Ozone layer

How the ozone layer protects Earth

The primary function of the ozone layer is to absorb harmful ultraviolet (ultraviolet, UV) radiation from the Sun, especially the most energetic and damaging types known as UV-B and UV-C.

• UV-C radiation is the most dangerous but is completely absorbed by oxygen (O₂) and ozone (O₃) in the stratosphere.
• UV-B radiation is the main cause of sunburn and can lead to skin cancer and cataracts. Most of it is absorbed by the ozone layer, though a small amount still reaches the surface. Without the ozone layer, UV-B levels would be high enough to be lethal to most life on Earth.

This protective function is a result of the ozone–oxygen cycle, a continuous process in which ultraviolet light from the Sun splits oxygen molecules (O₂) into individual oxygen atoms. 

These free atoms then combine with other O₂ molecules to form ozone (O₃). When ozone absorbs ultraviolet radiation it breaks apart again, so the cycle both creates ozone and removes the most harmful ultraviolet radiation from sunlight.

Good ozone vs. bad ozone

It is important to distinguish ozone by location:

• Good ozone: ozone in the stratosphere (the ozone layer) protects life by absorbing ultraviolet radiation.
  
• Bad ozone: ozone near the Earth’s surface, in the troposphere, is a harmful air pollutant and a key ingredient in photochemical smog; it can cause respiratory problems in humans and damage plants.
  

Ozone depletion and recovery

In the late 20th century, scientists discovered that certain human-made chemicals—particularly chlorofluorocarbons (CFCs)—were depleting the ozone layer. 

When CFCs reach the stratosphere, they release chlorine and bromine atoms that act as catalysts to destroy ozone molecules; a single chlorine atom can destroy over 100,000 ozone molecules. This led to severe thinning of the ozone layer and the seasonal “ozone hole” over Antarctica.

In response, the international community adopted the Montreal Protocol in 1987, a landmark treaty to phase out the production and use of ozone-depleting substances. The Montreal Protocol is widely regarded as one of the most successful international environmental agreements. 

Since its adoption, the concentration of many ozone-depleting substances in the atmosphere has declined and the ozone layer has been slowly healing. Scientists project that, assuming continued compliance with the treaty and control of other threats, the ozone layer could recover to pre-1980s levels by the middle of this century.

Is the ozone layer still in danger?

The ozone layer is not considered to be in immediate danger of collapse, but recovery remains fragile and ongoing. The major historical driver of ozone depletion—CFCs—has been largely phased out under the Montreal Protocol, which has allowed gradual healing. 

However, full recovery depends on continued global adherence to the treaty and on monitoring for other potential threats. Climate change can indirectly affect ozone through changes in temperatures and circulation, and the release of other ozone-harming chemicals would also pose new risks. Continued vigilance and scientific monitoring are therefore essential.

What does the ozone layer do?

The primary role of the ozone layer is to absorb and filter out harmful ultraviolet radiation from the Sun. It is particularly effective at blocking UV-B and UV-C radiation, the most energetic and damaging forms:

• UV-C radiation is completely absorbed by oxygen and ozone in the upper atmosphere.
• UV-B radiation is largely absorbed by the ozone layer; however, a small portion still reaches the surface and is responsible for sunburn and increased long-term health risks such as skin cancer and cataracts.

This absorption process is crucial for protecting terrestrial and marine life and for preserving the integrity of ecosystems and human health.

What happens if the ozone layer were destroyed?

If the ozone layer were to be destroyed, the consequences would be severe and wide-ranging:

• Increased health problems: Dramatic rises in skin cancer, cataracts, and weakened immune systems in humans and animals.
• Damage to ecosystems: Harm to single-celled organisms such as phytoplankton, which form the base of the marine food web; reduced crop yields and damage to terrestrial plants due to DNA damage from increased ultraviolet exposure.
• Changes in climate and atmospheric chemistry: The ozone layer contributes to the thermal structure of the atmosphere; its removal would alter temperature profiles and circulation patterns, potentially changing global wind patterns and weather systems with unpredictable consequences.

The role of the ozone layer in Earth’s future

The ozone layer is one of Earth’s most vital natural defenses, filtering the Sun’s most harmful ultraviolet radiation and helping to maintain the stability of the atmosphere. Human activity once caused significant depletion, but coordinated international action under the Montreal Protocol has set the ozone layer on a path to recovery. 

Nonetheless, that recovery is neither automatic nor guaranteed: it requires continued global commitment, careful monitoring, and attention to related threats such as climate change. Protecting the ozone layer remains an important example of how sustained, collective action can preserve planetary systems that are essential to life.