What this resource is
This framework re-presents the Diploma Programme Environmental Systems and Societies Standard Level course as a single, prerequisite-ordered teaching sequence — not eight syllabus topics taught in numerical order. It is built on three convictions: that topics should be taught in the order their dependencies require, that each should be taught to its full depth rather than its minimum, and that the connections between topics should be made explicit so that students see ESS as one connected discipline rather than twenty-seven separate ones.
ESS is unusual among Diploma subjects in that the syllabus already tells you what the connective tissue is. Perspectives, systems and sustainability are not three topics in Chapter 1 — they are the three unifying concepts the whole course is built from, and every later topic is an application of them. A sequence that treats Chapter 1 as an introduction to be finished and left behind wastes the single greatest structural advantage the subject has.
Inside this document
- 01The design principle — how topics are classified and ordered
- 02The Teaching Spine — the full 27-topic flow, at a glance
- 03Why the Syntheses Come Last — the prerequisite feeder map
- 04The Sequence at a Glance — every topic with syllabus code, role and hours
- 05The 27 Topics in Depth — depth, interconnection and inquiry angle for each
- 06Time Allocation & Pacing — reconciled to 150 hours, finishing by end of January
The design principle
Every topic is classified by the role it plays in the sequence:
- Foundational — self-contained, and a prerequisite for later topics.
- Developmental — extends one or more foundations.
- Synthesis — teachable at depth only once several strands are mature. Succession and resilience, climate responses, conservation strategy, aquatic food production, water pollution management, food security, energy choices and sustainable urban planning all sit here, and all are placed late by design.
The synthesis topics are scattered across all eight syllabus topics, yet their feeders cut straight across topic boundaries. Eutrophication sits in the water chapter but is unteachable without the nitrogen and phosphorus cycling from the ecology chapter. Food security sits in the land chapter but needs trophic efficiency from ecology, water security from the water chapter and climate impacts from the atmosphere chapter. Ocean acidification appears in the water chapter and again in the climate chapter, and makes sense in neither place until the carbon cycle is secure. Succession and resilience is the culmination of population dynamics, energy flow, nutrient cycling and feedback — and the syllabus places it fifth in the ecology chapter, before biodiversity has been met at all.
Two further principles govern the order. Climate change is taught early, not last. The syllabus places the atmosphere chapter sixth of eight, but climate change is the driver behind biodiversity loss, ocean acidification, water scarcity, food insecurity, energy choices and environmental migration — every one of which the course goes on to teach. Placed straight after ecology, where the carbon cycle and feedback mechanisms are still fresh, it becomes a lens available for the whole second half of the course rather than a late arrival that has to be retrofitted.
And the foundation topic is never finished. Perspectives, systems and sustainability are taught first because nothing else makes sense without them, but each subsequent topic is written here to name which of the three it is exercising. A student who can draw a systems diagram in week three and cannot draw one for a fishery in week thirty has not learned systems thinking; they have learned a diagram.
IB DEMYSTIFIEDExaminers · Moderators · Mentors
The Teaching Spine
Prerequisite-ordered flow · 27 topics · ESS SLTeach top to bottom — each phase is a prerequisite for the next. Colour shows each topic’s role; the pill shows the phase’s teaching hours.
PHASE A16 teaching hours
1Perspectives
2Systems
3Sustainability
▼
PHASE B22 teaching hours
4Populations
5Energy Flow
6Nutrient Cycles
7Climate & Biomes
8Succession
▼
PHASE C10 teaching hours
9Atmosphere
10Climate Change
11Climate Responses
12Ozone & UV
▼
PHASE D13 teaching hours
13Biodiversity
14Threats
15Conservation
▼
PHASE E20 teaching hours
16Hydrological Cycle
17Water Security
18Aquatic Food
19Water Pollution
20Soil Systems
21Food Security
▼
PHASE F19 teaching hours
22Natural Capital
23Energy Resources
24Solid Waste
25Human Populations
26Urban Systems
27Urban Air Quality
Role:
Foundational
Developmental
Synthesis — taught late
Total taught content: 100 h · + 50 h experimental programme = 150 h
The full 27-topic sequence. Teach top to bottom; each phase is a prerequisite for the next.
IB DEMYSTIFIEDExaminers · Moderators · Mentors
Why the Syntheses Come Last
How the applied topics build on the foundationsEach synthesis topic (right) can only be taught at depth once its feeders (left) are in place. A gold arrow means ‘is a prerequisite for’.
8. Succession & Resilience
2 · Systems — feedback & tipping points4 · Populations — colonisation & competition5 · Energy flow — productivity shifts6 · Nutrient cycles — soil development
➤
8 · Succession & Resilience
Placed fifth in the syllabus, but it is the culmination of the ecology topic.
11. Responding to Climate Change
3 · Sustainability — frameworks & justice6 · Nutrient cycles — the carbon cycle9 · Atmosphere — the greenhouse effect10 · Climate change — impacts to be reduced
➤
11 · Responding to Climate Change
Mitigation is meaningless until the mechanism it targets is understood.
