DSS5101 Chap.2 Planetary Boundaries and the Polycrisis
Planetary Boundaries and the Polycrisis
A polycrisis with physical limits
The course synopsis describes the present as a polycrisis in which climate change, conflict and biodiversity loss arrive together, and the Week 1 outline places it in the age of the Anthropocene. The planetary boundaries framework, presented in the Week 3 lecture in its 2025 update, gives that idea a physical form.
It identifies nine Earth-system processes, sets a boundary for each, and describes the region inside all nine as a safe operating space for humanity.
Where the nine processes stand
According to the lecture’s summary of the update, six boundaries are transgressed. Climate change is beyond its boundary and carbon dioxide is still rising.
Novel entities, synthetic chemicals and modified organisms released without adequate testing, sit in the high-risk zone. Biosphere integrity is outside safe levels on genetic diversity and on functional integrity. The flows of nitrogen and phosphorus, freshwater change across rivers and soil moisture, and land system change as forests shrink below safe levels complete the six.
Three remain inside: stratospheric ozone, now recovered; aerosol loading, just within the boundary on its global metric; and ocean acidification, inside but close to crossing as the sea absorbs carbon dioxide.
How to read a boundary
The lecture quotes the lead author of the 2023 assessment comparing the boundaries to blood pressure.
A reading above the line does not mean a certain collapse; it means higher risk, which is a reason to act. The ozone boundary shows the other half of the analogy. It was exceeded in the 1990s and is no longer transgressed, because the phase-out of ozone-depleting substances under the Montreal Protocol worked.
A boundary can therefore be recrossed in the safe direction, and a boundary still inside the line can be close to failing.
The chapter also works through the climate numbers the lecture presents, including the pace of emissions cuts needed for 1.5°C and 2°C and the warming implied by current national policies.
What this chapter covers
- 01
The polycrisis framing in the course synopsis
- 02
Nine Earth-system processes and the safe operating space
- 03
Six boundaries transgressed in the 2025 update
- 04
Ozone, aerosols and ocean acidification inside the line
- 05
The blood-pressure reading of a boundary
- 06
Emissions cuts needed for 1.5 and 2 degrees
Worked example · free
Checking the arithmetic behind a 2 degree pathway
- 1Convert the annual decline into a yearly multiplier.
- 2Apply it for six years and read off the total cut.
- 2Recompute for four years and state the consequence.
Key terms
- Planetary Boundaries
- Limits set for nine Earth-system processes, inside which the framework judges that humanity can operate with acceptable risk.
- Safe Operating Space
- The region inside all nine boundaries at once, where the Earth system is judged likely to remain stable.
- Novel Entities
- Synthetic chemicals, new materials and modified organisms released into the environment, placed in the high-risk zone because many are released without adequate testing.
- Biosphere Integrity
- The variety, abundance and condition of life and of ecosystems, which help regulate the planet's energy balance and chemical cycles.
- Ocean Acidification
- The fall in ocean pH as seawater absorbs atmospheric carbon dioxide, harming organisms that build shells and skeletons from calcium carbonate.
- Polycrisis
- A set of crises, such as climate change, conflict and biodiversity loss, that occur together and interact rather than arriving one at a time.
Planetary Boundaries and the Polycrisis FAQ
How many planetary boundaries have been crossed?
The 2025 update presented in the climate lecture reports six of the nine as transgressed: climate change, novel entities, biosphere integrity, nitrogen and phosphorus flows, freshwater change and land system change. Stratospheric ozone, aerosol loading and ocean acidification remain inside their boundaries.
Does crossing a boundary mean collapse is certain?
No. The lead author quoted in the lecture compares a boundary to a blood-pressure threshold: exceeding it raises the risk of harm without making harm certain. It is a signal to reduce pressure, not a countdown.
Can a boundary be brought back into the safe space?
Yes. Stratospheric ozone was beyond its boundary in the 1990s and is no longer transgressed, because the international phase-out of ozone-depleting substances under the Montreal Protocol reduced the pressure. It is the framework's clearest example of recovery through policy.
Why is ocean acidification listed as safe if carbon dioxide keeps rising?
Its indicator, the saturation state of a carbonate mineral in seawater, is still just inside the boundary. The lecture notes that rising atmospheric carbon dioxide means it is close to crossing, so safe here describes the present reading rather than the trend.
How fast would emissions need to fall to keep to 1.5 degrees?
The lecture cites a 42 per cent cut by 2030, about 8.7 per cent a year, against 28 per cent for 2 degrees. It also reports that current national policies point to about 3 degrees and that the chance of holding 1.5 degrees is around 14 per cent.
What does polycrisis mean in this course?
It is the synopsis's word for crises that arrive together and reinforce one another, naming climate change, conflict and biodiversity loss. The boundaries framework is one way of measuring the environmental side of it.
Assessment move
Memorise the nine processes as two lists, crossed and not crossed, and attach one sentence of reasoning to each. Then practise explaining a boundary with the blood-pressure analogy and the ozone precedent, since those two ideas answer most questions about what a crossed line does and does not mean.
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