September 16 is celebrated as World Ozone Day, marking four decades since representatives from around the world gathered in Vienna to lay the foundation for the Vienna Convention for the Protection of the Ozone Layer.

In the early 1970s, scientists Mario Molina and F. Sherwood Rowland discovered that chlorofluorocarbons (CFCs) -- chemicals used in aerosols, refrigerators, and air conditioners -- could rise into the stratosphere and destroy ozone molecules. Since the ozone layer protects life on Earth from harmful ultraviolet (UV) radiation, their discovery alarmed the scientific community.

By the late 1970s, evidence of ozone depletion was growing, including the discovery of the "ozone hole" over Antarctica in the early 1980s. The United Nations Environment Programme (UNEP) began addressing the issue in 1977, leading to the World Plan of Action on the Ozone Layer. On March 22, 1985, the Vienna Convention for the Protection of the Ozone Layer was adopted in Vienna, Austria, and entered into force in 1988. However, it was not a regulatory treaty -- it did not ban or restrict harmful chemicals.

The Vienna Convention provided the legal and institutional framework for further negotiations, eventually leading to the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987. The Montreal Protocol established binding targets to phase out CFCs and other ozone-depleting substances.

The convention's success is remarkable -- it remains the only environmental treaty to achieve universal ratification, signed by every country on Earth. This global cooperation led to significant recovery of the ozone layer and continues to protect both the ozone and the climate today.

However, there is now a new concern. As the ozone hole closes, scientists have identified a previously overlooked problem. A study led by the University of Reading suggests that ozone recovery could contribute up to 40% more warming. While the ozone layer shields Earth from harmful UV radiation, ozone itself is also a greenhouse gas that traps heat in the atmosphere.

According to the study, between 2015 and 2050, ozone is expected to cause 0.27 watts per square meter (W/m²) of additional warming. This measurement represents how much extra energy is trapped per square meter of Earth's surface -- making ozone the second-largest contributor to future warming by 2050, after carbon dioxide (1.75 W/m²).

This finding parallels the situation with air pollution. Some air pollutants, such as aerosols (tiny particles from burning coal, oil, and biomass), reflect sunlight back into space and help form clouds that also reflect sunlight, slightly cooling the Earth. When air quality improves and these particles decrease, this cooling effect disappears, causing a small increase in warming.

Of course, this does not mean we should continue polluting -- air pollution causes serious health and environmental damage. Instead, it highlights the complex balance between atmospheric chemistry and climate.

Protecting the ozone layer remains essential for human health, as it prevents skin cancer and protects ecosystems from harmful UV radiation. Still, new research suggests that climate policies may need to be updated to account for ozone's role in future warming.