15. Protecting Biodiversity
1 · Perspectives — competing value systems8 · Resilience — what makes recovery possible13 · Biodiversity — what is being protected14 · Threats — what it is protected from
➤
15 · Protecting Biodiversity
Reserve design is an ecological argument with an ethical premise.
21. Agriculture & Food Security
5 · Energy flow — trophic efficiency10 · Climate change — yields & growing seasons17 · Water security — irrigation limits20 · Soil systems — the productive base
➤
21 · Agriculture & Food Security
Four chapters converge on one question: can we be fed sustainably?
23. Energy Resources
3 · Sustainability — indicators & frameworks6 · Nutrient cycles — fossil carbon released11 · Climate responses — decarbonisation22 · Natural capital — income vs capital
➤
23 · Energy Resources
An energy choice is a climate, resource and equity decision at once.
How to read this
- Right-hand boxes are the five synthesis topics that draw the most strands together.
- Left-hand chips are the topics they depend on.
- A gold arrow means ‘is a prerequisite for’.
- Every feeder sits earlier in the spine, so by the time the topic is taught its feeders are done.
- The whole argument for the order in one image: sequence by dependency, not by chapter number.
Each synthesis topic can be taught at depth only once its feeders are in place.
The sequence at a glance
Every topic in teaching order, with its syllabus code, its role and its teaching hours. The suggested Year 1 / Year 2 boundary falls at the end of Phase E and is adjustable.
| # | Topic | Syllabus | Role | Hours |
|---|---|---|---|---|
| Phase A — Foundation: Perspectives, Systems, Sustainability (16 h) | ||||
| 1 | Environmental Perspectives and Values | 1.1 | Foundational | 3 h |
| 2 | Understanding Environmental Systems | 1.2 | Foundational | 5 h |
| 3 | Sustainability and Sustainable Development | 1.3 | Developmental | 8 h |
| Phase B — How Ecosystems Work (22 h) | ||||
| 4 | Organisms, Populations and Ecosystems | 2.1 | Foundational | 6 h |
| 5 | Energy Flow and Biomass | 2.2 | Foundational | 6 h |
| 6 | Nutrient and Biogeochemical Cycling | 2.3 | Developmental | 4 h |
| 7 | Climate Patterns and Global Biomes | 2.4 | Developmental | 3 h |
| 8 | Ecosystem Change, Succession and Resilience | 2.5 | Synthesis | 3 h |
| Phase C — The Atmosphere and a Changing Climate (10 h) | ||||
| 9 | The Atmosphere and Greenhouse Effect | 6.1 | Foundational | 2 h |
| 10 | Causes and Consequences of Climate Change | 6.2 | Developmental | 3 h |
| 11 | Responding to Climate Change | 6.3 | Synthesis | 3 h |
| 12 | Stratospheric Ozone and UV Radiation | 6.4 | Developmental | 2 h |
| Phase D — Biodiversity Under Pressure (13 h) | ||||
| 13 | Biodiversity and Evolutionary Change | 3.1 | Developmental | 5 h |
| 14 | Human Pressures on Biodiversity | 3.2 | Developmental | 4 h |
| 15 | Protecting and Restoring Biodiversity | 3.3 | Synthesis | 4 h |
| Phase E — Water, Soil and Food (20 h) | ||||
| 16 | The Hydrological System | 4.1 | Foundational | 3 h |
| 17 | Water Availability, Use and Security | 4.2 | Developmental | 3 h |
| 18 | Aquatic Food Production | 4.3 | Synthesis | 3 h |
| 19 | Managing Water Pollution | 4.4 | Synthesis | 3 h |
| 20 | Soil Systems and Functions | 5.1 | Foundational | 3 h |
| 21 | Agriculture and Food Security | 5.2 | Synthesis | 5 h |
| Phase F — Resources, People and Cities (19 h) | ||||
| 22 | Managing Natural Resources | 7.1 | Developmental | 3 h |
| 23 | Energy Resources and Sustainability | 7.2 | Synthesis | 4 h |
| 24 | Solid Waste and Resource Recovery | 7.3 | Developmental | 3 h |
| 25 | Human Population Dynamics | 8.1 | Developmental | 3 h |
| 26 | Urban Systems and Sustainable Planning | 8.2 | Synthesis | 3 h |
| 27 | Urban Air Quality and Pollution | 8.3 | Synthesis | 3 h |
| Total taught content | 100 h | |||
Scroll the table sideways on narrow screens.
The 27 topics in depth
Each topic carries its teaching depth, its interconnections with other topics, and the practical and inquiry angle through which it prepares a student for the individual investigation and for Papers 1 and 2.
Filter by role
Showing all 27 topics in teaching order.
Phase A — Foundation: Perspectives, Systems, Sustainability
The three unifying concepts of the course, taught first because every later topic is an application of them. The order within the phase matters: perspectives first, because a student who has not yet examined their own value system will mistake it for neutrality; then systems, because it supplies the analytical apparatus; then sustainability, which is the largest single topic in the foundation and depends on both.
1. Environmental Perspectives and Values
- Teach to this depth
- The factors that shape an environmental viewpoint — cultural, religious, economic, socio-political, education, media, personal experience — identified in real people rather than described in the abstract. Environmental value systems taught as inputs, processing and outputs, so the concept arrives already in systems form. Ecocentric, anthropocentric and technocentric worldviews placed on a spectrum rather than in three boxes, with real actors, policies and campaigns located on it. Then the historical development of the environmental movement through the events that shifted public perspective. The essential move is that students identify and defend their own position and then argue the opposing one credibly.
