The SDGs tell us what we want. The planetary boundaries model tells us what the Earth will tolerate. It identifies nine processes that kept the planet stable throughout the Holocene, and sets a limit on how far we can disturb each one.
📘 What you need to know
Sustainability models are simplified versions of reality, so they have uses and limitations.
The planetary boundaries model outlines nine critical processes that regulated Earth’s stability and resilience during the Holocene.
It identifies limits to human disturbance to prevent abrupt and irreversible change.
Uses: identifies science-based limits, highlights the need for action beyond climate change, raises awareness.
Limitations: ignores societal factors, is a work in progress, and its global focus may not suit local action.
Crossing one boundary can affect the others, sometimes unpredictably.
Models are simplifications
A range of frameworks and models support our understanding of sustainability. Like all models — as covered in 1.2 — they are simplified versions of reality, which means every one of them has both uses and limitations. The SDGs covered earlier in this sub-topic are one such model, created by the UN in 2015. The planetary boundaries model is another, and it approaches the problem from the opposite direction.
Two frameworks, two questions. The SDGs ask what should we achieve? and answer socially. Planetary boundaries ask what must we not exceed? and answer scientifically. Neither is complete on its own, which is exactly why the doughnut model in the next note combines them.
The planetary boundaries model
The planetary boundaries model outlines nine critical processes and systems that have regulated the stability and resilience of the Earth system during the Holocene epoch. Scientists created it to specify the ecological limits within which humanity could operate safely, identifying limits to human disturbance that prevent abrupt and irreversible change.
The length of each wedge shows how far that process has been pushed. Anything reaching past the dashed line is outside the safe operating space.
The nine boundaries
Boundary
What it means
Example
Climate change
Human-induced alteration of Earth’s climate system, seen in rising temperatures, sea level rise and extreme weather
Increasing frequency and intensity of hurricanes
Erosion of biosphere integrity
Reduction in Earth’s variety of life through habitat destruction, species extinction and ecosystem degradation
Deforestation of the Amazon rainforest causing loss of species diversity
Biogeochemical flows
Disruption of the nitrogen and phosphorus cycles by agricultural and industrial activity
The “dead zone” in the Gulf of Mexico from Mississippi River nutrient runoff
Stratospheric ozone depletion
Thinning of the ozone layer by chemicals such as CFCs, increasing UV exposure
The Antarctic ozone hole formed by CFC emissions
Ocean acidification
Falling ocean pH as oceans absorb more carbon dioxide, damaging organisms with calcium carbonate shells
Coral bleaching on the Great Barrier Reef
Freshwater use
Unsustainable extraction depleting aquifers, reducing river flows and degrading ecosystems
The Aral Sea shrinking from excessive irrigation withdrawals
Land system change
Conversion of natural ecosystems to urban, agricultural and industrial land
Amazon deforestation for cattle ranching and soy production
Chemical pollution
Release of synthetic chemicals, or novel entities, that harm health, ecosystems and wildlife
PCB contamination in rivers affecting fish populations
Atmospheric aerosol loading
Emission of particulate matter and aerosols affecting climate, air quality and health
Smog formation in cities from industrial emissions
Judging the model
That first limitation is the important one. A model of ecological limits alone cannot tell you who should cut back, or by how much.
Uses
Identifies science-based limits. It gives clear boundaries grounded in scientific understanding of Earth systems.
Highlights the need for comprehensive action. It shifts focus beyond climate change, which dominates public discussion, to other critical environmental issues.
Raises awareness. It alerts the public and policymakers to the urgency of protecting Earth’s systems.
Limitations
Ignores societal factors. It focuses only on ecological systems and does not consider the human dimension needed to act on environmental justice.
A work in progress. Assessments of the boundaries change constantly as new data becomes available.
Global focus may not suit local action. The boundaries may not align with local or national priorities, making them hard to implement at smaller scales.
You do not need to learn all nine boundaries by name, but you must understand that crossing one can affect the others, sometimes unpredictably. Crossing the freshwater use boundary disrupts aquatic habitats and reduces resources for freshwater-dependent species, which pushes on the biosphere integrity boundary. That interconnection is the systems thinking from 1.2 applied at planetary scale.
Worked examples
WE 1
The model explained
Outline the planetary boundaries model. (3 marks)
Step 1: what it identifies
It outlines nine critical processes and systems that regulated the stability and resilience of the Earth system during the Holocene epoch.
Step 2: what it is for
It specifies the ecological limits within which humanity can operate safely.
Step 3: why the limits matter
It identifies limits to human disturbance of these processes in order to prevent abrupt and irreversible change.
Nine processes, each with a limit humanity should not cross“abrupt and irreversible” links directly to tipping points in 1.2
WE 2
Interconnected boundaries
Explain, with an example, why crossing one planetary boundary may affect others. (3 marks)
Step 1: the principle
The nine processes are components of a single interconnected Earth system, so disturbing one alters conditions for the others.
Step 2: the example
Crossing the freshwater use boundary through excessive extraction depletes aquifers and reduces river flows.
Step 3: the knock-on effect
This disrupts aquatic habitats and reduces resources for species dependent on freshwater ecosystems, contributing to the erosion of biosphere integrity.
The boundaries are not nine separate problems but one system with nine measuresthe Aral Sea works well as a real-world illustration of this chain
WE 3
Comparing frameworks
Compare the SDGs and the planetary boundaries model as frameworks for sustainability. (4 marks)
Similarity
Both are simplified models of reality intended to guide action on sustainability, and both raise public and policy awareness.
Difference 1: what they measure
The SDGs set social and environmental objectives to be achieved. Planetary boundaries set science-based ecological limits not to be exceeded.
Difference 2: the human dimension
The SDGs address poverty, health, education and inequality directly. The planetary boundaries model ignores societal factors entirely, so it offers no route to environmental justice.
Shared weakness
Both are hard to apply locally: the SDGs may overlook local contexts, and the boundaries are global measures that may not align with national priorities.
One sets goals for people, the other sets limits for the planet; neither does boththis gap is precisely what the doughnut model was designed to fill
💡 Exam tips
Say nine boundaries and mention the Holocene.
Use the phrase abrupt and irreversible change when explaining why limits exist.
You do not need all nine names, but know three or four with examples.
Learn three uses and three limitations. They are examined directly.
The strongest limitation is that it ignores societal factors.
Be ready to explain that boundaries interact, with the freshwater and biodiversity example.
⚠ Common mistakes
Treating the nine boundaries as independent. Crossing one affects others.
Trying to memorise all nine with figures. Not required; the model itself is what is examined.
Saying the model covers social issues. It explicitly does not, and that is its main weakness.
Presenting the boundaries as fixed. They are revised as new data emerges.
Confusing the boundary with the process. Climate change is the process; the boundary is the safe limit on it.
Assuming only climate change matters. The model exists partly to correct that assumption.
Up next: Rethinking Economic Models. The planetary boundaries give the ecological ceiling; the social SDGs give the floor. The final page in this sub-topic is about the economic models built to fit between them.
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