RCP scenarios were the standard framework for climate projections from 2013 through 2021, powering the CMIP5 climate models used in IPCC AR5. While SSP scenarios have since replaced them in IPCC AR6, RCP scenarios remain widely referenced in existing climate studies, risk assessments, and regulatory documents. Understanding RCP scenarios is necessary to interpret the large body of climate research built on them and to understand how the newer SSP framework relates.
RCP stands for Representative Concentration Pathways. Each RCP scenario defines a trajectory for greenhouse gas concentrations and the resulting radiative forcing through 2100, without specifying the socioeconomic conditions that produce those emissions.
What Are RCP Scenarios?
RCP scenarios describe four pathways for radiative forcing, the amount of additional energy trapped in Earth’s atmosphere by greenhouse gases. Each pathway is named after its forcing level in watts per square meter (W/m2) by 2100: RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5.
Radiative forcing is the key variable because it determines how much the planet warms. Higher forcing means more trapped energy, higher temperatures, and more severe climate impacts. RCP scenarios provided the inputs to CMIP5 climate models, which generated the temperature, precipitation, and sea level projections used in thousands of climate studies and risk assessments between 2013 and 2021.
Unlike the newer SSP scenarios, RCP scenarios do not specify how emissions reach a particular level. RCP 4.5 defines the forcing pathway but does not describe whether that outcome results from strong climate policy, technological change, or economic shifts. The SSP framework added these socioeconomic narratives to provide richer context for climate scenario analysis.
The Four RCP Scenarios Compared
| Scenario | Forcing by 2100 | Warming by 2100 | CO2 Trajectory | Description |
|---|---|---|---|---|
| RCP 2.6 | 2.6 W/m2 | ~1.6 C | Peaks ~2020, declines sharply | Strong mitigation |
| RCP 4.5 | 4.5 W/m2 | ~2.4 C | Peaks ~2040, stabilizes | Moderate mitigation |
| RCP 6.0 | 6.0 W/m2 | ~2.8 C | Peaks ~2080, stabilizes | Limited mitigation |
| RCP 8.5 | 8.5 W/m2 | ~4.3 C | Continuous increase through 2100 | No mitigation (business as usual) |
RCP 2.6: The Strong Mitigation Pathway
RCP 2.6 represents the most ambitious mitigation pathway, where global emissions peak before 2020 and decline rapidly to near-zero by the second half of the century. Forcing peaks at approximately 3.0 W/m2 mid-century before declining to 2.6 W/m2 by 2100, producing roughly 1.6 degrees of warming.
Achieving RCP 2.6 requires aggressive decarbonization, widespread deployment of carbon capture technology, and significant changes to land use. Most climate scientists consider it the pathway most consistent with the Paris Agreement’s 2-degree target.
RCP 4.5 and RCP 6.0: Moderate Pathways
RCP 4.5 is the most commonly referenced moderate pathway in RCP scenarios. Emissions peak around 2040 and then stabilize, with forcing reaching 4.5 W/m2 by 2100. Warming of approximately 2.4 degrees represents a world that takes meaningful but incomplete climate action.
RCP 6.0 sits between RCP 4.5 and RCP 8.5. Emissions continue growing until 2080 before stabilizing, and forcing reaches 6.0 W/m2. With approximately 2.8 degrees of warming, RCP 6.0 represents limited mitigation effort and delayed action.
In practice, RCP 6.0 received less attention than the other three pathways because RCP 4.5 and RCP 8.5 provided a sufficient range for most risk assessments.
RCP 8.5: The High-Emission Baseline
RCP 8.5 represents a world with no climate mitigation policy. Fossil fuel use continues to grow, emissions increase throughout the century, and forcing reaches 8.5 W/m2 by 2100. Warming of approximately 4.3 degrees produces severe impacts across all climate hazards.