- Connects to
- Perspectives is one of the three unifying concepts, so it recurs in every topic: conservation priorities, energy choices, food ethics, climate justice and urban planning are all disagreements between value systems before they are disagreements about evidence.
- Practical & inquiry angle
- A structured worldview survey of the class or the wider school, analysed for pattern. Perspectives is the criterion most often thinly handled in the individual investigation, and it begins here.
2. Understanding Environmental Systems
- Teach to this depth
- Systems thinking as a way of seeing, then the technical apparatus built carefully: storages and flows, inputs, outputs and transfers against transformations. Open, closed and isolated systems distinguished with examples that are genuinely each. Negative feedback as the mechanism of stability and positive feedback as the mechanism of runaway change, both traced through real loops rather than described. Steady-state equilibrium, tipping points and resilience defined precisely, since these are the terms that make the second half of the course intelligible. Then models — their construction, their usefulness and their limitations — including the first and second laws of thermodynamics as constraints on every system that follows.
- Connects to
- The analytical spine of the entire course. Every subsequent topic is a system: an ecosystem, the carbon cycle, a fishery, a watershed, a city, the climate. Feedback and tipping points return decisively in climate change and in ecosystem resilience.
- Practical & inquiry angle
- Building and critiquing a systems diagram for a local system, with storages quantified where possible. Every strong individual investigation begins by defining the system boundary, and that skill is taught here or nowhere.
3. Sustainability and Sustainable Development
- Teach to this depth
- The largest topic in the foundation, and it earns the hours. Sustainability defined through the relationship between use and regeneration, then sustainable development as the contested political project it actually is, with the tension between the two named rather than smoothed over. Environmental justice taught with cases in which the people bearing the cost are not the people receiving the benefit. Sustainability indicators — ecological footprint, biocapacity, carbon footprint, water footprint — calculated and then criticised for what they leave out. The UN Sustainable Development Goals and planetary boundaries as two competing frameworks with different logics. Citizen science as a genuine data source with genuine limitations. Then alternative economic approaches — circular economy, doughnut economics, degrowth — taught as arguments to be evaluated, not endorsed.
- Connects to
- Builds on both preceding topics: sustainability is a systems property judged from a perspective. It supplies the evaluative criteria for every management strategy the course later proposes, and connects directly forward to natural capital, energy choices and urban planning.
- Practical & inquiry angle
- Calculating and interrogating a personal or school ecological footprint; comparing two countries against the same indicator set and explaining the divergence. A common and productive individual investigation context.
Phase B — How Ecosystems Work
The scientific engine room of the course. Populations first, because the vocabulary of limiting factors and carrying capacity recurs in fisheries, human demography and urban systems alike; then energy flow, which explains why trophic efficiency constrains food production; then nutrient cycling, which is the prerequisite for both climate change and eutrophication; then climate and biomes; and finally succession, which is the synthesis of all four.
4. Organisms, Populations and Ecosystems
- Teach to this depth
- Species, populations, communities, habitats and ecosystems distinguished precisely, and the abiotic–biotic distinction applied to a real site. Classification and taxonomy taught as a working tool — dichotomous keys used, not merely described. Then population dynamics done properly: limiting factors, carrying capacity, density-dependent and density-independent controls, and the S and J curves derived from what the organisms are actually doing rather than presented as shapes. Predator–prey and other species interactions. Human population patterns introduced here so the parallel with the later demography topic is deliberate. Field methods for estimating abundance — quadrats, transects, capture–mark–recapture — taught to the point of competent use, with the sources of error named by the students.
- Connects to
- Carrying capacity and limiting factors recur in fisheries and maximum sustainable yield, in human demography and in urban carrying capacity. The field methods here are the methods most individual investigations actually use.
- Practical & inquiry angle
- A quadrat or transect survey of a real site with abundance, percentage cover and a diversity index calculated, and the sampling limitations stated honestly. The single highest-value practical of Year 1.
5. Energy Flow and Biomass
- Teach to this depth
- Photosynthesis and respiration at the level ESS needs — as the two processes that fix and release energy and carbon — with word equations secure and the link to the carbon cycle flagged before it is taught. Trophic levels, food chains and food webs constructed from real ecosystems. Then the quantitative core: energy losses between trophic levels and why roughly ninety per cent disappears, gross and net productivity, primary and secondary productivity, and biomass. Pyramids of numbers, biomass and productivity drawn to scale and compared, with the inverted cases explained rather than excused. Finally, human impacts on flows of energy and matter, which is where the topic stops being abstract.
- Connects to
- Trophic efficiency is the argument behind lower-trophic-level diets in the food security topic and behind fishery collapse in the aquatic food topic. Productivity returns in succession, in soil systems and in ocean productivity.
- Practical & inquiry angle
- Measuring productivity in a bottle ecosystem or from published data; constructing and comparing pyramids for a terrestrial and an aquatic system. Reliably quantitative, and reliably examined in Paper 2.
