6.4 Stratospheric ozone

Solar electromagnetic radiation spans low-frequency radio waves through infrared, visible light and ultraviolet to high-frequency gamma radiation; shorter wavelength means higher frequency and photon energy.

Infrared transfers heat, visible light powers photosynthesis and supports vision, and ultraviolet can drive atmospheric chemistry but damage biological molecules at higher-energy bands.

UV-C has shorter wavelength and higher photon energy than visible red light.

Order by wavelength or frequency, then identify the biosphere process that actually absorbs or uses each band.

Brighter light is not automatically higher-energy light per photon.

UV harm depends on photon energy and dose; ozone blocks most UV-C and much UV-B, while UV-A reaches the surface more readily.

Higher-energy UV can damage DNA and tissue; total dose depends on intensity and exposure time. Ozone absorption changes the spectrum reaching organisms.

A short intense UV exposure can deliver similar dose to a longer weak exposure, but band and shielding still determine damage mechanism.

More biologically damaging UV-B/C can reach the surface; dose, behaviour and band determine actual risk.

UV index is not a direct measure of ozone alone; sun angle, clouds, altitude and surface reflection also matter.

Stratospheric ozone absorbs damaging UV, reducing the dose that reaches surface organisms while being continually formed and destroyed.

Ozone forms when UV splits O₂ and oxygen atoms combine with O₂; other reactions destroy it. Location matters: stratospheric ozone shields, near-surface ozone is a pollutant.

A temporary fall in stratospheric ozone can raise surface UV even if total atmospheric oxygen is unchanged.

It must be present in the stratosphere where it intercepts relevant UV before it reaches the surface.

‘Ozone’ is not always beneficial; location and concentration determine whether it shields or harms.

Less stratospheric ozone increases surface UV, creating separate health/DNA and ecosystem/photosynthesis pathways.

UV can damage DNA, causing mutations and increasing cancer risk; in humans it contributes to sunburn, premature skin ageing and cataracts. It can also reduce phytoplankton photosynthesis and change food-web productivity.

A rise in surface UVB can affect exposed skin and eyes while separately reducing phytoplankton photosynthesis; each pathway needs its own receptor and evidence.

Name the receptor, UV exposure and mechanism—photosynthetic inhibition, DNA damage, skin injury or lens damage.

More UV does not produce one identical impact in every species or habitat.

Equal formation and destruction rates can keep ozone concentration steady while reactions continue constantly.

Ozone concentration can remain stable when concurrent formation and destruction occur at equal rates. If destruction becomes faster without matching formation, concentration falls until the rate balance changes again.

A measured steady ozone concentration can therefore hide continuous molecular formation and destruction; the observation alone does not mean reactions have stopped.

Compare formation and destruction rates first, then infer whether ozone concentration rises, falls or remains steady.

A stable concentration does not prove no chemical reactions are occurring.

An ozone-depleting substance changes stratospheric chemistry; CO₂-driven warming changes infrared energy balance. Some gases can affect both, but mechanisms differ.

CFCs release chlorine radicals that catalytically destroy ozone and also have high greenhouse potential. CO₂ does not deplete ozone through that chlorine cycle.

Replacing CFCs protects ozone; reducing CO₂ addresses long-wave forcing. One policy can help both only if the substance and mechanism overlap.

Ozone chemistry and greenhouse forcing are different problems, even when a refrigerant policy affects both.

‘Atmospheric chemical’ does not mean every gas has the same target, lifetime or impact.

An ozone hole is a seasonal region of unusually low stratospheric ozone, especially over polar areas—not a literal opening in the atmosphere.

Polar stratospheric clouds support reactions that activate chlorine; spring sunlight drives rapid catalytic loss. The reduced shield raises surface UV and can affect health and ecosystems.

Antarctic spring measurements show a seasonal low-ozone area that expands and contracts rather than remaining a permanent hole.

A measured regional concentration anomaly over time, not a photograph or one day’s UV reading.

The ozone hole is not uniform global ozone disappearance; location and season are essential.

The Montreal Protocol is an international treaty controlling the production, trade and use of CFCs and other ozone-depleting substances; it is widely regarded as the most successful international environmental cooperation to date.

A scheduled phase-out, monitoring, differentiated responsibilities, finance and technology support, trade controls and national law aligned action around a specific measurable chemical pathway.

A phased CFC reduction gives manufacturers time to replace equipment while monitoring tests whether atmospheric ODS levels fall.

Its success depends on commitments, timelines, reporting, assistance, domestic implementation and revision as substitutes create new risks.

International agreement is not magic compliance; domestic law, finance and monitoring complete the chain.

Montreal Protocol action reduced ODS emissions and prevented the planetary boundary for stratospheric ozone depletion from being crossed, according to the evidence specified by the syllabus.

Evaluate the conclusion with ODS emissions and atmospheric concentrations, ozone-hole area or depth, regional and seasonal ozone measurements, baselines and uncertainty. One good year is not enough.

Declining CFC concentrations alongside long-term ozone recovery supports treaty effectiveness even when polar recovery varies with weather and atmospheric transport.

Boundary avoidance is a monitored long-term trend, not proof that recovery is complete or identical everywhere.

A successful agreement does not remove the need for compliance checks and new substitute assessment.