6.4 Stratospheric ozoneIB Environmental Systems and Societies SL: Revision notes

The Sun emits electromagnetic radiation across a range of wavelengths, from low-frequency radio waves to high-frequency gamma radiation. Most of the energy reaching the surface is visible light and infrared.

Shorter wavelengths have higher frequencies and more energy, so they are more dangerous to living things. Ultraviolet (UV) radiation has enough energy to damage biological molecules such as DNA.

Stratospheric ozone (O3) absorbs UV radiation from the Sun, reducing the amount reaching the surface and protecting living organisms.

Effects of UV on living things:

• Phytoplankton: UV reduces photosynthesis, so less energy and less food enters marine food webs.
• DNA damage causes mutations and cancer.
• In humans: sunburn, premature ageing of the skin and cataracts, as well as skin cancer.

Ozone is continually formed (UV splits O2 into atoms which combine with O2 to make O3) and continually destroyed (ozone absorbs UV and splits). The relative concentration has stayed constant over long periods because the rates are equal: a steady state equilibrium.

Ozone-depleting substances (ODSs), such as CFCs from old refrigerators, aerosols and foams, destroy ozone molecules and so augment the natural breakdown. Destruction exceeds formation and the concentration falls, most strikingly as the Antarctic ozone hole. More UVB reaches the surface, affecting ecosystems and human health.

The Montreal Protocol (1987) is an international treaty that regulates the production, trade and use of CFCs and other ODSs. Every UN member state has ratified it; about 99% of controlled ODSs have been phased out; developing countries had longer timetables and support from a Multilateral Fund.

Actions taken in response to the Protocol have prevented the planetary boundary for stratospheric ozone depletion from being crossed, and the ozone layer is projected to recover to 1980 levels over Antarctica by about 2066. Limits: CFCs persist for decades, so recovery is slow, and illegal use has occurred.