing oxygen free radicals. The oxygen radicals possess the sun’s high energy. Being so unstable, the oxygen radicals react with other O2 molecules, forming ozone.
In turn, when ozone is struck by UV radiation, it liberates a stable diatomic molecule (O2) and an oxygen free radical (O) in a process called photodissociation. As an energized radical, O quickly reacts with a nearby O2 molecule, forming a new ozone molecule. Then when the sun’s rays strike the newly formed O3, the entire process repeats.
These chain reactions happen continuously in the ozone layer (more appropriately called the O3-O2 layer). The global exchange between O3 and O2 in the ozone layer is approximately 300 million tons per day.
Here is a schematic that shows the chemical reactions between O3 and O2. (Note: The numerical subscripts indicate the number of atoms present in the molecule. When there is no subscript, there is just one atom.)
Besides the ability to block UV radiation, the ozone layer serves another critical function for the Earth. The pressure it exerts allows it to operate like a tight-fitting seal above the troposphere (the atmospheric layer closest to the Earth’s surface). In this capacity, the ozone layer affects Earth’s weather patterns and heat cycles.
Top 3 causes of ozone layer depletion
Ozone layer depletion refers to the situation during the O2-O3 inter-conversion in the stratospheric ozone layer (described in the preceding section) when the destruction of O3 exceeds the creation of O3. In other words, there is a net loss of ozone in favor of the formation of more O2.
Until the 1970s, people believed the ozone layer couldn’t be affected by other chemicals. This turned out to be false, as the ozone hole attests (see below).
In the ‘70s, scientists began to investigate how other chemicals could disrupt the delicate balance of chemical reactions in the stratosphere. They were surprised that ozone layer depletion could occur. Once scientists began publicly announcing their discoveries, it set off alarms.
Spurred on by citizens’ panic, some governments placed restrictions on the production and use of chlorofluorocarbons (CFCs), believed to be the major culprits in ozone layer depletion. Others called for more study before taking action, leading to further research into the ozone layer.
Decades of research resulted in conclusive evidence that chlorofluorocarbons, nitrous oxides, and halogens were the top 3 causes of ozone depletion.
1. Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs) is the name given to a broad class of chemical compounds used as refrigerants containing just three elements: chlorine, fluorine, and carbon. Thomas Ridgley, Jr., a mechanical engineer, and his assistant, chemist Albert L. Henne, invented many of the first-generation CFCs in 1928. They called them Freon.
Similar compounds called hydrochlorofluorocarbons (HCFCs) were invented later.
In 1974, chemists F. Sherwood Rowland and Mario Molina published a landmark paper in the distinguished journal, Nature. They announced their discovery that typically nonreactive CFCs, after spending decades in the troposphere close to the Earth, eventually are carried up into the stratosphere by atmospheric winds.
Like ozone, CFC molecules photodissociate upon being hit by solar ultraviolet radiation. But in this case, chlorine atoms, noted by their chemical symbol, Cl, are freed.
Encountering oxygen free radicals, present from O3 photodissociation as explained in the previous section, chlorine reacts with them, forming chlorine monoxide (ClO). In the process, the chlorine atoms bind up oxygen free radicals. This prevents them from reforming ozone.
To make matters worse, one molecule of ClO reacts with a second oxygen radical. Diatomic oxygen (O2) forms in a reaction that absorbs no UV light, leaving chlorine-free to react with yet another oxygen radical.
In other words, it’s as if chlorine hijacks oxygen free radicals, disrupting the normal O2-O3 equilibrium necessary to keep the ozone layer intact. While this is happening, solar radiation streams past, heading to Earth.
Here is a schematic that illustrates what is happening to ozone when it encounters a CFC molecule in the stratosphere:
In the stratosphere, the catalytic chain reaction involving chlorine occurs over and over until chlorine eventually encounters a methane or nitrogen dioxide molecule. They react, forming stable, heavier compounds that eventually fall back to the troposphere and land on Earth.
But the damage to the ozone layer has already been done by rogue chlorine from CFCs. In its stratospheric sojourn, a single chlorine atom destroys 100,000 or more ozone molecules. Expressed in another way, one pound of CFCs can destroy 70,000 lbs. of ozone.
Given the million or more tons of CFCs produced every year, Rowland and Marino initially predicted 20-40% of ozone layer depletion was likely if all of the CFCs made it to the stratosphere. At the time of their research, 70% of CFCs were used in aerosol spray cans for all sorts of products such as cooking spray, hair spray, and deodorant. With each spray, you’d release CFCs into the air. So most of them would eventually reach the stratosphere.
