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Representative Concentration Pathway
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Representative Concentration Pathway
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Different RCP scenarios result in different predicted greenhouse gas concentrations in the atmosphere (from 2000 to 2100). RCP8.5 would result in the highest greenhouse gas concentration (measured as CO2-equivalents).

Representative Concentration Pathways (RCP) are climate change scenarios to project future greenhouse gas concentrations. These pathways (or trajectories) describe future greenhouse gas concentrations (not emissions) and have been formally adopted by the IPCC. The pathways describe different climate change scenarios, all of which were considered possible depending on the amount of greenhouse gases (GHG) emitted in the years to come. The four RCPs – originally RCP2.6, RCP4.5, RCP6, and RCP8.5 – are labelled after the expected changes in radiative forcing values from the year 1750[1][2] to the year 2100 (2.6, 4.5, 6, and 8.5 W/m2, respectively).[3][4][5] The IPCC Fifth Assessment Report (AR5) began to use these four pathways for climate modeling and research in 2014. The higher values mean higher greenhouse gas emissions and therefore higher global surface temperatures and more pronounced effects of climate change. The lower RCP values, on the other hand, are more desirable for humans but would require more stringent climate change mitigation efforts to achieve them.

In the IPCC's Sixth Assessment Report the original pathways are now being considered together with Shared Socioeconomic Pathways. There are three new RCPs, namely RCP1.9, RCP3.4 and RCP7.[6] A short description of the RCPs is as follows: RCP 1.9 is a pathway that limits global warming to below 1.5 °C, the aspirational goal of the Paris Agreement.[6] RCP 2.6 is a very stringent pathway.[6] RCP 3.4 represents an intermediate pathway between the very stringent RCP2.6 and less stringent mitigation efforts associated with RCP4.5.[6] RCP 4.5 is described by the IPCC as an intermediate scenario.[7] In RCP 6, emissions peak around 2080, then decline.[8] RCP7 is a baseline outcome rather than a mitigation target.[6] In RCP 8.5 emissions continue to rise throughout the 21st century.[9]: Figure 2, p. 223 

For the extended RCP2.6 scenario, global warming of 0.0 to 1.2 °C is projected for the late 23rd century (2281–2300 average), relative to 1986–2005.[10] For the extended RCP8.5, global warming of 3.0 to 12.6 °C is projected over the same time period.[10]

Concentrations

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The RCPs are consistent with a wide range of possible changes in future anthropogenic (i.e., human) greenhouse gas emissions, and aim to represent their atmospheric concentrations.[11] Despite characterizing RCPs in terms of inputs, a key change from the 2007 to the 2014 IPCC report is that the RCPs ignore the carbon cycle by focusing on concentrations of greenhouse gases, not greenhouse gas inputs.[12] The IPCC studies the carbon cycle separately, predicting higher ocean uptake of carbon corresponding to higher concentration pathways, but land carbon uptake is much more uncertain due to the combined effect of climate change and land use changes.[13]

The four RCPs are consistent with certain socio-economic assumptions but are being substituted with the shared socioeconomic pathways which are anticipated to provide flexible descriptions of possible futures within each RCP. The RCP scenarios superseded the Special Report on Emissions Scenarios projections published in 2000 and were based on similar socio-economic models.[14]

Pathways used in modelling

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RCP 1.9

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RCP 1.9 is a pathway that limits global warming to below 1.5 °C, the aspirational goal of the Paris Agreement.[6]

RCP 2.6

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RCP 2.6 is a "very stringent" pathway.[6] According to the IPCC, RCP 2.6 requires that carbon dioxide (CO2) emissions start declining by 2020 and go to zero by 2100. It also requires that methane emissions (CH4) go to approximately half the CH4 levels of 2020, and that sulphur dioxide (SO2) emissions decline to approximately 10% of those of 1980–1990. RCP 2.6 is the only scenario that requires net negative CO2 emissions near the end of the century. Net negative CO2 emissions means that in total, humans absorb more CO2 from the atmosphere than they release. These negative emissions are on average 2 Gigatons of CO2 per year (GtCO2/yr).[15] RCP 2.6 is likely to keep global temperature rise below 2 °C by 2100.[7]

