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IPCC Scenarios: SSP1 to SSP5
Why scenarios matter
Scenarios are not predictions. They are structured descriptions of plausible futures that allow scientists, policymakers, and businesses to explore what the climate could look like depending on the choices humanity makes over the coming decades. The IPCC's Shared Socioeconomic Pathways (SSPs) are the primary tool for this kind of forward-looking analysis.
From Emissions to Futures: The Logic of Scenarios
The climate does not respond to intent or policy announcements. It responds to the physical concentration of greenhouse gases in the atmosphere. Whether those concentrations rise, stabilize, or fall depends on the cumulative choices made by governments, businesses, and individuals across every sector of the global economy. Scenarios make those choices explicit and tractable.
Since the IPCC's Fifth Assessment Report (AR5), the scientific community has used a framework called Shared Socioeconomic Pathways (SSPs). Each SSP describes a distinct global development narrative, covering how population grows, how economies expand, how energy systems evolve, how land is used, and how well international cooperation functions. These narratives are then combined with radiative forcing targets (measured in watts per square metre, W/m²) to produce specific temperature outcomes by 2100.
Analogy: Roads to the same destination, or very different ones
Think of SSPs like different routes you could take on a road trip. Some routes involve well-maintained highways with modern fuel-efficient vehicles; others wind through traffic-choked roads in ageing cars. The destination (the climate in 2100) is determined by which route you take and how fast you drive. The SSPs define the route; the radiative forcing level determines the speed.
The Five SSP Narratives
The five pathways span a spectrum from deep global cooperation and sustainable development to fossil-fuel-intensive fragmentation. Here is what each one represents.
| Scenario | Nickname | Core Narrative | Approx. Warming by 2100 |
|---|---|---|---|
| SSP1-1.9 | Sustainability | Strong global cooperation, rapid renewable energy transition, low population growth, circular economies | approx. 1.0-1.8°C |
| SSP1-2.6 | Sustainability (high) | Same sustainability narrative with somewhat higher near-term emissions before steep decline | approx. 1.3-2.4°C |
| SSP2-4.5 | Middle of the Road | Current trends continue; some improvement in energy efficiency; moderate international cooperation | approx. 2.1-3.5°C |
| SSP3-7.0 | Regional Rivalry | Nationalism, fragmentation, high emissions, slow technology transfer, inequality increases | approx. 2.8-4.6°C |
| SSP5-8.5 | Fossil-Fuelled Development | Rapid economic growth driven by fossil fuels; very high energy demand; no meaningful climate policy | approx. 3.3-5.7°C |
The numbers appended to each SSP (1.9, 2.6, 4.5, 7.0, 8.5) refer to the radiative forcing in W/m² that the scenario produces by the year 2100. Higher forcing means more energy trapped in the climate system, and therefore more warming.
SSP1: The Sustainability Pathway
SSP1 envisions a world that pivots decisively toward sustainable development. Population peaks below 9 billion and begins to decline. Dietary shifts toward less meat reduce pressure on land. Renewable energy becomes dominant. International institutions function well, enabling technology transfer and climate finance to flow to developing nations. Inequality falls both within and between countries.
In the most aggressive variant (SSP1-1.9), global CO₂ emissions reach net zero before mid-century, and carbon dioxide removal (CDR) technologies are deployed at scale. This is the only scenario consistent with the Paris Agreement's 1.5°C aspiration.
SSP2: Middle of the Road
SSP2 is often described as the "business as usual with improvements" scenario. Progress on social and environmental goals is uneven. Energy systems evolve, but fossil fuels remain significant for decades. Emissions peak around mid-century and then decline, but not steeply enough to avoid substantial warming. This is broadly consistent with current national policies and pledges without strong additional ambition.
SSP3: Regional Rivalry
SSP3 imagines a world reshaped by nationalism and competition. Nations focus on energy and food security within their own borders. Investment in education and technology stagnates in many regions. Emissions remain high throughout the century. This pathway produces severe climate impacts and heightens the risk of crossing multiple tipping points (covered in lesson 3.1).
SSP5: Fossil-Fuelled Development
SSP5 is the highest-emissions scenario. It assumes that rapid economic growth is achieved by exploiting abundant fossil fuel resources. Energy use is very high, living standards improve in many regions, but at enormous climate cost. Atmospheric CO₂ concentrations exceed 1,000 ppm by 2100. This scenario produces the most severe warming and is increasingly regarded as physically implausible given declining renewable costs, but it remains useful as an upper-bound reference.
Radiative forcing (RF) is the net change in energy flux in the atmosphere caused by a change in the concentration of a greenhouse gas or other climate driver. A positive RF means more energy is being retained by the Earth system, causing warming. It is measured in watts per square metre (W/m²), representing the additional energy trapped per unit of Earth's surface area.
Pre-industrial CO₂ levels produced a baseline RF of approximately 0 (by convention). Each SSP scenario targets a specific RF level by 2100: SSP5-8.5 represents 8.5 W/m² of additional forcing above pre-industrial levels, roughly equivalent to the energy output of a 8.5-watt lightbulb for every square metre of Earth's surface, continuously. That is an enormous amount of additional energy.
The Gap Between Current Policies and Paris Targets
A critical question is: where do current national policies and pledges (Nationally Determined Contributions, or NDCs) land on this spectrum? According to the IPCC AR6 WGIII assessment, the aggregate of current NDCs as of 2021 is most consistent with trajectories between SSP2-4.5 and SSP3-7.0, pointing toward roughly 2.5-3.5°C of warming by 2100 if no additional policies are adopted.
Limiting warming to 1.5°C requires a trajectory closer to SSP1-1.9, which demands that global emissions peak before 2025 and fall by roughly 43% by 2030 relative to 2019 levels, before reaching net zero CO₂ in the early 2050s.
Real-world application: Scenario analysis in corporate strategy
Under IFRS S2 (the ISSB's climate disclosure standard) and the Task Force on Climate-related Financial Disclosures (TCFD), companies are required to analyze their business resilience across multiple climate scenarios. Most scenario analyses use SSP1-2.6 (or a 2°C equivalent) as the "orderly transition" scenario, and SSP3-7.0 or SSP5-8.5 as the "delayed transition" or "hot house world" scenario. Understanding what each SSP narrative implies about policy, technology, and physical risk is therefore a core literacy for any sustainability professional.
Key Takeaways
- 1SSPs (Shared Socioeconomic Pathways) are structured narratives describing possible futures based on different development choices, not predictions
- 2The five SSPs range from deep sustainability (SSP1) to fossil-fuelled growth (SSP5), with warming projections from below 2°C to nearly 5.7°C by 2100
- 3The radiative forcing numbers (1.9 to 8.5 W/m²) quantify how much extra energy is trapped in the climate system under each scenario
- 4Current NDC pledges are broadly consistent with SSP2-4.5 to SSP3-7.0, pointing to 2.5-3.5°C of warming
- 5Limiting warming to 1.5°C requires the steepest decarbonisation trajectory (SSP1-1.9), with emissions falling 43% by 2030 and net-zero CO2 in the early 2050s