6. Nutrient and Biogeochemical Cycling
- Teach to this depth
- Biogeochemical cycles taught as systems diagrams with storages and flows — the vocabulary from topic 2 applied for the first time to something large. The carbon cycle in full: the atmospheric, oceanic, terrestrial and lithospheric stores, the fluxes between them, and the residence time of each. Then human disruption traced quantitatively — combustion, deforestation, land-use change and their effect on the atmospheric store — so that climate change arrives later as an already-explained consequence rather than a new claim. Strategies for reducing the disruption introduced here in outline and evaluated in the climate topics that follow.
- Connects to
- The direct prerequisite for the whole atmosphere and climate phase, and for ocean acidification, soil carbon and eutrophication. Placing it immediately before the climate topics is the central sequencing decision of this framework.
- Practical & inquiry angle
- Quantifying a carbon store — a tree, a plot, a school ground — and comparing it against a published flux. Systems diagrams with real numbers attached are what separates a strong Paper 2 answer from a labelled sketch.
7. Climate Patterns and Global Biomes
- Teach to this depth
- Weather distinguished from climate with the timescales made explicit. Atmospheric circulation built from the ground up — differential heating, Hadley, Ferrel and polar cells, and the resulting bands of high and low pressure — so that the global distribution of deserts and rainforests is derived rather than memorised. Ocean currents and the thermohaline circulation as the second distributor of heat. Then the major biomes characterised by temperature, precipitation, productivity and structure, and mapped onto the circulation pattern just derived. Climographs read and compared.
- Connects to
- Explains biome distribution, which explains where the productivity from the previous topic is highest, which explains where agriculture and biodiversity are concentrated. Circulation returns in the atmosphere phase and in urban air pollution.
- Practical & inquiry angle
- Constructing climographs from real station data and inferring the biome; predicting how a biome’s boundaries shift under a warming scenario.
8. Ecosystem Change, Succession and Resilience
- Teach to this depth
- Zonation distinguished from succession by whether the variable is space or time — a distinction students routinely collapse. Primary and secondary succession traced through a real sere, with the changes in soil depth, productivity, biomass, diversity and complexity tracked at each stage rather than asserted for the endpoint. Then resilience: what makes an ecosystem able to absorb disturbance and return, why diversity and size and speed of reproduction matter, and where the tipping points from topic 2 sit. Human activity treated as a factor that can arrest, divert or reset succession.
- Connects to
- The synthesis of the ecology phase: it needs population dynamics, energy flow, nutrient cycling and the feedback vocabulary from the foundation. Resilience is then the organising idea for conservation, restoration and rewilding in Phase D.
- Practical & inquiry angle
- A transect across a real successional or zonation gradient — a dune, a wall, an abandoned plot — with diversity measured at intervals. A classic and very defensible individual investigation.
Phase C — The Atmosphere and a Changing Climate
Moved forward from sixth in the syllabus to third in the teaching order. The carbon cycle and the feedback vocabulary are still fresh from Phase B, so the greenhouse mechanism can be built rather than asserted — and once climate change is in place, it is available as a driver for biodiversity loss, ocean acidification, water scarcity, food insecurity, energy choice and migration, all of which the course goes on to teach.
9. The Atmosphere and Greenhouse Effect
- Teach to this depth
- Atmospheric composition and the functions the atmosphere performs — radiation absorption, heat distribution, water transport. Then the energy budget handled honestly: incoming shortwave radiation, albedo and reflection, absorption at the surface, outgoing longwave radiation, and the greenhouse gases that intercept it. The natural greenhouse effect established as a precondition for life before the enhanced effect is mentioned, so students do not leave with the idea that the greenhouse effect is itself the problem — the most common and most damaging misconception in the topic.
- Connects to
- Sits on the carbon cycle from topic 6 and on the circulation from topic 7. It is the mechanism that the next two topics diagnose and treat, and it returns in ozone chemistry and in urban air quality.
- Practical & inquiry angle
- A comparative albedo or heat-retention experiment; interpreting a published energy-budget diagram and identifying which arrow human activity changes.
10. Causes and Consequences of Climate Change
- Teach to this depth
- Natural drivers of climate variation separated from anthropogenic ones, and the evidence for attribution examined rather than assumed — students should be able to say why the current change is not explained by the natural drivers. The major greenhouse gases compared on source, concentration and global warming potential. Then the feedback mechanisms that decide how bad it gets: ice–albedo, permafrost methane, water vapour, and the ocean and forest sinks weakening. Consequences taught in two registers — ecological (range shifts, phenology, coral bleaching, ocean acidification) and social and economic (agriculture, health, displacement, infrastructure) — with the distribution of harm explicitly unequal.
- Connects to
- Positive feedback and tipping points from the foundation reach their most important application here. The consequences listed become the driving pressures in biodiversity, water, food and population topics later in the course.
- Practical & inquiry angle
- Working with real temperature, CO₂ or sea-level series — plotting, describing trend, and distinguishing correlation from the mechanistic argument. Paper 1 case studies lean heavily on this skill.
