IB DP Environmental Systems and Societies – Atmosphere, climate change and stratospheric ozone Revision Notes

For full explanations and worked examples, start with the Topic 6 study guide. These notes condense Topic 6 of IB Diploma Programme Environmental Systems and Societies, aligned to the International Baccalaureate Diploma Programme Subject Brief: Environmental systems and societies, first assessment 2026. They cover syllabus sections 6.1–6.4 (the atmosphere, climate change causes and impacts, mitigation and adaptation, and stratospheric ozone), which are studied at both SL and HL; HL students study some topics in extra depth. They follow the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The brief gives Topic 6 Atmosphere and climate change 10 teaching hours at SL and 23 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.

Use them with the practice questions, the IB DP ESS course hub and the printable syllabus checklist.

Key definitions

• Atmosphere — a dynamic system of gases around Earth, with inputs, outputs, storages and flows.
• Albedo — the fraction of incoming solar radiation a surface reflects (0 to 1).
• Greenhouse gas — a gas that absorbs and re-emits long-wave (infrared) radiation: water vapour, CO₂, CH₄, N₂O, ozone, halocarbons.
• Natural greenhouse effect — the warming of Earth’s surface by naturally occurring greenhouse gases; keeps the average near 15 °C instead of about −18 °C.
• Enhanced greenhouse effect — extra warming caused by human additions of greenhouse gases.
• Weather / climate — short-term atmospheric conditions / long-term average weather (usually 30+ years).
• Global warming potential (GWP) — warming from 1 t of a gas relative to 1 t of CO₂, usually over 100 years.
• CO₂-equivalent (CO₂e) — mass of a gas × its GWP.
• Tipping point — a threshold beyond which a system shifts to a new, hard-to-reverse state.
• Mitigation — reducing emissions or removing greenhouse gases (tackles the cause).
• Adaptation — adjusting to reduce harm from climate change (tackles the effects).
• Carbon sink — a store that takes in more carbon than it releases (forests, peat, oceans).
• Ozone-depleting substance (ODS) — a gas that carries chlorine or bromine to the stratosphere (CFCs, halons, methyl bromide, carbon tetrachloride, HCFCs).
• Dobson unit (DU) — measure of total ozone in a column of air; below 220 DU counts as an ozone hole.

Key figures and formulas

Item	Value or formula
Dry air	N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ about 0.04%
CO₂ concentration	about 280 ppm pre-industrial; above 420 ppm now
Troposphere	surface to about 12 km (average); temperature falls about 6.5 °C per km
Stratosphere	to about 50 km; temperature rises with height (ozone absorbs UV)
Natural greenhouse warming	about 33 °C (−18 °C → 15 °C)
Earth’s overall albedo	about 0.30
Absorbed radiation	input × (1 − albedo)
CO₂e	mass × GWP
GWP (100-year, approximate)	CO₂ 1, CH₄ about 28, N₂O about 265
Rate of change	(final − initial) ÷ time
Percentage change	(final − initial) ÷ initial × 100
Warming to 2011–2020	about 1.1 °C above 1850–1900 (IPCC AR6)

6.1 The atmosphere in brief

• Composition changed over geological time; oxygen accumulated from photosynthesis, which allowed ozone to form.
• Troposphere: weather, almost all water vapour, most of the mass.
• Stratosphere: ozone layer; warms with height.
• Energy balance: short-wave in → some reflected (albedo) → rest absorbed → long-wave out → greenhouse gases absorb and re-emit some towards the surface.

6.2 Causes, evidence, feedback, impacts

Sources: CO₂ (fossil fuels, deforestation, cement); CH₄ (livestock, rice, landfill, fossil-fuel leaks); N₂O (fertilisers); halocarbons (refrigerants, foams).

Evidence: direct CO₂ measurements (rising trend with a seasonal wobble from Northern Hemisphere plants); ice-core air bubbles; instrumental temperature records.

Positive feedback (amplifies): ice–albedo; permafrost thaw releasing CH₄ and CO₂; more water vapour; forest dieback and fire.

Negative feedback (dampens): more plant growth taking up CO₂; more low cloud reflecting sunlight.

Impacts: sea-level rise (thermal expansion + land-ice melt); more extreme weather; species and biomes shift poleward and uphill; coral bleaching; ocean acidification; changes to crop yields, water supply and disease vectors; displacement of people. Impacts fall hardest on many low-emitting, low-income countries (climate justice).

Method in steps: CO₂e calculation

1. Multiply each gas's mass by its GWP.
2. Add the CO₂e values (CO₂ has GWP 1).
3. For a share: gas CO₂e ÷ total CO₂e × 100.
4. State units: t CO₂e.

Worked reminder: 4 t of CH₄ with GWP 28 = 4 × 28 = 112 t CO₂e.

Method in steps: albedo

Absorbed = incoming × (1 − albedo)
Reflected = incoming × albedo

Worked reminder: 300 W m⁻² on fresh snow of albedo 0.8 → absorbed = 300 × 0.2 = 60 W m⁻².

6.3 Mitigation and adaptation

Mitigation (cause)	Adaptation (effects)
Renewable energy, efficiency, public transport	Sea walls, flood barriers, raised buildings
Reforestation, protecting peat and soils	Drought-tolerant crops, better irrigation
Carbon capture and storage (CCS), direct air capture	Early-warning systems
Carbon tax, emissions trading (cap and trade)	Managed retreat from coasts
Diet change, less food waste	Heat-resilient building design

Agreements: UNFCCC (1992) framework → Kyoto Protocol (1997), binding targets for industrialised countries only → Paris Agreement (2015), well below 2 °C and pursuing 1.5 °C, nationally determined contributions (NDCs) reviewed every five years.

