Why Is Ozone Important in the Atmosphere?

Ozone is a trace gas that makes up only a few parts per million of the atmosphere, yet it performs two functions no other molecule can replace: it absorbs the sun’s most biologically damaging ultraviolet radiation before it reaches the surface, and it acts as a greenhouse gas that shapes temperature patterns from the ground up through the stratosphere. The balance of ozone in the atmosphere has direct consequences for human health, crop yields, marine ecosystems, and global climate, which is why its partial destruction over the past half-century prompted one of the most successful international environmental agreements ever enacted.

How the Ozone Layer Filters Ultraviolet Radiation

Most of the atmosphere’s ozone sits in the stratosphere, roughly 15 to 35 kilometers above the surface. Sunlight drives its creation: short-wavelength ultraviolet radiation splits ordinary oxygen molecules into individual oxygen atoms, which then combine with other oxygen molecules to form ozone. That ozone, in turn, absorbs ultraviolet radiation and breaks apart again, releasing heat. The cycle constantly builds and destroys ozone, maintaining a thin but remarkably effective shield.

Early atmospheric chemistry assumed this simple creation-and-destruction loop, known as the Chapman cycle, was the whole story. Research showed otherwise. The Chapman reactions and atmospheric circulation account for only about 20 percent of the ozone destroyed below 45 kilometers; additional chemical pathways involving nitrogen, hydrogen, chlorine, and bromine radicals are responsible for the rest.¹ That discovery was critical, because it meant human-made chemicals entering the stratosphere could accelerate ozone destruction far beyond what natural cycles would produce.

What Happens to Health When the Shield Thins

When stratospheric ozone declines, more ultraviolet-B radiation reaches the ground. UV-B is the wavelength band most responsible for sunburn, but its effects go well beyond red skin. Increased UV-B exposure damages the eyes, the immune system, and the skin. Chronic eye conditions expected to increase with ozone depletion include cataracts, corneal growths, and ocular cancers. For light-skinned populations, solar UV exposure is the most important environmental risk factor for basal cell carcinoma, squamous cell carcinoma, and melanoma.²

These are not hypothetical concerns. Modeling of the Montreal Protocol’s effects has estimated how many cases of skin cancer and cataracts were avoided in the United States alone by phasing out ozone-depleting chemicals.³ The immune system effects are subtler but equally concerning: UV-B suppresses certain immune responses at the skin’s surface, potentially reducing the body’s ability to fight infections and detect abnormal cells. The ozone layer, in other words, is not just preventing sunburns. It is holding back a broad spectrum of biological harm.

Effects on Ecosystems and Food Production

The consequences of ozone thinning extend far beyond human skin. In the Southern Ocean around Antarctica, where the ozone hole is most severe each spring, researchers have documented direct harm to the phytoplankton that form the base of the marine food web. A six-week cruise in the Bellingshausen Sea during the 1990 austral spring found that as the ozone layer thinned, UV-B inhibition of photosynthesis increased, with an estimated 6 to 12 percent reduction in primary production during the study period.⁴ Later modeling efforts looking at the entire Southern Ocean found that sea ice cover and UV attenuation within the water column limit the damage when integrated across depth, keeping the net loss of deep-water production under the ozone hole to less than a quarter of a percent.⁵ The surface waters, however, take a much harder hit, and surface productivity matters enormously for the organisms that feed there.

On land, plants face a similar threat. High-level UV-B radiation damages DNA, generates harmful reactive oxygen species inside cells, and impairs photosynthesis.⁶ These stresses overlap with another ozone-related problem that operates through an entirely different mechanism: ground-level ozone pollution, which enters leaves through their pores and damages them from the inside.

Ground-Level Ozone Is a Different Problem Entirely

The same molecule that protects life in the stratosphere is a pollutant at ground level. Tropospheric ozone is not emitted directly; it forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds from vehicle exhaust, industrial emissions, and other sources.⁷ Cities with heavy traffic and abundant sunshine are especially prone to high ozone days, and the pollution often drifts downwind into surrounding regions.