RCP 3.4

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RCP 3.4 represents an intermediate pathway between the "very stringent" RCP2.6 and less stringent mitigation efforts associated with RCP4.5.[6] As well as just providing another option a variant of RCP3.4 includes considerable removal of greenhouse gases from the atmosphere.[6]

RCP 4.5

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RCP 4.5 is described by the IPCC as an intermediate scenario.[7] Emissions in RCP 4.5 peak around 2040, then decline.[9]: Figure 2, p. 223  According to resource specialists, IPCC emission scenarios are biased towards exaggerated availability of fossil fuels reserves;[16] RCP 4.5 is the most probable baseline scenario (no climate policies) taking into account the exhaustible character of non-renewable fuels.[17]

According to the IPCC, RCP 4.5 requires that carbon dioxide (CO2) emissions start declining by approximately 2045 to reach roughly half of the levels of 2050 by 2100. It also requires that methane emissions (CH4) stop increasing by 2050 and decline somewhat to about 75% of the CH4 levels of 2040, and that sulphur dioxide (SO2) emissions decline to approximately 20% of those of 1980–1990. Like all the other RCPs, RCP 4.5 requires negative CO2 emissions (such as CO2 absorption by trees). For RCP 4.5, those negative emissions would be 2 Gigatons of CO2 per year (GtCO2/yr).[15] RCP 4.5 is more likely than not to result in global temperature rise between 2 °C and 3 °C, by 2100 with a mean sea level rise 35% higher than that of RCP 2.6.[18] Many plant and animal species will be unable to adapt to the effects of RCP 4.5 and higher RCPs.[19]

RCP 6

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In RCP 6, emissions peak around 2080, then decline.[8] The RCP 6.0 scenario uses a high greenhouse gas emission rate and is a stabilisation scenario where total radiative forcing is stabilised after 2100 by employment of a range of technologies and strategies for reducing greenhouse gas emissions. 6.0 W/m2 refers to the radiative forcing reached by 2100. Projections for temperature according to RCP 6.0 include continuous global warming through 2100 where CO2 levels rise to 670 ppm by 2100, making the global temperature rise by about 3–4 °C by 2100.[20]

RCP 7

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RCP7 is a baseline outcome rather than a mitigation target.[6]

RCP 8.5

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In RCP 8.5 emissions continue to rise throughout the 21st century.[9]: Figure 2, p. 223  RCP8.5 is generally taken as the basis for worst-case climate change scenarios. Since the publication of the IPCC Fifth Assessment Report (2014) the likelihood of this RCP has been debated, due to overestimation of projected coal outputs.[21][22] On the other hand, many uncertainties remain on carbon cycle feedbacks, which could lead to warmer temperatures than projected in representative concentration pathways.[23] RCP 8.5 is still used for predicting mid-century (and earlier) emissions based on current and stated policies.[24]

Projections based on the RCPs

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21st century

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Mid- and late 21st-century (2046–2065 and 2081–2100 averages, respectively) projections of global warming and global mean sea level rise from the IPCC Fifth Assessment Report (IPCC AR5 WG1) are tabulated below. The projections are relative to temperatures and sea levels in the late 20th to early 21st centuries (1986–2005 average). Temperature projections can be converted to a reference period of 1850–1900 or 1980–99 by adding 0.61 or 0.11 °C, respectively.[25]

AR5 global warming increase (°C) projections[25]
Scenario 2046–2065 2081–2100
Mean (likely range) Mean (likely range)
RCP2.6 1.0 (0.4 to 1.6) 1.0 (0.3 to 1.7)
RCP4.5 1.4 (0.9 to 2.0) 1.8 (1.1 to 2.6)
RCP6 1.3 (0.8 to 1.8) 2.2 (1.4 to 3.1)
RCP8.5 2.0 (1.4 to 2.6) 3.7 (2.6 to 4.8)

Across all RCPs, global mean temperature is projected to rise by 0.4 to 2.6°C (1.5°C) by the mid 21st century and by 0.3 to 4.8°C (2.55°C) by the late 21st century.