11. Responding to Climate Change
- Teach to this depth
- Mitigation and adaptation distinguished by what they target, then a full spread of strategies under each, evaluated on effectiveness, cost, timescale, equity and feasibility rather than listed. Decarbonisation treated concretely — energy supply, transport, industry, buildings, land use — with the scale of change each requires made honest. International climate agreements assessed on what they actually bind parties to and what enforcement exists. Throughout, the differing perspectives from topic 1 are put to work: the same policy looks different from an ecocentric and a technocentric standpoint, and from a high-emitting and a highly-exposed country.
- Connects to
- Needs the greenhouse mechanism, the carbon cycle, the impacts of the previous topic and the sustainability frameworks from Phase A. It is the direct precursor to energy choices in Phase F.
- Practical & inquiry angle
- Costing and evaluating a school or local decarbonisation measure against the emissions it actually avoids. A strong, tractable individual investigation with real data.
12. Stratospheric Ozone and UV Radiation
- Teach to this depth
- Ultraviolet radiation and its biological effects, then the ozone–oxygen cycle that absorbs it, taught as a dynamic steady state rather than a layer. Halogenated organic gases as catalysts of destruction, with the catalytic nature emphasised because it explains the disproportion between quantity released and damage done. Polar stratospheric conditions as the reason depletion is seasonal and regional. Then the Montreal Protocol assessed as the clearest case in the course of an effective international response — and used deliberately as the comparator against which climate agreements are judged.
- Connects to
- Shares atmospheric structure with topic 9 and offers the sharpest available contrast with topic 11 on why one international agreement worked and another has struggled. Frequently and unhelpfully confused with the greenhouse effect, which is why it is taught immediately after it.
- Practical & inquiry angle
- Interpreting ozone-column and CFC-concentration time series together, and using the lag between them to argue about policy effectiveness.
Phase D — Biodiversity Under Pressure
Placed after climate because climate change is one of the principal threats the phase has to evaluate, and after succession because resilience is what conservation is trying to preserve. The internal order is diagnostic: what biodiversity is and how it arose, what is destroying it, and only then what can be done — with the closing topic drawing on perspectives, resilience and the two topics before it at once.
13. Biodiversity and Evolutionary Change
- Teach to this depth
- Biodiversity separated into its three components — species, genetic and habitat diversity — and the relationship between diversity and resilience argued from the succession work rather than asserted. Evolution by natural selection at the level ESS requires, with speciation and the role of isolation, and human influence on evolutionary processes through selective breeding, habitat fragmentation and pollution. Then the measurement problem taken seriously: species richness against evenness, diversity indices calculated by hand at least once, and the reasons two sites with the same richness can have very different diversity. Approaches to biodiversity management introduced in outline for the two topics that follow.
- Connects to
- Rests on population dynamics, energy flow and succession from Phase B. The diversity index calculated here is the same statistic used in field practicals and in a large share of individual investigations.
- Practical & inquiry angle
- Calculating and comparing a diversity index across two habitats, with a defensible account of what the difference does and does not show.
14. Human Pressures on Biodiversity
- Teach to this depth
- The major threats — habitat loss and fragmentation, overexploitation, invasive species, pollution and climate change — with the relative importance of each argued from evidence rather than ranked by assertion. Conservation status assessed using real criteria, and the reasons a species is listed traced back to the threats. Then case studies handled properly: at least one extinction and at least one successful recovery, each examined for the specific mechanism that caused the outcome. The tragedy of the commons taught as a structural explanation for overuse — a property of the incentive arrangement rather than a failure of character — because that is what makes it transferable to fisheries, water and the atmosphere later on.
- Connects to
- Climate change from Phase C is one of the threats evaluated here. The tragedy of the commons returns explicitly in aquatic food production, in water security and in natural resource management.
- Practical & inquiry angle
- Building an evidence-based threat profile for one local species; a simulation or game modelling common-pool resource depletion and the rules that prevent it.
15. Protecting and Restoring Biodiversity
- Teach to this depth
- The arguments for preserving biodiversity set out in full — ecological, economic, cultural, aesthetic and ethical — and the point made that they are different arguments that support the same action for incompatible reasons. In-situ and ex-situ approaches compared on what each can and cannot achieve. Protected-area design taught as an applied ecological problem: size, shape, edge effects, corridors and buffer zones, each justified from the ecology already taught. Rewilding evaluated as a strategy with genuine opposition. The biodiversity planetary boundary connected back to the framework from topic 3. Finally, differing conservation perspectives — including the tension between conservation and the people who live in the protected place.
- Connects to
- Draws together perspectives, resilience, biodiversity measurement and threat analysis. Reserve design in particular is only teachable once succession, edge effects and population viability are secure.
- Practical & inquiry angle
- Designing a reserve for a real landscape on a real map and defending every boundary decision; evaluating a named protected area against its stated objectives.
Phase E — Water, Soil and Food
The two chapters that together answer whether humans can be fed. Water comes first because irrigation, aquatic productivity and eutrophication all depend on it; soil follows because it is the other productive base; and food security closes the phase as the synthesis that needs trophic efficiency, soil, water and climate simultaneously. Two topics here — eutrophication and ocean acidification — are the clearest evidence in the course that chapter order and teaching order should differ.