Breathing ground-level ozone is harmful. Its oxidative properties damage the cells lining the airways, triggering inflammation and hyperreactivity. Repeated exposure supports a pattern that combines features of asthma and emphysema, and ozone can worsen the effects of other air pollutants.⁸ Both short-term spikes and long-term exposure have been linked to increased death rates from respiratory and cardiovascular disease, with effects observed even at concentrations below current regulatory limits.⁹ Longitudinal studies have also tied long-term ozone exposure to faster declines in lung function and progression of emphysema.¹⁰

Crops suffer, too. Ozone enters plants through stomata on the leaf surface, where it triggers the production of reactive oxygen species that damage chloroplasts, block the machinery of photosynthesis, and cause visible yellowing. Over time, this reduces growth, nutrient uptake, and reproductive performance.¹¹ A 40-year analysis of U.S. agriculture estimated that ozone pollution has caused average yield losses of roughly 3.5 percent for maize and about 6 percent for soybeans, amounting to annual economic losses of approximately $2.6 billion.¹² In Europe, similar analyses have documented losses across staple crops including wheat, barley, and potatoes, with potatoes particularly vulnerable because of their high market value per hectare.¹³

This dual nature is one of the most counterintuitive things about ozone. Stratospheric ozone is essential and we want more of it; tropospheric ozone is dangerous and we want less. They are the same molecule in different places, doing very different things.

Ozone’s Role in Climate

Beyond UV shielding, ozone is a greenhouse gas in its own right, and its influence on the climate system is more complex than that of carbon dioxide or methane. The effect depends strongly on altitude. In the troposphere, an increase in ozone warms the surface through both longwave and shortwave radiation effects. In the stratosphere, the picture splits: more ozone absorbs more incoming shortwave radiation (cooling the surface by keeping energy higher up) while also trapping longwave radiation (a warming effect). The net result of stratospheric ozone changes depends on which effect dominates at a given altitude and latitude.¹⁴

Estimates of the total radiative forcing from ozone changes since the mid-1800s put the net warming effect at about 0.3 watts per square meter through the present day, with tropospheric ozone increases contributing the larger share.¹⁵ That figure is modest next to carbon dioxide’s forcing, but it is far from negligible. Changes in ozone are especially efficient at altering the radiation balance in the upper troposphere and lower stratosphere near the tropics, making this a region of outsized climate sensitivity.

Ozone depletion over Antarctica has also reshaped Southern Hemisphere weather. The loss of stratospheric ozone cooled the polar stratosphere, strengthening the westerly winds that circle Antarctica during summer. Model experiments show that this wind shift was driven more by ozone depletion than by greenhouse gas increases, and that recovering ozone is expected to largely reverse these circulation changes by the end of the century.¹⁶ Those wind shifts influence ocean currents, sea ice distribution, and precipitation patterns across the Southern Hemisphere, meaning the ozone hole has been quietly rearranging climate far from Antarctica’s borders.

What Created the Ozone Hole

The Antarctic ozone hole was the event that turned ozone from a chemistry curiosity into a global crisis. Its cause is now well understood. Chlorofluorocarbons and related industrial chemicals released chlorine atoms into the stratosphere, where those atoms catalytically destroy ozone: a single chlorine atom can break apart thousands of ozone molecules before it is finally deactivated.¹⁷

Antarctica became the epicenter because of its extreme winter cold. Temperatures in the polar stratosphere drop low enough to form polar stratospheric clouds, which are not ordinary clouds but thin sheets of ice and nitric acid crystals that appear at altitudes around 15 to 25 kilometers. These cloud particles provide surfaces for chemical reactions that convert relatively inert chlorine compounds into forms that are easily broken apart by sunlight. When spring arrives and sunlight returns, those reactive chlorine species go to work destroying ozone at extraordinary speed.¹⁸ Polar stratospheric clouds also remove nitrogen compounds from the gas phase by locking them into particles that settle downward, which eliminates a key chemical brake that would otherwise slow the catalytic destruction of ozone.¹⁹

The Montreal Protocol and Signs of Recovery

The 1987 Montreal Protocol phased out production and consumption of the major ozone-depleting substances, and the recovery of stratospheric ozone from past depletion is underway.²⁰ Atmospheric concentrations of regulated chemicals have been falling for decades, and modeling suggests the gradual ozone recovery over the past two decades has already made a measurable difference. A simulation of the unusually severe Arctic ozone depletion event of spring 2020 found that recovery since the mid-1990s ameliorated the depletion by about 20 Dobson units compared to what would have occurred at peak chlorine and bromine loading.²¹

Recovery is not uniform, though. Greenhouse-gas-induced cooling of the upper stratosphere actually helps ozone there, pushing it back toward historical levels ahead of schedule. But transport changes in the tropical and southern midlatitude lower stratosphere are moving ozone in the opposite direction, and modeling indicates that ozone in those regions may never fully return to pre-1980 values even after ozone-depleting substances are completely gone.²² The ozone layer is recovering, but climate change is reshaping what “recovered” will look like.