According to a 2021 study in which plausible AR5 and RCP scenarios of CO2 emissions are selected,[26]

AR5 and RCP Scenarios and temperature change projections
RCP Scenario Range of Global Mean Temperature Increase (Celsius) – 2100 from pre-Industrial baseline
RCP 1.9 ≈1 to ≈1.5
RCP 2.6 ≈1.5 to ≈2
RCP 3.4 ≈2 to ≈2.4
RCP 4.5 ≈2.5 to ≈3
RCP 6.0 ≈3 to ≈3.5
RCP 7.5 ≈4
RCP 8.5 ≈5
AR5 global mean sea level (m) increase projections[25]
Scenario 2046–2065 2081–2100
Mean (likely range) Mean (likely range)
RCP2.6 0.24 (0.17 to 0.32) 0.40 (0.26 to 0.55)
RCP4.5 0.26 (0.19 to 0.33) 0.47 (0.32 to 0.63)
RCP6 0.25 (0.18 to 0.32) 0.48 (0.33 to 0.63)
RCP8.5 0.30 (0.22 to 0.38) 0.63 (0.45 to 0.82)

Across all RCPs, global mean sea level is projected to rise by 0.17 to 0.38 meters (0.275 meters) by the mid 21st century and by 0.26 to 0.82 meters (0.54 meters) by the late 21st century.

23rd century

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The IPCC Fifth Assessment Report also projected changes in climate beyond the 21st century. The extended RCP2.6 pathway assumes sustained net negative anthropogenic GHG emissions after the year 2070.[11] Meanwhile, the extended RCP8.5 pathway assumes continued anthropogenic GHG emissions after 2100.[11] In the extended RCP 2.6 pathway, atmospheric CO2 concentrations reach around 360 ppmv by 2300, while in the extended RCP8.5 pathway, CO2 concentrations reach around 2000 ppmv in 2250, which is nearly seven times the pre-industrial level.[11]

For the extended RCP2.6 scenario, global warming of 0.0 to 1.2°C (0.6°C) is projected for the late 23rd century (2281–2300 average), relative to 1986–2005.[10] For the extended RCP8.5, global warming of 3.0 to 12.6°C (7.8°C) is projected over the same time period.[10]

Between these RCPs, global warming of 1.5 to 6.9°C (4.2°C) is projected for the late 23rd century (2281-2300 average).

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Representative Concentration Pathway

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Representative Concentration Pathways (RCPs) are standardized scenarios of future concentrations and associated levels, developed to enable consistent projections in general circulation models for assessing climate change impacts. These pathways, introduced for the Intergovernmental Panel on Climate Change's (IPCC) , encompass four variants—RCP2.6, RCP4.5, RCP6.0, and RCP8.5—named according to their projected end-of-century values in watts per square meter (W/m²) relative to 1750 levels. RCP2.6 assumes stringent with forcing peaking below 3 W/m² mid-century before declining, RCP4.5 and RCP6.0 represent intermediate stabilization cases with modest policy interventions, and RCP8.5 depicts high emissions without significant climate policies, leading to forcing exceeding 8 W/m². Unlike probabilistic forecasts, RCPs serve as illustrative "what-if" frameworks to explore a broad range of potential futures based on varying assumptions about , , technological change, and energy systems, without assigning likelihoods to specific pathways. They were generated using integrated assessment models and have been widely applied in the Phase 5 (CMIP5) to drive simulations of , , and other variables. Subsequent refinements, such as (SSPs), have built upon RCPs by linking concentration trajectories to detailed narratives of societal evolution. RCPs have faced scrutiny, particularly regarding RCP8.5, which relies on assumptions of sustained high use, slow technological progress, and rapid that diverge from observed trends like declining consumption and accelerating deployment. Critics argue that its frequent portrayal as a "business-as-usual" baseline exaggerates plausible high-end risks, as current emissions trajectories align more closely with lower-forcing scenarios like RCP4.5, rendering RCP8.5 increasingly implausible without major reversals in decarbonization efforts. This has prompted calls for updated scenario frameworks emphasizing realism over extremes to better inform with empirical grounding.

