IB DP Environmental Systems and Societies – Atmosphere, climate change and stratospheric ozone Study Guide

This study guide teaches Topic 6 of IB Diploma Programme Environmental Systems and Societies (ESS), aligned to the International Baccalaureate Diploma Programme Subject Brief: Environmental systems and societies, first assessment 2026. It covers syllabus sections 6.1–6.4, studied at both SL and HL. It follows 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.

The brief gives Topic 6 10 teaching hours at SL and 23 at HL, so HL students study some topics in extra depth. Your teacher’s copy of the full subject guide lists the detailed understandings.

Afterwards, use the revision notes and the practice questions. The IB DP ESS course hub and the printable syllabus checklist show where this unit sits in the course.

What this unit covers

Section	What you must be able to do	SL/HL
6.1 Introduction to the atmosphere	Composition and layers; energy balance; natural greenhouse effect	SL and HL
6.2 Climate change – causes and impacts	Enhanced greenhouse effect, CO₂e, feedback, tipping points, impacts	SL and HL
6.3 Climate change – mitigation and adaptation	Distinguish mitigation from adaptation; evaluate strategies and agreements	SL and HL
6.4 Stratospheric ozone	Ozone formation and destruction, UV effects, the Montreal Protocol	SL and HL

Topic 6 uses Topic 1’s systems language; revise it in the Topic 1 Foundations study guide.

6.1 Introduction to the atmosphere

The atmosphere as a system

The atmosphere is a dynamic system. Inputs include solar radiation and gases from volcanoes, respiration and combustion. Outputs include emitted radiation and gases taken up by photosynthesis or dissolved in the oceans. Its composition has changed over geological time: oxygen built up only after photosynthesising organisms appeared, which later allowed the ozone layer to form.

Composition of dry air

Gas	Approximate share by volume
Nitrogen (N₂)	78%
Oxygen (O₂)	21%
Argon (Ar)	0.93%
Carbon dioxide (CO₂)	about 0.04% (above 420 ppm)

Water vapour varies from almost zero to a few per cent, so it is left out of dry-air figures.

Layers

• Troposphere — surface to about 12 km on average (lower at the poles). It holds most of the atmosphere’s mass and almost all its water vapour, so weather happens here. Temperature falls by about 6.5 °C per km on average.
• Stratosphere — up to about 50 km. Temperature rises with height because ozone absorbs ultraviolet (UV) radiation. Most ozone lies here.
• Mesosphere and thermosphere lie above.

Earth’s energy balance and the natural greenhouse effect

Short-wave solar radiation is partly reflected by clouds, ice and bright surfaces; the fraction reflected is the albedo. The rest is absorbed and warms the surface, which then emits long-wave (infrared) radiation. Greenhouse gases — water vapour, CO₂, methane (CH₄), nitrous oxide (N₂O) and ozone — absorb some of this infrared and re-emit it in all directions, including back towards the surface.

This natural greenhouse effect keeps Earth’s average surface temperature at about 15 °C. Without it, the average would be about −18 °C, roughly 33 °C colder. It is necessary for life.

Worked example 1 — energy balance. The average solar input at the top of the atmosphere is about 340 W m⁻². Earth’s overall albedo is about 0.30. Find the reflected and absorbed amounts.

Reflected = albedo × input  = 0.30 × 340 = 102 W m⁻²
Absorbed  = (1 − albedo) × input = 0.70 × 340 = 238 W m⁻²

If greenhouse gases slow the loss of energy to space, the system warms until output again equals input.

6.2 Climate change – causes and impacts

Weather and climate

Weather is the state of the atmosphere at a place over hours or days. Climate is the average pattern of weather over a long period, usually 30 years or more.

The enhanced greenhouse effect

Human activity adds greenhouse gases faster than sinks remove them, strengthening the effect. Main sources:

• CO₂ — burning fossil fuels, deforestation, cement production.
• CH₄ — livestock, rice paddies, landfill, leaks from coal, oil and gas extraction.
• N₂O — nitrogen fertilisers, some industrial processes.
• Halocarbons — CFCs and their replacements (HFCs), used in refrigeration and foams.