New Threats to the Ozone Layer

Even as legacy ozone-depleting chemicals decline, new pressures have appeared. One of the most unexpected is wildfire smoke. Large wildfires can inject soot and combustion products directly into the stratosphere through pyrocumulonimbus clouds. Satellite measurements following the severe 2019–2020 Australian bushfires revealed that smoke particles hosted chemical reactions that increased reactive chlorine species and decreased ozone and nitrogen dioxide well beyond anything observed in 15 years of prior monitoring.²³ Separate satellite and modeling work confirmed that the chemistry occurring on wildfire smoke particle surfaces in the midlatitudes follows patterns consistent with ozone depletion.²⁴ Analysis of 13 years of airborne data shows that pyrocumulonimbus events already account for 10 to 25 percent of the black carbon and organic aerosols in the lower stratosphere.²⁵ If major wildfire seasons become more frequent in a warming climate, this pathway could slow ozone recovery in ways the Montreal Protocol was never designed to address.

The space industry presents another emerging concern. When satellites reach end of life and burn up during atmospheric reentry, the primary byproduct is aluminum oxide nanoparticles, which can catalyze the same chlorine-activation reactions that deplete ozone. A single 250-kilogram satellite generates roughly 30 kilograms of aluminum oxide particles during reentry, and these particles can persist in the atmosphere for decades. The entire population of satellites reentering in 2022 produced an estimated 17 metric tons of aluminum oxide compounds; projected mega-constellation operations could push that past 360 metric tons per year.²⁶ Rocket launches themselves also contribute, with nitrogen oxides from reentry heating and chlorine from solid rocket fuels both playing a role in stratospheric ozone loss.²⁷ Current modeling through 2029 suggests the total ozone loss from all space missions remains small compared to regulated sources, but the trajectory points sharply upward as launch rates accelerate.²⁸

Solar geoengineering proposals add yet another layer of uncertainty. The most commonly discussed approach involves injecting sulfate aerosols into the stratosphere to reflect sunlight and cool the planet. However, sulfate aerosols provide surfaces for heterogeneous chemical reactions that activate chlorine and destroy ozone, much as polar stratospheric clouds do. Models estimate that sustained injection at a scale comparable to the aftermath of a major volcanic eruption would reduce global ozone by 1 to 2 Dobson units.²⁹ The ozone loss from sulfate injection would also be accompanied by heating of the lower tropical stratosphere, which increases water vapor concentrations and causes additional ozone loss.³⁰ Any future deployment of stratospheric aerosol injection would need to weigh climate benefits against the possibility of partially undoing the ozone recovery the Montreal Protocol achieved.

When Earth Had No Stable Ozone Layer

The ozone layer as we know it is a relatively recent feature in the planet’s history. After the rise of atmospheric oxygen roughly 2.4 billion years ago, high marine iodide concentrations would have produced significant inorganic iodine emissions that catalytically destroyed ozone, keeping atmospheric ozone levels unstable or persistently low. Research into the iodine cycle suggests this condition lasted for about two billion years, with a stable ozone layer not emerging until roughly 500 million years ago in the early Phanerozoic era.³¹ The timing is significant because the colonization of land by complex life accelerated around the same period. Without a stable UV shield, the surface would have been too hostile for organisms that lacked the protection of water or soil. The implication is striking: the evolution of terrestrial life as we know it may have been gated not just by the availability of oxygen, but by whether that oxygen could sustain a durable ozone layer overhead.

Ozone as a Tool Outside the Atmosphere

Ozone’s powerful reactivity, the same property that makes it dangerous as a ground-level pollutant, has made it useful in controlled industrial settings. It is one of the strongest antimicrobial agents available and has been applied across food processing, drinking water treatment, pharmaceuticals, textiles, and healthcare for decontamination purposes.³² Unlike chlorine-based disinfectants, ozone breaks down quickly into ordinary oxygen, leaving minimal chemical residues. Municipal water systems in many countries use ozone as a primary or secondary disinfection step, and food producers use it to extend shelf life and reduce pathogen loads on fresh produce and meat. The same molecule that forms smog in hot urban air turns out to be an effective sanitizer when generated and applied in controlled doses, and then allowed to decompose harmlessly.