Definition and Purpose

Radiative Forcing Framework

is defined as the change in net downward minus upward (in watts per square meter, W/m²) at the or top of the atmosphere due to a driver, such as increased concentrations of gases, with all other factors held constant, including fixed sea-surface temperatures and extent. This metric captures the imbalance in induced by anthropogenic or natural perturbations, providing a standardized measure of how these changes influence the system's tendency to warm or cool. In practice, effective (ERF) refines this by accounting for rapid atmospheric adjustments, such as responses, which instantaneous forcing may overlook; ERF values for RCPs are typically close to but slightly higher than traditional forcing estimates. The Representative Concentration Pathways (RCPs) framework centers on specified trajectories of to standardize inputs for modeling, emphasizing end-of-century forcing levels relative to pre-industrial baselines around 1750. Each RCP is named for its approximate forcing in 2100—RCP2.6 at 2.6 W/m² (a "peak-and-decline" reaching ~3.1 W/m² mid-century before declining), RCP4.5 stabilizing near 4.5 W/m², RCP6.0 at about 6.0 W/m², and RCP8.5 rising to roughly 8.5 W/m²—spanning a range from stringent to unmitigated emissions growth. These levels integrate contributions from long-lived greenhouse gases (e.g., CO₂, CH₄, N₂O), short-lived species (e.g., aerosols, tropospheric ), and land-use changes, derived from integrated assessment models that simulate emissions, concentrations, and forcing over time. This forcing-based approach enables consistent comparison across models by focusing on physical climate drivers rather than prescribing specific emission sources or socioeconomic paths upfront, though full RCP datasets include of concentrations, emissions, and to achieve the targeted forcings. The "representative" designation underscores that each RCP exemplifies one viable pathway among many that could yield the designated forcing, facilitating harmonized projections in IPCC assessments while allowing later linkage to narrative-driven scenarios like (SSPs). Uncertainties in forcing calculations arise from incomplete knowledge of effects and indirect feedbacks, but the framework prioritizes empirical models for quantification.

Role in IPCC Climate Assessments

The Representative Concentration Pathways (RCPs) were developed specifically to standardize greenhouse gas and aerosol concentration trajectories for use in the Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report (AR5), published between 2013 and 2014, replacing the earlier Special Report on Emissions Scenarios (SRES) that focused primarily on emissions rather than concentrations. In AR5, RCPs served as inputs to the Coupled Model Intercomparison Project Phase 5 (CMIP5), enabling consistent simulations across global climate models to project future climate responses under four radiative forcing levels—2.6, 4.5, 6.0, and 8.5 W/m² by the year 2100—spanning a range from stringent mitigation to high-emissions futures without climate policy interventions. This framework allowed the IPCC Working Group I to assess physical science basis projections, such as global temperature increases and sea level rise, by decoupling concentration pathways from specific socioeconomic narratives, which were addressed separately in AR5's impacts and mitigation volumes. In subsequent IPCC assessments, including the Sixth Assessment Report (AR6) released between 2021 and 2022, RCPs continued to play a complementary role, particularly in evaluating legacy CMIP5 model outputs and providing continuity with AR5 findings, even as the primary scenarios shifted to Shared Socio-economic Pathways (SSPs) integrated with similar levels for CMIP6. AR6 explicitly notes that RCP-based studies from AR5 supplement SSP-driven assessments, aiding in the synthesis of near-term and long-term projections, such as those for events and heat uptake, while highlighting uncertainties in high-forcing scenarios like RCP8.5, which assume continued reliance on fossil fuels without technological breakthroughs. This dual use underscores RCPs' function as exploratory tools rather than predictive baselines, emphasizing plausible ranges derived from integrated assessment models rather than probabilistic outcomes. Critiques of RCP application in IPCC reports have centered on the selection of scenarios, with some analyses arguing that intermediate pathways like RCP4.5 and RCP6.0 better align with observed emissions trends post-2000, while extreme cases like RCP2.6 require implausibly rapid decarbonization and RCP8.5 assumes unchecked coal expansion unlikely under current policy trajectories. Nonetheless, the IPCC maintains RCPs' utility for bounding uncertainty in climate sensitivity and forcing feedbacks, informing policy-relevant assessments without endorsing any single pathway as most likely. Their role has thus evolved from core AR5 drivers to referential benchmarks in AR6, facilitating cross-report comparisons amid ongoing debates over scenario realism.