Global warming potential (GWP) compares the warming from 1 tonne of a gas with 1 tonne of CO₂, usually over 100 years: about 28 for methane and 265 for nitrous oxide (values differ slightly between IPCC reports). Multiplying a mass by its GWP gives CO₂-equivalent (CO₂e), so different gases can be added together.

Worked example 2 — CO₂-equivalent. A farm emits 500 t of CO₂, 12 t of CH₄ and 1.5 t of N₂O in a year. Use GWPs of 28 and 265.

CH₄:  12  × 28  = 336 t CO₂e
N₂O:  1.5 × 265 = 397.5 t CO₂e
Total = 500 + 336 + 397.5 = 1233.5 t CO₂e
Methane share = 336 ÷ 1233.5 × 100 = 27.2%

Methane is only about 2% of the farm’s emissions by mass but over a quarter of its warming effect.

Evidence

Atmospheric CO₂ has risen from about 280 ppm before industrialisation to above 420 ppm. The measured record rises and falls each year because Northern Hemisphere plants take in CO₂ in summer and release it in winter. Air bubbles in ice cores give CO₂ and temperature records hundreds of thousands of years long. The IPCC’s Sixth Assessment Report found that global surface temperature in 2011–2020 was about 1.1 °C above 1850–1900.

Feedback loops and tipping points

• Positive feedback (amplifies change): ice–albedo feedback; thawing permafrost releasing CH₄ and CO₂; warmer air holding more water vapour; drought and fire killing forests, which then store less carbon.
• Negative feedback (dampens change): faster plant growth with more CO₂ taking up more carbon; more evaporation forming low clouds that reflect sunlight.

A tipping point is a threshold beyond which positive feedback shifts a system to a new state that is hard to reverse, such as loss of the Greenland ice sheet or Amazon dieback.

Worked example 3 — ice–albedo feedback. Sunlight of 250 W m⁻² falls on sea ice with albedo 0.60. The ice melts to open ocean with albedo 0.06. Compare absorbed energy.

Sea ice:    250 × (1 − 0.60) = 100 W m⁻² absorbed
Open ocean: 250 × (1 − 0.06) = 235 W m⁻² absorbed
Increase = 135 W m⁻² (2.35 times as much)

Warmer water melts more ice, lowering albedo further: a positive feedback loop.

Impacts

• Sea-level rise from thermal expansion of seawater and melting land ice, threatening low-lying coasts and small island states.
• More extreme weather — heatwaves, heavier rainfall in some regions, longer droughts in others.
• Ecosystems — species and biomes shift towards the poles and uphill; coral bleaching increases; the ocean absorbs CO₂ and becomes more acidic.
• Societies — changes to crop yields and water supply, spread of disease vectors, displacement of people.

Impacts are unequal: many countries with the lowest emissions per person are among the most exposed and least able to pay for responses. This is the climate justice perspective.

6.3 Climate change – mitigation and adaptation

Mitigation reduces the cause: it cuts greenhouse gas emissions or removes gases from the atmosphere. Adaptation reduces the harm: it changes how people and systems live with a changing climate.

Mitigation	Adaptation
Switching to renewable energy	Sea walls and flood barriers
Energy efficiency, insulation, public transport	Drought-tolerant crop varieties
Reforestation and protecting peatland (carbon sinks)	Water storage and more efficient irrigation
Carbon capture and storage (CCS)	Early-warning systems for storms and heatwaves
Carbon taxes and emissions trading	Managed retreat from eroding coasts

Geoengineering proposals include direct air capture of CO₂ and reflecting sunlight (solar radiation management). Reflecting sunlight treats the symptom, not the cause, and does nothing about ocean acidification.