Currently, most aerosol sprays do not use CFCs. There are several alternatives including hydrofluorocarbons (HFCs) and carbon dioxide. Today, the major use of CFCs and related compounds like HFCs is in refrigerants and air conditioners. It is also used to fluff up foam and clean electronic parts.
What is the ozone hole?
The ozone hole over Antarctica in the Southern Hemisphere is not really a hole. Rather, it is an area of the stratosphere where the ozone layer has thinned considerably, allowing harmful UV radiation to reach the Earth. It begins to enlarge near the beginning of Antarctica’s spring of every year as explained below.
A similar event occurs in the Arctic, but not as dramatic. 1980s measurements revealed a 40% depletion in the ozone layer in September (the end of the Antarctic winter).
Joseph Farman at the British Antarctic Survey first observed a drastic reduction of ozone in 1981 compared to all the data he had collected since 1956. Skeptical of their veracity, he hid the data, which repeated in the following four years. Finally, in 1985, he published his measurements and shocked the world.
In 2022, the Antarctic ozone hole was approximately 9 million square miles, slightly smaller than in 2021. For comparison, that is more than twice the area of the continental U.S. The ozone hole peaked in size in 2006. Scientists state that the Montreal Protocol, the only international treaty signed by all countries, is responsible for the reduction in the size of the ozone hole.
Here is an image of it:
The ozone hole appears due to the chlorine chemistry during the frigid polar vortex that settles over Antarctica during its winter. But how does chlorine get to the remote Antarctic?
As described in the preceding section on CFCs, chlorine atoms break away from chlorofluorocarbons (CFCs) in the presence of stratospheric UV radiation. They later attach to oxygen free radicals, leading to ozone layer depletion, in a catalytic chain reaction.
However, chlorine atoms do not immediately cause ozone depletion once they arrive in the stratosphere. In fact, the freed chlorine atoms usually become part of two other chemicals and remain in the stratosphere for some time in those relatively stable forms. These chemicals are hydrochloric acid (HCl) and chlorine nitrate (ClNO3).
However, in the winter over Antarctica, polar stratospheric clouds form within the whirlpooling winds of frigid air in the polar vortex centered over the South Pole. Under these extraordinary conditions, HCl and ClNO3 react. One of the major products formed is chlorine gas (Cl2).
When spring arrives in the Antarctic, the sun’s UV radiation breaks Cl2 gas molecules apart, liberating chlorine atoms. It is these free chlorine atoms (along with bromine, another halogen) that interfere with the O2-O3 equilibrium and deplete the ozone layer. Up to 2-3% of the ozone layer can be destroyed per day by this process.
Here’s a flow diagram that illustrates the entire pathway of ozone depletion caused by the halogens, chlorine, and bromine:
Has the Montreal Protocol fixed the ozone hole?
Although the Montreal Protocol and its amendments, including the most recent Kigali Amendment added in 2016 banning hydrofluorocarbon (HFC) production go far in abolishing the manufacture of ozone layer depleters, much work remains to be done to eradicate all chemicals and practices that deplete the ozone layer.
The Protocol allows the continued use of existing CFCs and does not mandate their removal or destruction. So, in 1995, the year before US production was to end, a multi-million dollar CFC black market emerged. It continues to this day and now includes HFCs.
Further, slow phaseouts of chemicals and rule exemptions slow down the reversal of ozone layer depletion. Additionally, there are some companies illegally producing some of the most ozone-destructive CFCs in recent years. Constant surveillance and enforcement are essential. If this doesn’t happen, the ozone hole could begin to enlarge again.
Furthermore, since ozone-depleting nitrous oxides (see below) are not regulated by the Montreal Protocol, their escalating use could cause further destruction of the ozone layer well into the 21st century and beyond.
2. Nitrous oxides
In 1970, atmospheric chemist Paul Crutzen studied nitrous oxide (N2O) in the stratosphere. There, the sun’s radiation splits it into nitric oxide (NO) and nitrogen dioxide (NO2).
Further, in a series of catalytic chain reactions, NO and NO2 interfere with the O2-O3 equilibrium of the ozone layer in the stratosphere, blocking ozone formation. As a result, there is ozone layer depletion.
Here is a schematic showing these reactions:
The major source of stratospheric nitrous oxide is agriculture, especially fertilized soil and livestock manure. (It’s also used in dental offices as laughing gas.)