Historical Development

Evolution from SRES Scenarios

The (SRES), released by the IPCC in 2000, comprised 40 baseline emission trajectories derived from four socioeconomic storylines (A1, A2, B1, B2), projecting future greenhouse gas emissions under assumptions of limited or no new mitigation policies beyond those existing at the time. These scenarios emphasized integrated narratives linking demographics, economics, technology, and energy use to emissions, serving as inputs for climate modeling in the IPCC's Third Assessment Report (AR3, 2001) and Fourth Assessment Report (AR4, 2007). However, by 2005, the IPCC recognized limitations in SRES, including outdated socioeconomic assumptions and the absence of low-emission pathways reflecting emerging mitigation possibilities, necessitating new scenarios aligned with advances in integrated assessment modeling (IAM) and to support timely preparation for AR5. IPCC sessions in (April 2006) and (October 2007) formalized the call for updated scenarios, catalyzing a parallel development process coordinated by III (Mitigation) and involving the Integrated Assessment Modeling Consortium (IAMC). An IPCC expert meeting in Noordwijkerhout, (July 2008), outlined a "two-pronged" strategy: first, generate Representative Concentration Pathways (RCPs) focused on atmospheric concentrations and for physical climate modeling; second, develop associated socioeconomic narratives post hoc. This approach, detailed in Moss et al. (2010), prioritized RCPs for the Phase 5 (CMIP5), ensuring availability by 2010 for AR5's I (physical science basis). Key differences from SRES include RCPs' emphasis on end-of-century levels (e.g., 2.6, 4.5, 6.0, 8.5 W/m² in 2100 relative to pre-industrial), rather than emissions tied to prescriptive storylines, allowing broader coverage of outcomes and flexibility for to reverse-engineer socioeconomic drivers. Unlike SRES, which excluded deliberate climate policies and focused on "business-as-usual" baselines, RCPs incorporate a from aggressive (e.g., RCP2.6 peaking emissions before 2020) to high-emission continuations, without implying forecasts or policy prescriptions. This shift enabled decoupled workflows: climate models used RCP forcing directly, while analyses linked back via , addressing SRES critiques on rigidity and integration challenges. The RCP database, hosted by IIASA, provided harmonized inputs for AR5, marking a foundational evolution toward modular scenario frameworks later extended with (SSPs) in AR6.