16. The Hydrological System
- Teach to this depth
- The hydrological cycle drawn as a systems diagram with the vocabulary from topic 2 — stores and flows, not a picture with arrows. The relative size and residence time of each store made explicit, since the fact that almost all fresh water is locked in ice and groundwater is the premise of the entire water security topic. Evaporation, transpiration, condensation, precipitation, infiltration, percolation, run-off and groundwater flow each explained by what drives it. Then human interference: abstraction, dams and reservoirs, deforestation, urbanisation and irrigation, each mapped onto the specific flow or store it alters.
- Connects to
- Applies systems thinking directly, and is the prerequisite for water security, water pollution and irrigated agriculture. Land-use change here connects back to the carbon cycle and forward to soil erosion.
- Practical & inquiry angle
- Comparing infiltration rates across land-use types on the school site — cheap, quantitative and a genuinely good individual investigation.
17. Water Availability, Use and Security
- Teach to this depth
- Why fresh water is unevenly available in space and time, and why availability and access are different problems — the second being political and economic rather than hydrological. Domestic, agricultural and industrial demand compared, with agriculture’s dominant share made explicit because it drives the food topics later. Supply-side responses — reservoirs, redistribution, desalination, rainwater harvesting, groundwater abstraction — evaluated on cost, energy, equity and sustainability. Demand-side responses given equal weight. Shared water resources and transboundary tension taught as the commons problem from topic 14 in a new setting.
- Connects to
- Needs the hydrological cycle and the sustainability indicators from Phase A. Feeds irrigation constraints in food security, and connects to climate change through altered precipitation and glacial storage.
- Practical & inquiry angle
- Auditing school or household water use against a per-capita benchmark; evaluating a real transboundary water dispute from both parties’ positions.
18. Aquatic Food Production
- Teach to this depth
- Aquatic food webs and the reasons marine productivity is concentrated where it is. Rising demand set against the biology, and overexploitation explained through maximum sustainable yield and what happens when harvest exceeds recruitment — the population dynamics from topic 4 doing real work. Mitigation approaches — quotas, gear restrictions, closed seasons and areas, marine protected areas — evaluated on enforceability as well as biology. Aquaculture assessed on both sides, including feed conversion, escapes, disease and effluent. Then climate change and ocean acidification as pressures that act on the same stocks, with the acidification chemistry connected explicitly back to the carbon cycle.
- Connects to
- A four-way synthesis: population dynamics and energy flow from Phase B, the commons problem from Phase D, the carbon cycle and climate impacts from Phases B and C. Taught in chapter order it would arrive before most of these; taught here it arrives after all of them.
- Practical & inquiry angle
- Modelling a fish stock under different harvest rates using published landing data; evaluating a named fishery’s management against its stock trend.
19. Managing Water Pollution
- Teach to this depth
- Point and non-point sources distinguished because they demand different management. The main pollutant classes — organic, inorganic nutrients, suspended solids, thermal, plastics, pathogens — with a real source for each. Plastic pollution taught with the microplastic pathway followed to its ecological endpoint. Water quality assessed both chemically and biologically: biochemical oxygen demand, dissolved oxygen, turbidity, and indicator species with biotic indices actually used. Then eutrophication in full — nutrient enrichment, algal bloom, light limitation, decomposition, oxygen depletion, death — as a positive feedback cascade, which is what makes it the best worked example of feedback in the course. Management evaluated at the three levels of altering activity, treating discharge and remediating damage.
- Connects to
- Eutrophication is unteachable without the nitrogen and phosphorus cycling from topic 6 and the feedback vocabulary from topic 2, and its principal cause is the fertiliser use taught in the next topic but one. It is the strongest single argument in this framework for a dependency-ordered sequence.
- Practical & inquiry angle
- Sampling a local watercourse for dissolved oxygen, nitrate, phosphate, turbidity and invertebrate indicators, and relating the results to upstream land use. Among the most commonly chosen and most successful individual investigations.
20. Soil Systems and Functions
- Teach to this depth
- Soil as a system with inputs, outputs, storages and transfers, and its composition — mineral matter, organic matter, water, air and organisms — quantified rather than listed. Texture, structure, porosity, permeability, pH and water-holding capacity taught as properties with agricultural consequences, using a soil texture triangle on real samples. The soil profile and its horizons. Then the functions soil performs: supporting primary production, storing and filtering water, cycling nutrients, and holding carbon — the last connecting directly back to topic 6 and forward to land management.
- Connects to
- Depends on nutrient cycling and on the soil development stage of succession. It is the productive base for the food security topic that follows and a carbon store in the climate topics already taught.
- Practical & inquiry angle
- Measuring texture, pH, organic content and infiltration across contrasting land uses. Simple apparatus, genuinely quantitative, and a reliable investigation context.
21. Agriculture and Food Security
- Teach to this depth
- Agricultural systems compared as systems: inputs, outputs, energy efficiency and productivity per hectare and per unit of labour, across subsistence and commercial, extensive and intensive, arable and pastoral. Terrestrial food production compared with aquatic on the same terms. Soil degradation — erosion, salinisation, compaction, nutrient depletion — traced to specific practices, and conservation measures matched to the specific cause rather than offered as a list. Then food security taken apart into availability, access, utilisation and stability, so students stop treating it as a synonym for production. Diet and trophic level, food waste, distribution and the political economy of hunger, with the trophic efficiency argument from topic 5 made quantitative.
