The climate planetary boundary at Higher Level: Different examples from SL: the 2025 Planetary Health Check, three radiative forcing studies, NASA's sea level record and 350.org. At HL, link the boundary to tipping points (6.2.13) and to who is responsible (6.2.15).
Practise this as you read
- Quote both limits with their latest values.
- Evaluate how strong the evidence is.
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A safe operating space: The planetary boundaries model sets a safe limit for each process. Climate change is one of the nine.
The points to remember
- Inside the limits is a safe operating space for humanity.
- Past a limit, the risk of large, possibly irreversible change rises.
- Each boundary is measured with a control variable.
- Limits are set with the precautionary principle: at the low end of the danger range.
Real example (HL): the first Planetary Health Check, in 2025, found seven of the nine boundaries crossed. Ocean acidification had just become the seventh.
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The climate boundary has two limits. Crossing either one means the boundary has been passed.
The points to remember
- Carbon dioxide: at most 350 ppm (ppm). Before industry it was 280 ppm.
- Radiative forcing: at most +1.0 W/m².
- Between the boundary and 450 ppm or +1.5 W/m² is a zone of increasing risk.
- The limits are set at the low end of that zone, to be safe.
Remember it as: 350 and 1: the two numbers of the climate boundary.
Real example (HL): radiative forcing adds up every human effect: carbon dioxide, methane and nitrous oxide warm, while some air pollution (aerosols) cools. It shows the total push on the energy balance.
Published measurements show that both limits have been passed, and the gap is still growing.
The points to remember
- Carbon dioxide passed 350 ppm in 1988; in 2025 the yearly average was 427 ppm.
- That is about 22% above the boundary, and it still rises by about 3 ppm a year.
- Radiative forcing is about +3.0 W/m² (2024): three times the limit.
- Both are past the boundary; radiative forcing is past the high-risk level too.
Real example (HL): three studies show the trend: 1.5 W/m² in 2009, 2.91 W/m² in 2023 and about 3.0 W/m² in the 2025 Planetary Health Check.
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Crossing the boundary does not mean instant disaster. It means we have left the safe zone, and the evidence of harm is growing.
The points to remember
- Global temperature is now more than 1.2 °C above pre-industrial levels; 2024 passed 1.5 °C.
- Ice sheets in Greenland and Antarctica are losing ice; sea level rise is speeding up.
- Extreme events, such as heatwaves, floods and wildfires, are more frequent and intense.
- The risk of passing tipping points, such as ice sheet collapse, rises.
- Carbon dioxide stays in the air for centuries, so the overshoot lasts.
Real example (HL): NASA's satellites show sea level rising 2.1 mm a year in 1993 but 4.5 mm a year in 2024, as warmer water expands and ice sheets melt faster.
The claim that the boundary has been passed is strong, but the boundary itself has limits.
The evidence is strong
- Direct, long records: Mauna Loa since 1958
- Both limits passed, in every study
- Matches the warming and melting seen
But the boundary has limits
- The exact safe level is uncertain
- One global number hides local harm
- It says nothing of who caused it
The points to remember
- Some argue a temperature limit, such as 1.5 °C or 2 °C, is easier to act on.
- Negative feedback, such as plants taking up carbon dioxide, slows warming a little.
- Fast mitigation (renewables, less methane, forests) can limit the overshoot.
- Getting back below 350 ppm needs net zero and removing carbon dioxide for decades.
Real example (HL): the campaign group 350.org, founded in 2008, is named after the 350 ppm limit. It argues the world must bring carbon dioxide back down, not just stop it rising.
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How this comes up: Paper 2 Section B (b) [7] can ask you to explain the evidence: one mark per point explained, with data where you have it.
The 2025 Planetary Health Check found that seven of the nine planetary boundaries have been crossed.
Explain how scientists know that the planetary boundary for climate change has been passed.
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