Unlike CFCs, nitrous oxides aren’t regulated by an international agreement. So, scientists A.R. Ravishankara, J.S. Daniel, and Robert W. Portmann of NOAA’s Earth System Research Laboratory (ESRL) concluded that, left unregulated, nitrous oxide will become the leading chemical depleting the ozone layer in the 21st century. In fact, nitrous oxide emissions are twice as high as CFC emissions today.
Catalytic chain reactions in ozone layer depletion
In working on nitrous oxide chemistry, Crutzen elucidated the general series of chemical reactions that result in ozone layer depletion.
In this article, we describe how chlorine monoxide, bromine monoxide, and nitric oxide serve as the catalysts for ozone destruction in the presence of short-wavelength UV radiation. A fourth catalyst, the hydroxyl radical (OH), produced in polar stratospheric clouds, is believed to play a key role in the formation of the other radicals and in ozone layer depletion.
All catalysts share one major thing in common: they are neither consumed or destroyed in a chemical reaction. Thus, a single catalyst is free to repeat the chain reactions hundreds of thousands of times, resulting in a net loss of stratospheric ozone. This is how ozone depletion occurs. (Note: A catalyst is noted by “R” in the following reaction series from ChemTube3D.)
R + O3 → RO + O2
RO + O → O2 + R
RO + O3 → R + 2O2
Net Reaction (in UV light): 2 O3 → 3 O2
Source: ChemTube3D
Crutzen, along with Rowland and Morina mentioned in the preceding section, won the Nobel Prize in Chemistry for their work on stratospheric ozone chemistry and the discovery of the ozone-depleting properties of CFCs and nitrous oxide in 1995.
When announcing the winners, the Royal Swedish Academy of Sciences stated:
“Even though ozone occurs in such small quantities, it plays an exceptionally fundamental part in life on earth. This is because ozone, together with ordinary molecular oxygen (O2), is able to absorb the major part of the sun’s ultraviolet radiation and therefore prevent this dangerous radiation from reaching the surface. Without a protective ozone layer in the atmosphere, animals and plants could not exist, at least not upon land.”
3. Halons (bromocarbons)
Halons are molecules containing bromine and carbon. Bromine is a member of the chemical family known as halogens, along with fluorine and chlorine.
Since chlorine atoms in chlorofluorocarbons (CFCs) are rapid ozone depleters, you might think that bromine atoms from halons would have the same destructive effect on ozone. Actually, bromine is far worse as an ozone depleter. In fact, one pound of a common halon, called 1211, can destroy 25 tons of ozone.
In the atmosphere, bromine commonly exists in the stable forms of hydrogen bromide (HBr) and bromine nitrate (BrONO2). However, when exposed to UV radiation, they break apart. Like free chlorine reacts with an oxygen radical to form the highly reactive ClO that causes a net loss of ozone, free bromine atoms do the same thing as BrO. Bromine atoms are 40 to 100 times as effective as chlorine at depleting ozone.
Here are reactions involving both chlorine and bromine showing how ozone is depleted and diatomic oxygen (O2) is formed. These coupled reactions are responsible for 30-40% of ozone layer depletion in the Antarctic.
Human-made products release up to 60% of all halons in the stratosphere. The use of methyl bromide fumigants and halon fire extinguishers are the major ways bromocarbons cause ozone layer depletion. Halons are regulated under the Montreal Protocol.
Top 3 effects of ozone layer depletion
Ozone layer depletion adversely affects humans, wildlife, marine ecosystems, plants, insects, and agriculture. When the ozone layer is thinned, more harmful UV-B radiation arrives to the Earth’s surface. Furthermore, research in 2007 suggested that more deleterious UV-C radiation, commonly believed to be blocked completely by the ozone layer, was also hitting Earth’s surface. Since then, other scientists have corroborated this conclusion.
1. Human health effects
According to the Environmental Protection Agency (EPA), the four major human health effects from ultraviolet radiation exposure are:
- Skin cancer: As the most common form of cancer, skin cancer affects 20% of people living in the United States. It is also easily preventable. Using sunscreen or avoiding the sun are proactive measures you can take to protect yourself from this carcinogen.
- Premature aging and other skin problems: Up to 90% of all skin changes attributable to aging are actually caused by the sun. UV radiation from the sun causes thick, wrinkled, and leathery skin. A common skin growth called actinic keratosis is a pre-cancer that could develop into full-blown cancer if not removed.
- Cataracts and other eye damage: UV radiation increases the likelihood that you’ll develop cloudy lenses (cataracts) that may lead to blindness if left untreated. Other eye damage includes pterygium (vision-blocking growth of tissue) and macular degeneration. To avoid these problems, wear sunglasses with 100% UV-A and UV-B protection.
