A scenario is not a prediction. It is a sentence of the form “if the world does this, the climate does that”. The IPCC builds five of them because the biggest uncertainty in the climate future is not the physics — it is us. How much we cooperate, how fast technology moves and what policies get passed all sit inside those five storylines.
📚 What you need to know
Emissions scenarios help predict future climate outcomes and set clear targets for reducing risk.
They are produced by the IPCC, the Intergovernmental Panel on Climate Change.
The five Shared Socioeconomic Pathways (SSPs) combine emissions with social and economic factors: cooperation, economic policy, population growth and technology.
SSP1 sustainability, SSP2 middle of the road, SSP3 regional rivalry, SSP4 inequality, SSP5 fossil-fuelled development.
Scenario names combine the SSP with the radiative forcing expected in 2100, in watts per square metre — so SSP1-1.9 up to SSP5-8.5.
Scenarios vary because of uncertainty in future actions, technological development and policy change.
The five storylines
Each SSP describes a different kind of world. Learn them as short stories rather than as numbers, because that is how they were designed and it makes them far easier to recall under pressure.
SSP1 — Sustainability, “Taking the Green Road”. Strong global cooperation, reduced inequality, heavy investment in green technology and low-carbon energy. Lowest emissions, warming held around 1.5–2°C.
SSP2 — “Middle of the Road”. Current trends continue with no dramatic change, moderate growth, limited progress on inequality and a mixed energy investment. Moderate emissions and moderate warming.
SSP3 — Regional Rivalry, “A Rocky Road”. A fragmented world with regional conflicts and minimal cooperation, little technology sharing and heavy reliance on fossil fuels. High emissions and substantial risk of severe impacts.
SSP4 — Inequality, “A Road Divided”. Large disparities between and within countries; wealthy nations adapt well while poorer regions struggle. Moderate to high emissions with very uneven adaptation success.
SSP5 — Fossil-fuelled Development, “Taking the Highway”. Rapid growth driven by high fossil fuel use and technological advance, with the environment a lower priority. Very high emissions and significant warming.
Drawn to show the shape and spacing of the pathways rather than to reproduce exact model output. The point to take from it is the fan: identical physics, wildly different outcomes, and the difference is entirely human decisions.
The detail almost everyone misses is that the lines stay bunched together until roughly 2040. That is not a drawing error. Carbon dioxide already released keeps warming the planet for decades, so the next twenty years are largely locked in whatever we do. Emissions cuts made now change the second half of the century, not the next one. Say that and you have explained why both mitigation and adaptation are needed at once.
Reading the scenario names
The name has two halves. SSP1 to SSP5 is the social and economic storyline. The number after the dash — 1.9, 2.6, 4.5, 7.0, 8.5 — is the radiative forcing expected in the year 2100, measured in watts per square metre. Radiative forcing is the extra energy the atmosphere holds onto for each square metre of Earth’s surface, so a bigger number means more trapped energy and more warming. SSP5-8.5 is therefore the fossil-fuelled world producing 8.5 watts per square metre of extra forcing.
What the numbers actually say
Scenario
Emissions storyline
Warming 2041–2060
Warming 2081–2100
Very likely range
SSP1-1.9
Very low; carbon dioxide reaches net zero around 2050
1.6°C
1.4°C
1.0–1.8°C
SSP1-2.6
Low; net zero reached around 2075
1.7°C
1.8°C
1.3–2.4°C
SSP2-4.5
Intermediate; near current levels to 2050, then falling but not to net zero
2.0°C
2.7°C
2.1–3.5°C
SSP3-7.0
High; emissions roughly double by 2100
2.1°C
3.6°C
2.8–4.6°C
SSP5-8.5
Very high; emissions roughly triple by 2075
2.4°C
4.4°C
3.3–5.7°C
Look at SSP1-1.9 carefully. Warming is higher in 2041–2060 than in 2081–2100. That is the only scenario where temperature comes back down, and it happens because emissions reach net zero and removals start to exceed them. It is the clearest possible illustration that overshoot followed by recovery is physically possible — if the cuts are deep enough and early enough.
Why every number comes with a range
Two separate things widen these bars: we do not know exactly how much will be emitted, and we do not know precisely how sensitive the climate is to each extra tonne. Push emissions higher and both uncertainties grow together.
Where the uncertainty comes from
Future actions. Nobody knows what governments and businesses will actually do.
Technological development. Cheap storage, working carbon removal or a breakthrough in clean fuels would change the picture; so would none of those arriving.
Policy changes. Elections, treaties and economic shocks all move emissions.
WORKED EXAMPLE
Explain why the IPCC produces several emissions scenarios rather than a single projection. [3]
Point 1 — human behaviour is unknownFuture emissions depend on political, economic and social choices that cannot be predicted, so no single pathway can be assumed.Point 2 — scenarios isolate the consequences of choicesEach SSP holds a different storyline about cooperation, growth and technology, showing what climate outcome follows from each set of decisions.Point 3 — use in policyThe comparison lets governments see the risk avoided by stronger action, which supports setting targets such as limiting warming to 1.5°C.3 / 3Note the framing: scenarios are decision tools, not forecasts. That distinction is the heart of the question.
WORKED EXAMPLE
Using the data in the table, compare SSP1-2.6 and SSP3-7.0. [3]
Point 1 — quote both figuresSSP1-2.6 gives about 1.8°C of warming by 2081–2100, while SSP3-7.0 gives about 3.6°C, roughly double.Point 2 — link to the storylineSSP1-2.6 assumes strong cooperation and net zero around 2075, whereas SSP3-7.0 assumes regional rivalry, little technology sharing and emissions doubling.Point 3 — comment on uncertaintyThe very likely range is also much wider under SSP3-7.0, at 2.8–4.6°C compared with 1.3–2.4°C, so the higher-emission future is less predictable as well as hotter.3 / 3In data questions, quote the actual figures with units. Describing the trend without numbers rarely gets full marks.
💡 Exam tip
Learn the five storylines by their nicknames — green road, middle of the road, rocky road, road divided, highway. Far easier to recall than the numbers.
Explain the naming system if asked: SSP number = social storyline, second number = radiative forcing in 2100.
In data questions, always quote figures with units and give a comparison, not just a direction.
Use the phrase “scenarios are not predictions”. It is the single most important idea on this page.
Mention that ranges widen at higher emissions. It is a sophisticated observation and it is visible in any version of the table.
If asked why action is urgent, use the fact that the pathways barely separate before 2040.
⚠️ Common mix-up
Calling SSP5-8.5 “what will happen”. It is a high-emission storyline, not a forecast.
Reading 8.5 as degrees. It is watts per square metre of radiative forcing, not temperature.
Forgetting SSP4 in a list. There are five pathways even though warming tables often show only four.
Assuming scenarios differ only in emissions. They differ in population, inequality, cooperation and technology too.
Ignoring the ranges. The best estimate is a midpoint; the very likely range is part of the answer.
Mixing up the IPCC and the UNFCCC. The IPCC assesses the science and builds the scenarios; the UNFCCC negotiates the response.
Up next: Geoengineering the Climate — the technologies proposed for when mitigation is not moving fast enough, and the reasons most scientists treat them with caution.
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