IB ESS SL1.3 SustainabilityPaper 1 & 2Core idea~12 min read
What Sustainability Means
Sustainability is one of those words that gets used so often it starts to mean nothing. In ESS it has a precise meaning, and it comes straight out of systems thinking: a system is sustainable when the flows out of it are not bigger than the flows back in.
📚 What you need to know
Sustainability is using resources at a rate that allows natural regeneration and does not damage the environment.
Natural capital is the stock of natural resources. Natural income is the yield that stock produces each year.
A system is sustainable if you live on the income and leave the capital intact.
Sustainability has three dimensions or pillars: environmental, social and economic.
Natural capital can be renewable, non-renewable or replenishable.
Natural capital also provides ecosystem services, not just materials.
Strong sustainability says natural capital cannot be replaced by human-made capital; weak sustainability says it sometimes can.
The definition, and why it works
DefinitionSustainability is the use of resources at a rate that allows natural regeneration and minimises damage to the environment.
Read that twice, because there are two separate conditions hiding in it. The resource has to be able to replace itself as fast as you take it, and the taking must not wreck the system that produces it. Fail either one and it is not sustainable, however efficient you are.
This is really a systems idea. A stock with an outflow bigger than its inflow shrinks. Every time. There is no clever way round it.
Natural capital and natural income
The easiest way to hold on to this is to think about money.
Natural capital is the savings account — the stock of natural resources: forests, fish stocks, soil, fresh water, minerals, the atmosphere.
Natural income is the interest that account pays each year — the new growth, the new fish, the water that recharges the aquifer.
Spend only the interest and you can keep spending forever. Start dipping into the savings and the interest gets smaller every year, so the collapse speeds up as it goes.
The red line gets steeper as it falls. A smaller stock produces less income, so the same harvest takes an ever bigger bite out of what is left.
Three kinds of natural capital
Type
What it means
Examples
Renewable
Replaces itself naturally, and can be used sustainably if the harvest stays within the natural income
Timber, fish, crops, wind and solar energy
Non-renewable
Forms far too slowly to be replaced on a human timescale, so any use runs the stock down
Fossil fuels, metal ores, minerals
Replenishable
Not alive, but restored by natural processes as long as those processes keep working
Groundwater, soil, the ozone layer
Watch the word “renewable”. A forest is renewable, but that does not make logging automatically sustainable. Cut faster than the trees grow back and you are treating a renewable resource exactly like a non-renewable one. The label describes the resource; sustainability describes how you use it.
Natural capital does more than supply things
It is easy to think of natural capital only as stuff you can take. Most of what it provides is not stuff at all. These are ecosystem services.
Regulating: climate regulation, flood control, pollination, water purification, carbon storage.
Supporting: soil formation, nutrient cycling, photosynthesis — the processes everything else depends on.
Cultural: recreation, beauty, spiritual and religious value, a sense of place.
Why this matters in an exam. A wetland drained for farmland loses its provisioning value in a way that is easy to count, and its flood control, water purification and nursery-habitat value in a way that is not. Answers that mention only the timber or the crops miss most of what was actually lost.
The three pillars
Sustainability is not just an environmental question. A solution that protects a forest but leaves local people with no income will not survive contact with reality, and neither will one that makes money but poisons a river.
Most real disputes are about trade-offs between two pillars. Naming which two is often the whole answer to a Paper 2 question.
Strong and weak sustainability
Here is the argument that sits underneath most environmental debates, and it is worth knowing because it explains why two sensible people can disagree completely.
Strong sustainability
Weak sustainability
The claim
Natural capital cannot be replaced by human-made capital
What matters is the total stock of capital, whatever kind it is
So
Natural capital must be maintained separately and protected
Losing natural capital is acceptable if enough human-made capital is gained
Example view
No amount of money replaces a lost species or a drained aquifer
Money from mining can build schools and technology that do the same job
Weakness
Can block development that people badly need
Assumes substitutes exist, and some things have no substitute
🧠
Easy way to remember it
Strong sustainability puts a strong fence round nature: it is not for sale. Weak sustainability lets the fence bend if the swap looks like a good deal.
Worked examples
WE 1
Explain sustainability using natural capital
Explain what is meant by sustainability, using the terms natural capital and natural income. (4 marks)
Point 1: natural capital
Natural capital is the stock of natural resources, such as a forest or a fish population.
Point 2: natural income
Natural income is the yield that stock produces, such as new growth or new fish each year.
Point 3: the condition
Use is sustainable when the harvest does not exceed the natural income, so the capital is left intact.
Point 4: the consequence of failing it
If harvest exceeds natural income the stock shrinks, so future income falls and the decline accelerates.
Live on the income and the capital lasts foreverthe fourth point — that a shrinking stock yields less income — is what turns a definition into an explanation
WE 2
Apply the three pillars
A company plans to clear an area of mangrove forest to build a shrimp farm. Evaluate the sustainability of this plan. (4 marks)
Environmental
Clearing mangroves destroys natural capital and the ecosystem services it provides, including coastal protection, water filtering and nursery habitat for fish.
Economic
The farm creates income and jobs in the short term, but shrimp ponds often become unproductive after a few years, so the income is not long term.
Social
Local fishing communities may lose the fish stocks and the storm protection they depend on, so benefits and costs fall on different people.
Judgement
It fails the environmental and social pillars, and the economic gain is short lived, so the plan is not sustainable.
Short-term profit paid for with long-term natural capitalevaluate means judge — finish with a clear verdict, not a list of points
WE 3
Classify natural capital
Distinguish between renewable, replenishable and non-renewable natural capital, giving one example of each. (3 marks)
Renewable
Living resources that regenerate naturally, such as a fish population — sustainable if harvest stays within natural income.
Replenishable
Non-living resources restored by natural processes, such as groundwater or soil.
Non-renewable
Resources that form far too slowly to be replaced, such as fossil fuels, so any use depletes the stock.
Renewable regrows, replenishable refills, non-renewable simply runs outadd the harvest condition to the renewable one — it shows you know renewable is not the same as sustainable
💡 Exam tips
Learn the definition of sustainability word for word. It appears constantly.
Use natural capital and natural income in the same sentence — the contrast is where the marks are.
When judging a plan, walk through all three pillars and then give a verdict.
Mention ecosystem services, not just resources you can carry away.
Know the difference between strong and weak sustainability, and which one an argument is assuming.
Link back to 1.2: a sustainable system is one whose stores are not being run down by its flows.
⚠ Common mistakes
Treating renewable as automatically sustainable. It depends entirely on the rate of use.
Saying sustainability just means protecting nature. It has three pillars, not one.
Mixing up natural capital and natural income. Capital is the stock; income is the yield.
Forgetting the second half of the definition. Minimising environmental damage matters as much as the rate of use.
Calling soil and groundwater renewable. They are replenishable.
Vague verdicts. “It is quite sustainable” scores nothing. Say which pillars pass and which fail.
Up next: Sustainable Development and the SDGs — what happens when the world tries to turn this idea into an actual plan with targets and deadlines.
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