Climate Scenarios Explained: Reading SSP Labels

What do climate scenarios actually describe?

Climate scenarios describe possible futures under stated assumptions, not forecasts of which future will occur. Shared Socioeconomic Pathway labels combine a societal storyline with an approximate radiative-forcing level in 2100. In SSP2-4.5, the 2 identifies the storyline and 4.5 denotes watts per square metre, not degrees of warming. Read the complete label alongside the chart's variable, baseline and time window; neither the highest scenario nor the middle one is automatically the expected outcome.

This guide decodes the labels used in the Intergovernmental Panel on Climate Change's Sixth Assessment Report, or IPCC AR6. It does not run a climate model or update the probability of today's policy outcomes.

How is a scenario different from a projection?

A scenario supplies assumptions about how conditions develop. A projection describes a response conditional on those assumptions and on the model or assessment method.

The US Fifth National Climate Assessment gives examples of scenario inputs: emissions, population, energy and land use. It explicitly distinguishes climate projections from forecasts or predictions. Future choices and the climate system's response both matter. NCA5 scenarios appendix

Consider the statement “under SSP2-4.5, this assessment projects a change.” The words “under SSP2-4.5” are part of the claim. Removing them does not simplify the same result; it changes a conditional statement into an unconditional one.

For the broader process, including models and assessment ranges, see our climate-projections guide.

What do the two parts of an SSP label mean?

SSP stands for Shared Socioeconomic Pathway. The storyline describes a possible societal development path, including factors such as population, economic development and urbanization. Researchers can combine a storyline with different mitigation assumptions; the storyline number alone does not specify the resulting emissions.

The five families are broadly described as sustainability (SSP1), middle-of-the-road development (SSP2), regional rivalry (SSP3), inequality (SSP4) and fossil-fuel-intensive development (SSP5). These are scenario descriptions, not rankings of countries. IPCC Chapter 1, section 1.6.1.1

The suffix describes the approximate radiative-forcing level resulting in 2100. Radiative forcing measures a change in Earth's energy balance, expressed in watts per square metre, written W/m². It is different from the resulting temperature change.

The IPCC's own label definition calls the forcing level approximate. A label is not a promise that every climate model calculates exactly that forcing. IPCC Summary for Policymakers, footnote 22

How do the familiar labels compare?

The five illustrative scenarios used extensively in AR6's physical-science assessment are:

These are forcing categories relative to pre-industrial conditions, not extra warming from today. IPCC radiative-forcing assessments commonly use 1750 as their reference; a temperature chart may instead use 1850–1900. Record the reference for the quantity actually plotted.

Read the first two rows together: one societal family appears with two forcing levels. That alone shows why “SSP1” is incomplete when identifying a climate-model experiment. Read the third and fifth rows together: both the family and forcing label change, so the comparison is not an experiment in changing only one policy.

The IPCC describes SSP2-4.5 as intermediate emissions and SSP5-8.5 as very high emissions within this illustrative set. Neither name supplies a probability. IPCC scenario overview, Box SPM.1

Is the highest scenario the worst-case forecast?

No. The IPCC attaches no likelihood to the scenarios assessed in its physical-science report. Their presence in a figure does not mean equal odds, and a central-looking line is not automatically a median forecast.

A high scenario can help investigate consequential conditions without being selected as the expected future. Conversely, choosing a low scenario for a chart does not demonstrate that its assumptions will be achieved. Evaluating current policies requires current policy evidence, not a judgment based on a legend's position or color.

Nor does an endpoint describe the entire journey. The original Scenario Model Intercomparison Project design explicitly includes questions about peak-and-decline forcing and long-term outcomes. Two paths need not have the same earlier conditions just because their endpoints look similar. ScenarioMIP experimental-design paper

Are RCP4.5 and SSP2-4.5 interchangeable?

RCP means Representative Concentration Pathway, an earlier scenario framework. The Met Office's UKCP18 guidance explains that RCP names also incorporate forcing targets for 2100 relative to pre-industrial levels. One RCP can be consistent with different socioeconomic assumptions. Met Office RCP guidance

Matching suffixes do not make old and new datasets identical. IPCC Chapter 1 identifies differences in gas composition, trajectories and effective forcing between SSP and RCP scenarios. A difference between their projected temperatures cannot automatically be attributed only to a change in climate models.

Record the full scenario and dataset generation. Do not silently replace “RCP4.5” with “SSP2-4.5” while keeping another assessment's numbers.

How can you audit a caption without running a model?

Use this original reading exercise. Imagine a draft caption saying: “SSP2-4.5 predicts 4.5 degrees in 2100.” No dataset accompanies it.

Three checks reveal the problem:

1. Label check: 4.5 is the nominal forcing suffix, not a temperature result.
2. Claim check: “predicts” discards the conditional character of the projection.
3. Evidence check: no variable definition, temperature baseline, averaging period or source result supports the claimed number.

Do not repair it by guessing a different temperature. A defensible replacement is: “SSP2-4.5 identifies the scenario used; the temperature result must be read from the cited assessment with its reference period and uncertainty.”

For a real figure, make a short record:

The record does not calculate a result. It identifies what is missing before comparing two results.

Why do versions and warming-level maps matter?

Version information can change the interpretation. IIASA's legacy SSP database documents that its December 2018 release added low scenarios targeting 1.9 W/m² and harmonized emissions with historical inventories. That is a specific historical release, not a claim that its old update notice describes the latest database today. Copy the version associated with the figure you are reading. IIASA SSP database release notes

A global warming level is another organizing device, not another SSP suffix. A map at 2°C global warming asks about conditions at that warming level rather than identifying one calendar year. The Met Office explains that warming-level comparisons separate the amount of warming from when it is reached; it also cautions that effects such as sea-level rise do not stabilize simply because temperature does. Met Office projection guide

This distinction makes comparisons more useful, not less urgent. Read scenarios as conditions to examine, keep evidence about present policies separate, and retain uncertainty where the source places it. For location-specific engineering or emergency decisions, use qualified local professionals and current official guidance. A global scenario label cannot establish whether a particular place is safe.

Our climate basics section explains the underlying physical terms.

Sources

• IPCC AR6 Working Group I: Chapter 1, scenario framework and limitations.
• IPCC AR6 Working Group I: Summary for Policymakers, Box SPM.1 and footnote 22.
• US Fifth National Climate Assessment: Appendix 3, Scenarios and Datasets.
• O'Neill and colleagues: ScenarioMIP experimental design.
• Met Office: UKCP18 Representative Concentration Pathways guidance.
• IIASA: SSP database Version 2.0 release notes.
• Met Office: Making sense of climate change projections.