Formulation Process for AR5

The development of the Representative Concentration Pathways (RCPs) for the IPCC's Fifth Assessment Report (AR5) was initiated following a 2007 IPCC request for updated scenarios to support climate modeling, reflecting advances in integrated assessment models (IAMs) and a need to explore policy-relevant radiative forcing ranges beyond the emission-focused Special Report on Emissions Scenarios (SRES). The process emphasized concentration trajectories over emissions, enabling broader applicability across IAMs and facilitating parallel development of socioeconomic narratives. Coordination was led by the International Institute for Applied Systems Analysis (IIASA), with four independent IAM teams selected in 2008 to produce pathways targeting specific 2100 levels: 2.6 W/m² (low, by PBL Environmental Assessment Agency using ), 4.5 W/m² (intermediate, by Joint Global Change Research Institute using GCAM/MiniCAM), 6.0 W/m² (intermediate-high, by , , using AIM), and 8.5 W/m² (high, by IIASA using ). These teams drew from an initial database of over 300 IAM-generated pathways, narrowing to 37 that met basic criteria for plausibility and range coverage, before refining the selected four to ensure separation of approximately 2 W/m² between pathways. Development occurred iteratively from 2008 to 2010, incorporating historical data back to 2000 as a base year and extending projections to 2300 for extended concentration pathways (ECPs). Key formulation steps included independent trajectory generation for greenhouse gases, aerosols, and ; harmonization of gridded emissions and land-cover data at 0.5° × 0.5° resolution; and conversion of emissions to concentrations using the MAGICC6 simple coupled with CAM3.5 for . Challenges addressed during this phase involved reconciling discrepancies across models—such as varying assumptions on and —for consistency, while avoiding premature socioeconomic narrative integration to maintain flexibility for later (SSPs). Final datasets, including emissions, concentrations, and forcing components, were archived in IIASA's RCP Database by mid-2011, enabling intercomparisons like CMIP5 ahead of AR5's Working Group I report in 2013. This marker-based approach, prioritizing empirical forcing endpoints over narrative-driven emissions, marked a shift from SRES by providing higher-resolution, policy-inclusive inputs verifiable against observed trends.

Methodology and Inputs

Concentration and Forcing Trajectories

The Representative Concentration Pathways (RCPs) consist of time-dependent projections of atmospheric concentrations for the full suite of greenhouse gases (GHGs), including (CO₂), (CH₄), (N₂O), and , as well as reactive gases, aerosols, and land-use changes that influence . These trajectories span from a 2005 baseline (harmonized across models) to 2100, with extensions to 2300 via Extended Concentration Pathways (ECPs) for long-term modeling. Concentrations are derived from integrated assessment models (IAMs) such as IMAGE for RCP2.6, MiniCAM for RCP4.5, AIM for RCP6.0, and MESSAGE for RCP8.5, ensuring consistency in emissions-to-concentration linkages using tools like the reduced-complexity model MAGICC6. Radiative forcing trajectories are calculated from these concentrations, excluding direct land-use albedo effects but incorporating GHG and aerosol contributions via IPCC Assessment Report 4 (AR4)-consistent formulations or radiative efficiencies. Forcing levels rise nonlinearly across RCPs due to varying emission assumptions: RCP2.6 exhibits an overshoot, peaking at approximately 3.1 W/m² mid-century before declining to 2.6 W/m² by 2100 through aggressive ; RCP4.5 increases steadily to stabilization near 4.5 W/m² post-2100; RCP6.0 reaches about 6.0 W/m² after 2100 without early overshoot; and RCP8.5 ascends monotonically to 8.5 W/m² by 2100 under high-emission conditions. ECPs beyond 2100 apply simple stabilization or continuation rules to facilitate system model experiments. Key GHG concentration endpoints in 2100 illustrate the divergence: CO₂ reaches roughly 421 ppm in RCP2.6 (after peaking near 450 ppm mid-century), 538 ppm in RCP4.5, 670 ppm in RCP6.0, and 936–940 ppm in RCP8.5. CH₄ and N₂O follow suit, with RCP2.6 assuming sharp reductions (e.g., CH₄ declining post-peak), intermediate stabilization in RCP4.5 and 6.0, and continued rises in RCP8.5 (e.g., CH₄ exceeding 2500 ppb, N₂O over 400 ppb). These pathways reflect model-specific socioeconomic and policy inputs but are designed for forcing consistency rather than probabilistic likelihood. Detailed , including annual values, are archived in the IIASA RCP Database for verification and model input.
RCP ScenarioCO₂ (ppm, 2100)Approximate Forcing Trajectory DescriptionPrimary Model
RCP2.6~421Peak mid-century, decline to 2.6 W/m²IMAGE
RCP4.5~538Rise and stabilize ~4.5 W/m² post-2100MiniCAM
RCP6.0~670Gradual rise to 6.0 W/m² after 2100AIM
RCP8.5~936–940Steady increase to 8.5 W/m²MESSAGE

