Britannica Sources used in this answer

How did the buildup of oxygen lead to the formation of the ozone layer?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

The formation of the ozone layer was a critical development in Earth's history, facilitated by the rise of atmospheric oxygen.

The process began approximately two billion years ago as oxygen levels in the atmosphere increased, largely due to the activity of photosynthetic organisms. As these oxygen molecules ( $O_{2}$ ) accumulated in the upper atmosphere, they were subjected to high-energy solar photons.12

This interaction triggers a process called photodissociation, where the solar radiation breaks the chemical bonds of the $O_{2}$ molecules, releasing single oxygen atoms. These highly reactive atoms then collide and bond with intact oxygen molecules to form ozone ( $O_{3}$ ).1

The resulting buildup of ozone created a protective layer in the stratosphere that screens out lethal levels of UVB radiation. This shielding effect was essential for the evolution of life, as it allowed marine organisms to safely migrate from the oceans and colonize land.12