6.4 Stratospheric Ozone

Every September and October, scientists at NOAA and NASA measure the seasonal hole in the ozone layer above Antarctica. In 2025, that measurement made headlines again.

What actually happened to the ozone hole in 2025?

The Sun emits electromagnetic radiation across a huge range of wavelengths, from low-frequency radio waves to high-frequency gamma radiation. Within that spectrum, infrared, visible light and ultraviolet (UV) radiation each play a distinct role in the biosphere: infrared drives much of the planet's heating, visible light powers photosynthesis, and UV, in smaller doses, drives processes like vitamin D synthesis in humans, but becomes dangerous at higher intensities.

Shorter wavelengths of radiation have higher frequencies and therefore carry more energy, posing an increased danger to living organisms. UVA, UVB and UVC radiation all damage organisms to different degrees, but stratospheric ozone absorbs all incident UVC, the shortest and most energetic wavelength, and most incident UVB, leaving mostly the least damaging UVA to reach the surface in significant quantities.

Stratospheric ozone absorbs UV radiation from the Sun, reducing the amount that reaches Earth's surface and protecting living organisms from its harmful effects. UV is damaging specifically because it is high-energy radiation, and the shortest wavelengths within the UV range are the most damaging of all.

UV radiation reduces photosynthesis in phytoplankton, the base of most marine food webs, and damages DNA directly, causing mutations and cancer. In humans, UV exposure causes sunburn, premature skin ageing, and cataracts, real and measurable health outcomes that scale directly with UV exposure.

The relative concentration of ozone molecules has stayed roughly constant over long periods of time, because a steady state of equilibrium exists between the concurrent processes of ozone formation and ozone destruction. Ozone is continuously being both created and broken down in the stratosphere; under natural conditions, these two processes balance out.

Ozone-depleting substances (ODSs) destroy ozone molecules, augmenting the natural breakdown process beyond what natural formation can match. When the rates of formation and depletion become unequal, the equilibrium tips toward whichever process is now faster, in this case, toward net destruction. It is worth stating clearly: ozone depletion is not itself a cause of global warming, even though both are atmospheric problems tied to human emissions.

Ozone depletion allows increasing amounts of UVB radiation to reach Earth's surface, with consequences for ecosystems and human health. Depletion has affected the stratosphere across the whole planet, but at the poles, seasonal ozone "holes" with much greater depletion appear every spring, driven by the combined effect of ODSs and seasonal atmospheric weather patterns unique to the polar stratosphere.

The Montreal Protocol is an international treaty that regulates the production, trade and use of chlorofluorocarbons (CFCs) and other ODSs. It is widely regarded as the most successful example of international cooperation yet achieved in managing a significant global environmental issue.

Actions taken in response to the Montreal Protocol have prevented the planetary boundary for stratospheric ozone depletion from being crossed. The steady, measurable shrinkage of the ozone hole since its early-2000s peak is exactly the kind of long-term data used as evidence for this conclusion: a boundary that was under serious threat, addressed early enough by coordinated global policy to pull it back from the brink.