Solar radiation management reflects sunlight: it treats a symptom and leaves ocean acidification untouched.

Method in steps: “evaluate” or “to what extent” on a strategy

1. Say what the strategy is and whether it is mitigation or adaptation.
2. Give strengths (scale of cut or protection, speed, co-benefits).
3. Give limitations (cost, who pays, technology risk, side effects, equity).
4. Bring in a perspective (technocentric vs ecocentric; richer vs poorer countries).
5. End with a clear, conditional judgement.

6.4 Stratospheric ozone

Formation:     O₂ + UV → O + O        O + O₂ → O₃
Absorption:    O₃ + UV → O₂ + O
Destruction:   Cl + O₃ → ClO + O₂     ClO + O → Cl + O₂

• Chlorine is regenerated, so it is a catalyst: one atom can destroy many thousands of ozone molecules.
• Antarctic hole: polar stratospheric clouds form in the polar winter; chlorine is released when sunlight returns in spring (September–October).
• More UV-B → skin cancer, cataracts, weaker immune response; harm to phytoplankton and some crops; plastics degrade faster.
• Management: Vienna Convention (1985) → Montreal Protocol (1987), ratified by every UN member state, with later amendments → HCFCs then HFCs as substitutes → Kigali Amendment (2016) phases down HFCs because they are strong greenhouse gases.
• Recovery: the 2022 WMO/UNEP assessment expects the Antarctic ozone layer to return to 1980 levels around the 2060s. ODS last decades in the atmosphere, so recovery lags behind the ban.

Must-know distinctions

• Natural vs enhanced greenhouse effect — the first is essential; the second is the problem.
• Ozone depletion vs climate change — different mechanisms (UV vs infrared). The ozone hole does not cause warming. The link is that CFCs and HFCs are also greenhouse gases.
• Stratospheric vs tropospheric ozone — high up it protects; at ground level it is a pollutant (Topic 8).
• Mitigation vs adaptation — cause vs effects. Reforestation is mitigation; a sea wall is adaptation.
• Positive vs negative feedback — amplifies vs dampens; neither word means good or bad.
• Weather vs climate — one hot summer is weather.
• Short-wave vs long-wave — in from the Sun vs out from Earth.

Quick self-test

1. State the two most abundant gases in dry air, with their percentages.
2. Explain why temperature rises with height in the stratosphere.
3. The surface temperature is 25 °C. Using a rate of 6.5 °C per km, estimate the temperature at 6 km.
4. Calculate the CO₂e of 4 t of methane (GWP 28).
5. Snow of albedo 0.8 receives 300 W m⁻². Calculate the energy absorbed.
6. CO₂ rose from 280 ppm to 420 ppm. Calculate the percentage increase.
7. Classify: (a) planting mangroves to store carbon; (b) building a flood barrier.
8. Give one positive feedback that speeds up warming.
9. Write the two equations for catalytic destruction of ozone by chlorine.
10. An ozone column fell from 300 DU to 120 DU. Calculate the percentage decrease and say whether this is an ozone hole.
11. Why did the Kigali Amendment target HFCs, which do not deplete ozone?
12. State one reason the Montreal Protocol succeeded that is harder to repeat for CO₂.

Answers

1. Nitrogen about 78%, oxygen about 21%.
2. Ozone absorbs UV radiation, which heats the air, and there is more UV to absorb higher up.
3. 25 − 6.5 × 6 = −14 °C.
4. 4 × 28 = 112 t CO₂e.
5. 300 × (1 − 0.8) = 60 W m⁻².
6. (420 − 280) ÷ 280 × 100 = 50%.
7. (a) Mitigation (carbon sink). (b) Adaptation.
8. Any one: ice–albedo; permafrost releasing CH₄; more water vapour; forest dieback.
9. Cl + O₃ → ClO + O₂ and ClO + O → Cl + O₂.
10. (300 − 120) ÷ 300 × 100 = 60%; yes, 120 DU is below 220 DU.
11. HFCs are strong greenhouse gases, so the phase-down cuts future warming.
12. Any one: few producers; cheap substitutes; direct, clear health harm; funding for poorer countries. Fossil fuels are used everywhere and underpin whole economies.

Where marks are usually lost

• Writing that greenhouse gases absorb UV or “reflect heat”. They absorb and re-emit long-wave infrared.
• Blaming the ozone hole for global warming, or saying CO₂ destroys ozone.
• Calling adaptation measures (sea walls, drought-tolerant crops) mitigation.
• Adding tonnes of CH₄ and N₂O to tonnes of CO₂ without converting to CO₂e first.
• Describing a feedback loop without closing it: the change must feed back to the start.
• Writing “CFCs break down ozone” without the UV release of chlorine and its catalytic role.
• Giving percentage change from the wrong base (final instead of initial value).
• Omitting units: ppm, DU, W m⁻², t CO₂e, °C.
• “Evaluate” answers that list only advantages, or finish without a judgement.
• Quoting a data trend without figures from the table or graph.

Related pages

See the practice questions, the assessment revision notes, the Topic 1 Foundations revision notes for systems vocabulary and the Topic 2 Ecology revision notes for carbon cycling.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.