RCP 8.5 has been the subject of debate. Some researchers argue it overestimates future emissions given the rapid decline in renewable energy costs. Others point out that it remains valuable as a stress test scenario and that feedback loops like permafrost thaw could push actual forcing toward RCP 8.5 levels even with moderate mitigation.
Regardless of its likelihood as a central estimate, RCP 8.5 (and its successor SSP5-8.5) continues to serve an important role in risk assessment by defining the upper bound of potential climate impacts.
RCP vs SSP: What Changed and Why
The transition from RCP scenarios to SSP scenarios in IPCC AR6 addressed a key limitation: RCPs defined only the physical forcing pathway, not the human story behind it. Two very different socioeconomic worlds could produce the same forcing level, but with different implications for vulnerability and adaptation capacity.
| Feature | RCP Scenarios | SSP Scenarios |
|---|---|---|
| IPCC Report | AR5 (2013) | AR6 (2021) |
| Climate Models | CMIP5 | CMIP6 |
| Defines | Forcing pathway only | Forcing + socioeconomic narrative |
| Number of pathways | 4 (2.6, 4.5, 6.0, 8.5) | 5 (SSP1 through SSP5) |
| Climate sensitivity | Lower (CMIP5 mean ~3.2 C) | Higher (CMIP6 mean ~3.7 C) |
| Approximate mapping | RCP 2.6 / 4.5 / 8.5 | SSP1-2.6 / SSP2-4.5 / SSP5-8.5 |
For organizations still working with RCP-based data, the forcing levels map approximately to their SSP equivalents. However, CMIP6 models used with SSPs generally project slightly higher warming for the same forcing level due to updated climate sensitivity estimates. New physical climate risk assessments should use SSP scenarios and CMIP6 data where available.
Frequently Asked Questions
What are RCP scenarios?
RCP scenarios (Representative Concentration Pathways) define four trajectories for greenhouse gas concentrations and radiative forcing through 2100. Used in IPCC AR5 and CMIP5 climate models, they range from RCP 2.6 (strong mitigation, 1.6 degrees warming) to RCP 8.5 (no mitigation, 4.3 degrees warming). They have been replaced by SSP scenarios in IPCC AR6.
What does RCP 8.5 mean?
RCP 8.5 represents a high-emission pathway where radiative forcing reaches 8.5 watts per square meter by 2100 with no climate mitigation. It produces approximately 4.3 degrees Celsius of warming and serves as the worst-case stress test in climate risk assessments. Its successor in the SSP framework is SSP5-8.5.
What is the difference between RCP and SSP scenarios?
RCP scenarios define only the radiative forcing pathway, while SSP scenarios add socioeconomic narratives that explain how emissions reach a given level. SSPs use newer CMIP6 climate models with updated climate sensitivity, generally projecting slightly higher warming. RCPs were used in IPCC AR5 (2013), SSPs in AR6 (2021).
Is RCP 4.5 the same as SSP2-4.5?
RCP 4.5 and SSP2-4.5 target the same radiative forcing level (4.5 W/m2 by 2100), but they are not identical. SSP2-4.5 adds a socioeconomic narrative (middle-of-the-road development) and uses CMIP6 models that project slightly higher climate sensitivity. The forcing pathway is similar, but the projected warming and regional impacts can differ.
Are RCP scenarios still used?
RCP scenarios are still referenced in older climate studies, regulatory frameworks, and risk assessments built on CMIP5 data. However, new assessments should use SSP scenarios and CMIP6 data. Many organizations are transitioning from RCP to SSP-based analysis to align with IPCC AR6 and current TCFD guidance.
Conclusion
RCP scenarios provided the foundation for a decade of climate risk research and assessment. While SSP scenarios have replaced them as the current standard, understanding RCP scenarios remains important for interpreting existing studies and comparing legacy risk assessments. The key takeaway: RCP 2.6 maps to SSP1-2.6, RCP 4.5 to SSP2-4.5, and RCP 8.5 to SSP5-8.5, with SSP-based projections generally showing slightly higher warming due to updated climate models.