- Connects to
- The synthesis of the phase and one of the most connected topics in the course: energy flow, soil, water security, climate impacts, biodiversity loss from land conversion and eutrophication from fertiliser all converge here.
- Practical & inquiry angle
- Comparing the land, water and emissions footprint of two diets using published coefficients; a soil erosion or conservation trial on a real plot.
Phase F — Resources, People and Cities
The applied conclusion of the course. Natural capital supplies the economic vocabulary for evaluating everything already taught; energy is the choice on which climate mitigation depends; waste is what the system does with what it has finished using; and the closing three topics turn the systems lens on the human population itself and on the cities most of it now lives in. Taught here, these can be taught as strategy rather than as description.
22. Managing Natural Resources
- Teach to this depth
- Natural capital and natural income defined precisely, with the distinction between living off the income and consuming the capital made the organising idea of the topic — it is the same distinction sustainability rested on in topic 3, now given an economic form. Renewable, non-renewable and replenishable resources classified, with the awkward cases argued rather than avoided. Valuation of ecosystem services attempted and then criticised, including the objection that some things should not be priced. Then the dynamic nature of resource value: how a substance becomes a resource when technology, culture or scarcity changes, illustrated with real reversals.
- Connects to
- Formalises the sustainability concept from Phase A and supplies the framework for evaluating energy, waste, fisheries, forests and soil. The commons problem from topic 14 is its central failure mode.
- Practical & inquiry angle
- Valuing an ecosystem service on the school site — shade, drainage, pollination — and stating clearly what the valuation omits.
23. Energy Resources and Sustainability
- Teach to this depth
- Every major energy source compared on a consistent set of criteria — energy density, emissions, land and water use, intermittency, waste, cost and social acceptability — so that comparison is possible rather than anecdotal. Patterns of consumption examined across countries and over time, with the inequality made explicit. Then the choice itself treated as the multi-factor decision it is: geography, economics, politics, culture and environmental values all bear on it, and students should be able to explain why two countries with the same resources make different choices. Storage and conservation given real weight, since intermittency is the central technical obstacle to decarbonisation.
- Connects to
- Needs natural capital, the carbon cycle, climate mitigation from Phase C and the perspectives framework from Phase A. It is where the decarbonisation strategy proposed in topic 11 either becomes concrete or does not.
- Practical & inquiry angle
- A full comparison of two national energy mixes against a shared criteria set, using real generation and emissions data; auditing and modelling school energy use.
24. Solid Waste and Resource Recovery
- Teach to this depth
- Waste types and sources quantified, with the sharp differences in generation and composition between countries used as evidence of consumption patterns rather than as trivia. Environmental and social impacts of disposal — leachate, methane, incineration emissions, marine plastic, and the export of waste to countries that did not generate it. Landfill, incineration with energy recovery, composting, anaerobic digestion and recycling compared on cost, emissions, energy and residual volume. Then the waste hierarchy applied as a decision rule, with the circular economy from topic 3 returning as the systemic alternative to managing waste better.
- Connects to
- Applies natural capital and the circular economy; connects to water pollution through plastics and leachate and to climate through landfill methane. Consumption patterns link it directly to the population topic that follows.
- Practical & inquiry angle
- A school waste audit by mass and category, with a costed intervention proposed and its diversion rate predicted.
25. Human Population Dynamics
- Teach to this depth
- Crude birth and death rates, fertility, natural increase and doubling time calculated, not merely defined. Age–sex pyramids read as predictions rather than descriptions — a student should be able to say what a given pyramid implies for the next forty years. The demographic transition model taught with its limitations named, including the countries it does not describe. Population policies of both kinds evaluated on effectiveness and on ethics. Then the connection the topic exists to make: population size, affluence and technology together determine environmental pressure, and treating population alone as the variable is a perspective, not a finding.
- Connects to
- The human application of the population dynamics from topic 4 — carrying capacity, limiting factors and S-curves, now with the complication that humans change their own carrying capacity. Feeds urbanisation and, through consumption, energy and waste.
- Practical & inquiry angle
- Constructing and interpreting pyramids for two contrasting countries from real census data and projecting the dependency burden forward.
26. Urban Systems and Sustainable Planning
- Teach to this depth
- The city analysed as a system with the vocabulary from topic 2 — inputs of food, water, energy and materials, outputs of waste, sewage, heat and emissions, and almost no internal cycling. Urbanisation and its drivers, and the forms urban expansion takes, including informal settlement. The urban heat island explained mechanistically from albedo, thermal mass and reduced evapotranspiration. Then planning responses evaluated: density and transport, green and blue infrastructure, sustainable drainage, and the ecological design of buildings and districts, each judged on what it does to a specific input or output rather than on how it sounds.
- Connects to
- A systems synthesis: it draws on the hydrological cycle for drainage and run-off, on energy and waste from earlier in the phase, on demography from the previous topic, and on the sustainability indicators from Phase A.