- Immune system suppression: Although sunlight stimulates the production of vitamin D in the skin, believed to boost immunity, UV-B radiation is known to suppress the immune system. As a result, skin infections and cancer are more likely to occur.
2. Ocean ecosystem effects
Microscopic marine plants called phytoplankton are the foundation on which all marine food webs depend. Their populations are reduced by UV-B exposure. Consequently, there will be reduced fish stocks. Humans who rely on fish as their major source of protein and/or livelihood will experience malnutrition or economic hardship.
Phytoplankton researchers estimated that a 16% reduction in the ozone layer could result in a 5% die off of phytoplankton. Although 5% may not seem significant, this equals a loss of approximately 7 million tons of fish per year.
Source: Wikimedia / Prof. Gordon T. Taylor
UV-B radiation also negatively affects ocean productivity in other ways. For example, photosynthesis by red, brown, and green benthic algae is significantly reduced by solar radiation. Since marine photosynthesis is responsible for producing more than half of the oxygen that humans breathe, ozone depletion is a real concern when it comes to human survival.
As previously mentioned in the section on the ozone hole, early spring is when the ozone layer is the thinnest. This happens to be the time when the early developmental stages of fish, shrimp, crabs, amphibians, and other animals occur. During these critical stages, organisms are highly sensitive to adverse environmental effects. UV-B radiation exposure interrupts normal development as well as the reproductive capacity of adults, resulting in greater mortality and smaller offspring.
3. Agricultural losses
Plants are adversely affected by UV-B radiation in several ways. Research shows some of these ways are:
- Reduction in leaf area
- Decreased stem growth
- Inhibited photosynthesis
- DNA damage
- Change in the time of flowering
- Reduction in the number of flowers
Studies done on the most common agricultural species, such as rice, soybeans, winter wheat, cotton, and corn, indicate that overexposure to UV-B reduces their size, productivity, and quality. If this occurs, food insecurity or famine could result.
Agriculture is also negatively affected by the climate crisis. With global heating comes disruptions in plant flowerings. When insects aren’t present to pollinate crops that flower out of sync with insect life cycles, crop productivity plummets.
Furthermore, several chemicals known to deplete ozone, including CFCs and nitrous oxide, are potent greenhouse gases. In effect, agriculture, on which human civilization depends, is served a double whammy.
It is difficult to determine whether crop losses are directly due to global heating, or specifically due to ozone depletion. To ensure crop productivity and quality, the elimination of CFCs and nitrous oxide will help curb the climate crisis, and preserve the ozone layer as well as biodiversity.
Top 3 solutions for ozone layer depletion
There are numerous actions that governments, corporations, and private citizens can take to prevent ozone layer depletion. Here are the top 3 solutions to ozone layer depletion.
1. Use alternatives to ozone layer-depleting chemicals
Since CFCs, halons, and nitrous oxide are commonly used or produced in most countries, you can begin to reduce your dependence on them through research to assess all the ways these chemicals directly or indirectly affect your life today. Industry is searching for replacements (with less ozone depleting potential) and alternatives (with no effect on the ozone layer) all the time, so it’s necessary to stay informed.
Note: Products or services labeled “non-CFC” or “ozone layer-friendly” often contain chemicals that destroy the ozone layer to a lesser degree than CFCs. So, you may choose to opt out.
Here are some ways that you can limit your personal contribution to ozone layer depletion.
Air conditioning
Today, most coolants are hydrofluorocarbons (HFCs). Once thought to be non-ozone layer depleting, they have been found to be ozone-damaging like their CFC cousins.
As a private citizen, you can eschew conventional air conditioning in your home and car (if you own one). Instead, rely on natural methods to stay cool. These include drinking cold water, taking cold showers, using ceiling fans or opening windows when it’s cooler outside, keeping windows shaded when it’s hot outside, or planting trees around your home.
If you work in an air-conditioned office, you can request that the thermostat be set at 82℉ or higher. Wear light, loose cotton clothing, and keep an iced beverage close by.
Fire extinguishers
Although production of halons is now banned, they are still used. A common replacement is hydrofluorocarbons (HFCs). These do not deplete the ozone layer as much as CFCs, and they are powerful greenhouse gases. Some may have up to 14,000 times the warming potential of carbon dioxide. By their very nature, they are gases made to be released to the environment during firefighting.
Incidentally, carbon dioxide is an alternative material used in fire extinguishers. Investing in a home sprinkling system is an alternative way to protect yourself and your home in case of fire.
