Ozone depletion is the thinning of Earth’s protective ozone layer in the stratosphere, roughly 15 to 35 kilometers above the surface. This layer absorbs most of the sun’s harmful ultraviolet radiation before it reaches the ground. Since the 1970s, human-made chemicals have been breaking down ozone molecules faster than nature can replenish them, with the most dramatic loss appearing as a seasonal “hole” over Antarctica. NOAA and NASA scientists project the ozone layer could fully recover by 2066, but newer chemical threats could complicate that timeline.
What the Ozone Layer Does
Ozone is a molecule made of three oxygen atoms. In the stratosphere, it forms a diffuse shield that absorbs the most energetic form of ultraviolet light, called UV-B. Without this shield, UV-B reaches Earth’s surface at levels that damage DNA in living cells, from human skin to the single-celled organisms at the base of the ocean food chain. The ozone layer also generates heat by absorbing both incoming ultraviolet radiation and infrared radiation rising from below, which means it plays a direct role in shaping atmospheric temperature and wind patterns.
How Ozone Gets Destroyed
The core problem is reactive chlorine and bromine atoms released from synthetic chemicals. Once these atoms reach the stratosphere, they don’t just destroy one ozone molecule and stop. They act as catalysts, cycling through the same reaction over and over. A single chlorine atom strips an oxygen atom from ozone, turning it into ordinary oxygen. That chlorine then releases the extra oxygen atom and is free to attack another ozone molecule. One chlorine atom can destroy thousands of ozone molecules before anything removes it from the cycle.
Over the poles, the chemistry gets more aggressive. At extremely cold temperatures, chlorine monoxide molecules react with each other or with bromine monoxide, opening up additional destruction pathways that don’t require the free oxygen atoms present in warmer parts of the stratosphere. These polar cycles are the reason ozone loss is so severe at high latitudes.
Which Chemicals Cause It
The main culprits are a family of industrial chemicals called ozone-depleting substances, or ODS. The most damaging are chlorofluorocarbons (CFCs), which were once widely used in refrigerators, air conditioners, aerosol sprays, foam packaging, and insulation. Halons, used in fire extinguishers, carry bromine atoms that are even more efficient at destroying ozone on a per-atom basis. Other ODS include carbon tetrachloride (an industrial solvent), methyl chloroform, and methyl bromide (a crop fumigant).
These chemicals share a key trait: they are extremely stable in the lower atmosphere, which means they don’t break down before drifting up into the stratosphere. Once there, intense ultraviolet light finally splits them apart, releasing the chlorine and bromine atoms that do the damage. A second class of chemicals, hydrochlorofluorocarbons (HCFCs), were introduced as temporary CFC replacements. They still deplete ozone but are less potent because they partially break down before reaching the stratosphere.
Why Antarctica Gets an Ozone Hole
Ozone-depleting chemicals are spread throughout the global atmosphere, yet the most severe depletion happens over Antarctica each spring (September through November). Three factors unique to the South Pole explain why.
First, the Antarctic winter stratosphere gets extraordinarily cold, dropping below minus 78°C. At those temperatures, polar stratospheric clouds form. These aren’t ordinary clouds. Their icy surfaces host chemical reactions that convert stable chlorine compounds into highly reactive forms, essentially loading the atmosphere with a reservoir of ozone-destroying chlorine. Second, the polar vortex, a ring of strong winds circling Antarctica, isolates this chemically primed air from the rest of the stratosphere for months. Third, when sunlight returns in spring, it triggers the reactive chlorine into action, and ozone destruction proceeds rapidly within the isolated vortex.
The Arctic experiences a similar process, but its winter stratosphere is warmer and more variable. Polar stratospheric clouds form less consistently, and the Arctic vortex is less stable, so ozone loss there is real but typically far less dramatic. Antarctic temperatures remain below cloud-formation thresholds much longer, which is why the Southern Hemisphere consistently produces the larger hole. The most intense depletion period ends when temperatures rise above those thresholds, usually by mid-October in Antarctica.