- Practical & inquiry angle
- Mapping the inputs and outputs of a real neighbourhood; measuring a heat island transect across a city with a thermometer and a route.
27. Urban Air Quality and Pollution
- Teach to this depth
- Primary and secondary pollutants distinguished, with the main urban pollutants traced to specific sources. Photochemical smog built as a chain of reactions rather than named, and the meteorological and topographic conditions — temperature inversion, basin location, calm — that concentrate it. Health and ecological impacts quantified where data allow. Management evaluated at the three levels of altering activity, regulating emissions and treating effects. Then acid deposition in full: the sulphur and nitrogen chemistry, transboundary transport, the effects on soils, water bodies and vegetation, and the reasons the pollution and the damage occur in different countries — the last making it the closing example of the course’s persistent theme that cause and consequence are unevenly distributed.
- Connects to
- The final synthesis: atmospheric structure and circulation from Phases B and C, urban systems from the previous topic, energy choices from earlier in this phase, and soil and water sensitivity from Phase E all bear on it. Acid deposition also returns to the transboundary problem first met as the tragedy of the commons.
- Practical & inquiry angle
- Measuring particulates or nitrogen dioxide along a traffic gradient with diffusion tubes or a low-cost sensor; using lichen distribution as a biological indicator of air quality.
Time allocation & two-year pacing
The IB recommends 150 teaching hours for ESS at Standard Level: 100 hours of syllabus content across the eight topics, and 50 hours for the experimental programme. The allocations in the sequence table above distribute those 100 content hours across all 27 topics; below they are reconciled topic by topic against the guide’s own figures and then paced so that teaching is complete by the end of January in Year 2.
Reconciliation to the official IB allocation
| IB syllabus component | Topics | IB hours | Allocated here |
|---|---|---|---|
| Topic 1 — Foundation | 3 | 16 h | 16 h |
| Topic 2 — Ecology | 5 | 22 h | 22 h |
| Topic 3 — Biodiversity and conservation | 3 | 13 h | 13 h |
| Topic 4 — Water | 4 | 12 h | 12 h |
| Topic 5 — Land | 2 | 8 h | 8 h |
| Topic 6 — Atmosphere and climate change | 4 | 10 h | 10 h |
| Topic 7 — Natural resources | 3 | 10 h | 10 h |
| Topic 8 — Human populations and urban systems | 3 | 9 h | 9 h |
| Syllabus content subtotal | 27 | 100 h | 100 h |
| Experimental programme — practical work | — | 30 h | 30 h |
| Experimental programme — collaborative sciences project | — | 10 h | 10 h |
| Experimental programme — individual investigation | — | 10 h | 10 h |
| SL course total | 27 | 150 h | 150 h |
Because this framework re-sequences topics across chapter boundaries, a phase’s hours will not match a syllabus topic’s. The reconciliation above proves that nothing has been added, lost or reweighted: every one of the eight syllabus topics receives exactly the hours the guide allocates it, and all 27 subtopics are present. Only the order has changed. The guide gives per-subtopic hours for the foundation topic (perspectives 3, systems 5, sustainability 8), which are used unaltered; within topics 2 to 8 the subtopic split is this framework’s.
The two-year pacing plan
Built on roughly two-and-a-half to three teaching hours per week across the two years, with practical work distributed through the phases rather than blocked. The cumulative column tracks progress toward the 150-hour total; the gold rows fall outside the teaching budget.
| Period | Focus | Hours | Cumul. |
|---|---|---|---|
| YEAR 1 | |||
| Autumn term | Phase A — Foundation · Phase B to nutrient cycling · fieldwork skills and the first practicals (8 h) | 40 | 40 |
| Spring term | Finish Phase B (biomes, succession) · Phase C — atmosphere and climate · Phase D to human pressures · practical work (10 h) | 35 | 75 |
| Summer term | Finish Phase D (conservation) · Phase E — water, soil and food · practical work (6 h) · collaborative sciences project (10 h) | 40 | 115 |
| YEAR 2 (to end January) | |||
| Autumn term | Phase F to human population dynamics · practical work (6 h) · individual investigation researched, written and submitted (10 h) | 29 | 144 |
| To end of January | Phase F completed — urban systems and urban air quality, taught as whole-course synthesis | 6 | 150 |
| FEBRUARY – APRIL | Dedicated revision: past papers, Paper 1 case-study drills and Paper 2 structured and extended response practice, and timed mocks (additional to the 150 teaching hours) | — | — |
| MAY | IB examinations | — | — |
A note on the individual investigation. The guide allocates 10 hours to it, but the fieldwork that makes a good investigation possible is spread across the 30 hours of practical work, not concentrated in those 10. This pacing deliberately front-loads technique — quadrats and transects in Phase B, water chemistry in Phase E, soil analysis alongside it — so that by the start of Year 2 a student has already run every method they are likely to need and knows from experience which ones produce usable data. The 10 hours are then spent on the investigation itself rather than on learning to hold a quadrat. The collaborative sciences project sits at the end of Year 1, where it can draw on a full year of systems thinking without competing with the investigation for attention.
Revision time (February–April) is deliberately additional to the 150 teaching hours, in line with the subject guide’s reminder that adequate time must be set aside for examination revision.