The ozone layer is essential to life on Earth. Although ozone (O₃) is present throughout the atmosphere, it reaches its highest concentrations in the stratosphere. This region is commonly referred to as the “ozone layer” (Figure 1) [1].
Figure 1: Ozone concentration as a function of altitude. The vast majority of ozone molecules are concentrated in the stratosphere. This “layer” protects the Earth from harmful UV-B radiation.
It plays a critical role in maintaining the habitability of the planet by absorbing harmful solar UV-B radiation (ultraviolet radiation between 280 and 320 nm). These radiations are particularly damaging to living organisms, as they can alter cellular DNA. High levels of UV-B radiation reduce photosynthesis and limit the growth of vegetation and crops. In humans, UV-B exposure increases the risk of skin cancer and cataracts, and weakens the immune system [2].
Are there really “holes” in the ozone layer?
During the 1970s, 1980s, and 1990s, human activities released large quantities of ozone-depleting substances (ODS). Ozone depletion refers to the thinning of the ozone layer in the stratosphere. This phenomenon led to the formation of so-called “ozone holes” over the polar regions. The term “hole” does not imply a complete absence of ozone, but rather a significant and rapid decrease in ozone concentration.
In the 1980s, the international community adopted agreements aimed at reducing—and ultimately eliminating—the emissions of these ozone-depleting substances. These policy measures proved highly effective (Figure 2). Since then, global emissions have decreased by more than 99% (Figure 3) [3], excluding certain fugitive emissions (i.e., unintended leaks).
Figure 2: Excess skin cancer cases attributable to ozone depletion in the United States. Estimates of additional cases per million inhabitants resulting from stratospheric ozone depletion. The study (Slaper et al., 1996) compares scenarios with no regulation, the Montreal Protocol, and the Copenhagen Amendment (more stringent). The results highlight the time lag between ozone depletion and health impacts.
Figure 3: Changes in the consumption of ozone-depleting substances. Consumption is expressed relative to 1986 levels (index = 100).
Which molecules are responsible?
Ozone depletion occurs when the natural balance between ozone production and destruction in the stratosphere is disrupted in favor of destruction (Figure 4).
Figure 4: Stratospheric ozone concentration, expressed in Dobson Units (DU). One Dobson Unit corresponds to the number of ozone molecules required to form a 0.01 mm thick layer of pure ozone at standard temperature and pressure. Higher DU values indicate a healthier ozone layer. A sharp decline is observed during the 1980s–1990s due to ODS emissions, followed by stabilization and a slight increase between 1995 and 2022.
Ozone-depleting substances (ODS) are the primary drivers of this imbalance. These synthetic compounds are chemically stable in the lower atmosphere and contain chlorine and/or bromine, allowing them to reach the stratosphere intact. Chlorofluorocarbons (CFCs) are among the most well-known and abundant ODS. A single chlorine atom released from a CFC molecule can destroy up to 100,000 ozone molecules through catalytic reactions [4].
Can the ozone layer recover?
ODS typically take several years to reach the stratosphere after being emitted. Despite this delay and associated uncertainties, the natural balance between ozone production and destruction can gradually be restored if the concentration of ozone-depleting substances is reduced.
This recovery process may require the near-complete elimination of these compounds. Some studies suggest that increasing greenhouse gas concentrations could influence the rate of ozone recovery. However, scientific assessments indicate that significant recovery trends are expected to become clearly detectable only after 2030 [4].
Nevertheless, observations already show a stabilization—and even a slight increase—in stratospheric ozone concentrations following the phase-out of ODS.
Bibliography
[1] Vidal, G. (2000, December 15). La structure de l’atmosphère — Planet-Terre. Planet-Terre.ens-Lyon.fr. https://planet-terre.ens-lyon.fr/ressource/structure-atmosphere.xml
[2] Canada, E. et C. climatique. (2008, December 27). Appauvrissement de la couche d’ozone : effets sur la santé et l’environnement. Www.canada.ca. https://www.canada.ca/fr/environnement-changement-climatique/servies/pollution-atmospherique/enjeux/couche-ozone/appauvrissement-consequences/effets-sante-environnement.html
[3] notre-environnement. (2024, May 14). Appauvrissement de l’ozone stratosphérique. Notre-Environnement. https://www.notre-environnement.gouv.fr/themes/societe/limites-planetaires-ressources/article/appauvrissement-de-l-ozone-stratospherique
[4] Canada, E. et C. climatique. (2008a, December 2). Appauvrissement de la couche d’ozone : causes, situation et restauration. Aem. https://www.canada.ca/fr/environnement-changement-climatique/services/pollution-atmospherique/enjeux/couche-ozone/appauvrissement-consequences/causes-situation-restauration.html