International action

• UNFCCC (1992) — the framework treaty that set up annual negotiations.
• Kyoto Protocol (1997) — binding emission targets for industrialised countries only.
• Paris Agreement (2015) — aims to keep warming well below 2 °C and to pursue 1.5 °C. Every country sets its own nationally determined contribution (NDC), reviewed every five years.

Evaluating a strategy

Ask: how much does it cut emissions or reduce harm, what does it cost and who pays, how quickly does it work, and who gains or loses? A technocentric value system may favour CCS; an ecocentric one may favour lower consumption and protecting ecosystems.

6.4 Stratospheric ozone

How ozone forms and absorbs UV

In the stratosphere, UV radiation splits oxygen molecules; a free oxygen atom then joins an O₂ molecule to form ozone:

O₂ + UV → O + O
O + O₂ → O₃
O₃ + UV → O₂ + O

Ozone is made and broken down all the time, giving a steady concentration. Each breakdown absorbs UV, so the layer stops most UV-B and all UV-C from reaching the surface. Ozone column thickness is measured in Dobson units (DU). Values of around 300 DU are typical; an area below 220 DU is called an ozone hole.

Ozone-depleting substances

Ozone-depleting substances (ODS) are human-made gases that carry chlorine or bromine into the stratosphere: CFCs (old refrigerants, aerosol propellants, foam-blowing agents), halons (fire extinguishers), methyl bromide (a pesticide) and carbon tetrachloride. They are stable, so they last long enough in the troposphere to reach the stratosphere. There, UV breaks them apart and releases chlorine atoms:

Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂

The chlorine atom comes out unchanged, so it acts as a catalyst. One atom can destroy many thousands of ozone molecules before it is locked away in a less reactive compound.

Depletion is worst over Antarctica. In winter, very cold polar stratospheric clouds form; reactions on them prepare chlorine that is released when sunlight returns in spring (September to October).

Effects of more UV-B

• Humans: more skin cancer and cataracts, weakened immune response.
• Ecosystems: damage to phytoplankton, reducing productivity at the base of marine food webs; lower yields in some crops.
• Materials: faster breakdown of plastics and paints.

Managing ozone depletion

The Vienna Convention (1985) set up co-operation; the Montreal Protocol (1987) set ODS phase-out timetables, tightened by later amendments, and every UN member state has ratified it. Replacements were HCFCs (less damaging, now being phased out), then HFCs, which spare ozone but are strong greenhouse gases. The Kigali Amendment (2016) adds an HFC phase-down, linking ozone and climate policy. The 2022 WMO/UNEP scientific assessment expects the Antarctic ozone layer to recover to 1980 levels around the 2060s.

Montreal worked because there were few producers, affordable substitutes, clear science and a direct harm (skin cancer), and richer countries funded poorer ones through the Multilateral Fund. Fossil fuels, by contrast, underpin whole economies, so climate agreements are much harder.

Do not confuse the two issues. The ozone hole does not cause global warming. It lets in more UV, not more heat-trapping. CFCs are, however, also greenhouse gases. Ground-level (tropospheric) ozone is a pollutant, covered with urban air pollution in Topic 8.

Common errors

• Saying greenhouse gases “trap UV” or “reflect heat back”. They absorb and re-emit long-wave infrared.
• Treating the greenhouse effect itself as harmful. The enhanced effect is the problem.
• Mixing up ozone depletion and climate change, or blaming the ozone hole for warming.
• Calling a sea wall “mitigation”. It is adaptation because it does not reduce emissions.
• Adding tonnes of different gases without converting to CO₂e first.
• Writing that CFCs destroy ozone directly. Chlorine atoms released by UV do, as catalysts.

How this unit is examined

Paper 1 uses data on an unseen case study; Paper 2 has short-answer and data-based questions, then structured essays. See the exam preparation guide, the assessment revision notes, the syllabus guide, the subject guide and the Topic 2 Ecology guide.

Next: the revision notes, then the practice questions.

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.