Ozone depletion and its effects at Higher Level: The same ideas as SL, with different cases: the global numbers from the 2022 assessment, the record Arctic hole of 2020, the EPA's health estimates and UVB-damaged frog eggs in Oregon. At HL, practise the long answer that links the thinner ozone to people and to biodiversity, step by step.
Practise this as you read
- Chain every effect back to more UVB.
- Separate effects on people from effects on ecosystems.
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Thinner shield, more UVB: The ozone layer is a shield that absorbs most UVB. When ODSs thin it, more UVB gets through to the ground, where it harms people, plants and animals.
The points to remember
- Ozone absorbs most UVB; all UVC is absorbed, even when ozone is thin.
- Less ozone means less UVB absorbed, so more UVB reaches the surface.
- Depletion has thinned ozone over the whole Earth, not just at the poles.
- UVB has high energy: it damages DNA in skin, eyes, leaves and algae.
- Say the chain: less ozone, more UVB, a named effect on life. 'More UV' alone is not enough.
Real example: the 2022 WMO/UNEP assessment found that ozone between 60°S and 60°N was still about 2-3% below its 1964-1980 level, and about 5% below it over southern mid-latitudes such as New Zealand and southern Chile.
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Ozone has thinned everywhere, but the biggest losses happen over the poles every spring. These areas of very low ozone are called ozone holes.
The points to remember
- The ozone hole is an area of very low ozone (below 220 DU) over a pole, in the stratosphere.
- It appears every spring: over the Antarctic in September and October.
- In winter, the polar vortex traps very cold air (below -78 °C) over the pole.
- Polar stratospheric clouds form in the cold; on them, chlorine from ODSs is turned into active forms.
- When sunlight returns in spring, the active chlorine destroys ozone very fast.
- By December the air warms, the vortex breaks up and the hole closes until next spring.
- The Arctic is warmer, so its holes are smaller and less frequent.
Winter: dark and cold
The polar vortex traps air over the Antarctic. It cools below -78 °C and polar stratospheric clouds form.
Spring: sunlight returns
In September the Sun rises again. Chlorine freed on the clouds destroys ozone fast.
Summer: the hole closes
By December the air warms, the vortex breaks up and ozone-rich air flows back in.
Real example: in spring 2020 an unusually cold and stable polar vortex formed over the Arctic, and scientists measured the largest ozone loss ever recorded there, with a hole that lasted into April.
UVB and human health: Extra UVB damages the DNA in skin and eye cells. This causes mutations, which can lead to cancer, and harms the immune system.
The points to remember
- Sunburn: skin burns faster under strong UVB.
- Skin cancer, including melanoma: UVB causes mutations in skin cells.
- Cataracts and other eye damage, which can lead to blindness.
- Premature ageing of the skin: wrinkles and spots.
- A weaker immune system, so the body fights infections and cancer less well.
Remember it as: Skin, eyes, defences: burn, cancer, cataracts, ageing, a weak immune system.
Real example: the US Environmental Protection Agency (EPA) estimated in 2020 that, by protecting ozone, the Montreal Protocol will prevent about 443 million cases of skin cancer, 2.3 million skin cancer deaths and 63 million cases of cataracts in people born in the United States between 1890 and 2100.
Humans only, when asked: If a question asks about human health, give effects on people: melanoma, sunburn, cataracts. Effects on plants or plankton do not count there.
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UVB also damages the DNA and the photosynthesis of other living things. On land and in the sea, the damage to producers passes up the food chain.
The points to remember
- Phytoplankton photosynthesise less and are damaged, so ocean productivity falls.
- Phytoplankton are the base of the food web: less food for krill, then fish, penguins, seals, whales.
- Plants: damaged leaves, smaller leaves, less photosynthesis and growth; lower crop yields.
- Animals: DNA damage kills eggs and larvae, e.g. of frogs and toads; zooplankton and krill die.
- So biodiversity can fall as sensitive species decline.
- Not global warming: thinner ozone does not warm the planet.
Thin ozone
- More UVB reaches the sea surface.
Phytoplankton
- phytoplankton: less photosynthesis.
Krill
- krill: less food.
Predators
- Penguins, seals, whales: less food.
Real example: in Oregon's Cascade Mountains, scientists placed frog and toad eggs in ponds, some under UVB filters (Blaustein, 1994). Fewer eggs of the Cascades frog and the western toad hatched in full sunlight, so UVB may help explain why these amphibians were declining.
Not global warming: Never say that ozone depletion or extra UV causes global warming: they are separate problems.
How this comes up: Paper 2, Section B (b) [7]: explain how a change in ozone affects living things and biodiversity, link by link; Section A: list two risks to human health from a graph of the ozone hole [2].
Since the 1980s an ozone hole has formed over the Antarctic every spring, and ozone has also thinned over the rest of the Earth.
Explain how ozone depletion can affect human health and biodiversity.
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