Health Effects of Increased UV Exposure
When the ozone layer thins, more UV-B radiation reaches the surface. UV is the established cause of roughly 90% of non-melanoma skin cancers and about 65% of melanomas. Together, the three main types of skin cancer (basal cell carcinoma, squamous cell carcinoma, and melanoma) affect more than a million Americans each year. More than five sunburns in a lifetime doubles your overall skin cancer risk.
Beyond skin cancer, UV-B exposure suppresses the immune system’s ability to detect and fight abnormal cells, which compounds cancer risk and can reduce the effectiveness of vaccines. Chronic UV exposure also contributes to cataracts, the clouding of the eye’s lens that is a leading cause of vision loss worldwide.
Damage to Ecosystems
The effects ripple well beyond human health. Phytoplankton, the microscopic organisms that form the foundation of ocean food webs, are highly sensitive to UV-B. Even at current levels, many phytoplankton species show signs of UV stress. Increased radiation impairs their ability to photosynthesize, grow, incorporate nitrogen, and fix carbon dioxide. Since phytoplankton produce a significant share of Earth’s oxygen and serve as the primary food source for marine ecosystems, reduced productivity at this level cascades upward through the entire food chain.
Terrestrial plants are also affected. UV-B can damage leaf tissues, reduce growth rates, and alter the timing of key developmental stages. Crop yields for UV-sensitive species decline under elevated radiation, making ozone depletion both an environmental and a food security concern.
The Link Between Ozone and Climate Change
Ozone depletion and climate change are distinct problems, but they interact in important ways. Because ozone generates heat in the stratosphere, losing it causes the stratosphere to cool. This cooling has been happening at the same time that greenhouse gases trap more heat in the lower atmosphere. The growing temperature difference between these two layers of the atmosphere has increased stratospheric wind speeds, altering circulation patterns that propagate downward and influence weather at the surface.
The relationship works in the other direction too. Rising greenhouse gas levels accelerate the upwelling of air in the tropics, pushing ozone-poor air higher into the stratosphere and thinning concentrations there. This means that even as CFC levels decline, climate change itself is working against ozone recovery in certain regions.
The Montreal Protocol and Recovery Progress
The 1987 Montreal Protocol banned the production of CFCs and other major ODS, and it is widely considered the most successful international environmental treaty ever enacted. Global production of CFCs has dropped by more than 99% since its peak. Atmospheric chlorine levels have been slowly declining for decades, and the Antarctic ozone hole has been shrinking. In 2024, the Antarctic ozone hole ranked as the seventh smallest since recovery monitoring began.
Full recovery, however, takes time. CFCs can persist in the atmosphere for 50 to 100 years, so the chlorine already up there will continue destroying ozone for decades. Scientists expect the ozone layer to return to 1980 levels and eliminate polar ozone holes in the second half of this century, with full recovery projected around 2066.
Newer Threats to the Ozone Layer
The recovery timeline assumes no new sources of ozone-destroying chemicals emerge at scale, but that assumption is being tested. A class of chemicals called very short-lived substances (VSLS) is drawing increasing concern. These are reactive halogenated compounds with atmospheric lifetimes under six months. Because they break down quickly, they were not regulated under the Montreal Protocol. But in the tropics, powerful thunderstorms can loft them into the stratosphere before they decay.
Dichloromethane, a common industrial solvent, is the most abundant of these short-lived chlorine compounds and has the fastest-growing emissions, particularly from Asia. A 2023 study published in Nature Climate Change found that VSLS chemistry may account for roughly a quarter of the observed ozone decline in the tropical lower stratosphere between 1998 and 2018. The same study projected that VSLS will continue contributing to ozone loss throughout the 21st century, making the tropical lower stratosphere the only region of the global stratosphere not projected to recover by 2100. Bromine and iodine compounds from ocean biological processes also contribute natural VSLS, but the rising industrial chlorine sources are the ones within human control.
These findings have prompted calls for mitigation strategies targeting anthropogenic VSLS emissions, a gap in the current international framework that could delay or partially undermine the recovery the Montreal Protocol set in motion.