You may find fire extinguishers that use recycled halon. Contact the Halon Alternative Research Corporation (HARC) for more information. They may be able to help you recycle your used fire extinguisher (with residual halon still inside).
Polystyrene (Styrofoam)
Foam cups and containers, as well as household insulation, are usually made with CFC replacements today. Note that they have a reduced ozone layer depletion potential.
You could consider eco-friendly home insulation instead. As for cups and containers, try glass or metal.
2. Practice and consume food grown by regenerative (non-industrial) agriculture
If you’re not a farmer, the most effective thing you can do to avoid contributing to nitrous oxide emissions that head to the stratosphere (unlike those in smog that stay in the troposphere) is to stop supporting industrial agriculture. Choosing an all-organic diet, grown, by law, without synthetic fertilizers and pesticides, is key. Although organic fertilizers like mulch or compost contribute nitrous oxide, too, it’s not as bad compared to what synthetic fertilizers do. Since manure also contributes to nitrous oxide emissions, going vegan (no meat or dairy) is ideal.
Farmers who wish to reduce nitrous oxide emissions from their profession could transition to regenerative agriculture. With a focus on restoring soil health through minimal disturbance, indigenous knowledge, carbon sequestration, cover cropping, planned grazing, and enhanced biodiversity, regen ag holds promise as a way to safeguard the land for future generations.
You can get involved by supporting food products with regenerative organic certified labels. In 2023, as Congress writes the next Farm Bill, establishing policy for the next five years, sign the petition to give regen ag a prominent place in U.S. agriculture.
3. No flying
There has been speculation about creating a fleet of supersonic jets as recently as 2020. But, so far, it is considered cost prohibitive. The jets would dump nitrous oxide and water vapor (source of hydroxyl radicals, another catalyst) directly into the stratosphere. So, these jets would undoubtedly deplete the ozone layer.
Subsonic passenger planes produce nitrous oxide, but it likely stays in the troposphere where it creates the pollutant, ground level ozone. Nitrous oxide is a powerful greenhouse gas and contributes to the climate crisis. For this reason, abstaining from flying to reduce your personal carbon footprint is an important way to do so.
Key takeaways on ozone layer depletion
Without the ozone layer encircling the Earth 10-12 miles above it, life as we know it would not exist. In constant flux from the chemical reactions converting diatomic oxygen (O2) to triatomic oxygen (O3) and vice versa, the wispy belt just two pennies thick reflects harmful ultraviolet (UV) radiation back into space.
Chlorine in chlorofluorocarbons (CFCs), other halogens like bromine, and nitrous oxide interfere with the O2-O3 chemical equilibrium in the stratosphere as catalysts in a series of chain reactions. In fact, the catalysts, when bonded to oxygen radicals, effectively halt the process of net ozone formation. Lacking an ozone filter, harmful UV-B and UV-C rays travel to Earth unobstructed.
As if this wasn’t bad enough for life on Earth, CFCs are powerful greenhouse gases. As such, they contribute substantially to the climate crisis.
Today, increased release of nitrous oxide, another potent greenhouse gas, is causing significant disruption of the O2-O3 equilibrium in Earth’s stratosphere. Consequently, ozone layer depletion is still occurring. Nitrous oxide is formed during the manufacture and use of synthetic pesticides and fertilizer for industrial agriculture.
The effects of ozone layer depletion are nothing short of catastrophic. Increased cancer rates and cataracts in humans occur from UV-B exposure. Oceanic phytoplankton populations, responsible for most of the oxygen that humans breathe, plummet from UV-B exposure. Agricultural crops wither, leading to food insecurity and potentially famine.
Solutions to ozone layer depletion require system-changing adjustments to several industries supporting 21st century lifestyles in rich countries like the USA. The air conditioning sector must stop production and use of all CFCs and their chemical cousins in refrigerants. Agribusiness must stop manufacture and application of synthetic pesticides and fertilizer.
If you’re an individual striving to lead a sustainable lifestyle, you can help prevent ozone layer depletion by not relying on conventional air conditioning and agriculture. Transition to alternative ways to keep cool. Grow your own food organically or support farmers practicing regenerative agriculture.
Above all, demand that your governmental representatives take steps to write, enact, and enforce policies that require the air conditioning, refrigeration, and agricultural industries to follow ozone layer-friendly practices. Raise awareness of these problems through public protest. Encourage your friends and family to get involved.
It’s only when the necessary changes summarized here occur on all these levels that ozone layer depletion will permanently end, and the related climate crisis can be somewhat offset.