Underlying Socioeconomic Assumptions

The Representative Concentration Pathways (RCPs) were derived from integrated assessment models (IAMs) that simulate interactions between socioeconomic drivers—such as , (GDP) growth, energy intensity, technological advancement, and land-use changes—and emissions of greenhouse gases and aerosols, ultimately yielding specified levels by 2100. Unlike the earlier (SRES), which emphasized narrative-based storylines, the RCPs prioritize concentration trajectories while allowing a range of compatible socioeconomic assumptions; each RCP originates from a distinct IAM run with internally consistent but non-unique socioeconomic inputs, spanning high-to-low forcing without prescribing a single global narrative. These inputs reflect baseline projections without coordinated policies for higher RCPs, incorporating factors like availability and regional development disparities. For RCP8.5, generated by the model from the International Institute for Applied Systems Analysis (IIASA), assumptions include high global population growth peaking near 12 billion by 2100, relatively slow income growth in developing regions, and sluggish technological progress in energy efficiency, leading to energy demand exceeding 900 exajoules (EJ) annually by 2100 with heavy reliance on unabated fuels. This pathway aligns with an updated version of the SRES A2 scenario, featuring regional self-reliance and no explicit climate mitigation, resulting in continuously rising emissions. RCP6.0, produced by the AIM model from Japan's , employs moderate population and GDP growth assumptions, with global energy demand stabilizing between 750 and 900 EJ by 2100 through a balanced mix of fuels, renewables, and efficiency improvements, alongside declining due to agricultural intensification. Stabilization of forcing occurs after 2100 without mid-century overshoot, reflecting intermediate socioeconomic development and limited policy intervention focused on non-CO2 gases. In RCP4.5, developed using the GCAM model from the Joint Global Change Research Institute (JGCRI), intermediate population projections (aligned with UN medium variants) and GDP growth drive potential output via labor productivity assumptions, yielding energy demand of 750–900 EJ by 2100, supported by , reduced cropland expansion, and yield-enhancing agricultural practices. Forcing stabilizes post-2100 without overshoot, incorporating dietary shifts toward less land-intensive consumption and moderate technological deployment for emission controls. RCP2.6, from the IMAGE model of the PBL Environmental Assessment Agency, assumes intermediate population and GDP trajectories but stringent mitigation policies, including rapid declines in and widespread adoption of bioenergy with (BECCS) to achieve negative emissions after , keeping energy demand at 750–900 EJ while shifting to low-carbon systems and intensive practices. This pathway requires global emissions to peak around 2020 and decline sharply, enabled by aggressive technological and policy assumptions not present in higher RCPs.
RCPModelPopulation ProjectionGDP/Economic GrowthEnergy Demand (2100)Key Features
8.5MESSAGE (IIASA)High (~12 billion peak)Slow in developing regions>900 EJNo mitigation, high fossil fuels
6.0AIM (NIES)ModerateIntermediate750–900 EJTech mix, ag intensification
4.5GCAM (JGCRI)Intermediate (UN medium)Intermediate (labor productivity-driven)750–900 EJReforestation, yield gains
2.6IMAGE (PBL)IntermediateIntermediate750–900 EJBECCS, low intensity

Specific RCP Scenarios

RCP 2.6

RCP 2.6 delineates a stringent mitigation pathway achieving of 2.6 W/m² above pre-industrial levels by 2100, representing the lowest forcing among the original four Representative Concentration Pathways developed for IPCC assessments. This scenario assumes aggressive global reductions in , with CO₂ emissions peaking near 40 GtCO₂ around 2020 before declining rapidly to near zero by 2100 and turning net negative thereafter, primarily through deployment of with (BECCS) and other negative emission technologies. Atmospheric CO₂ concentrations under this pathway peak at approximately 440 ppm in the mid-21st century, then fall to around 360 ppm by 2300 in extended projections, driven by sustained net removals exceeding residual emissions. The pathway's emissions trajectory derives from the integrated assessment model, incorporating assumptions of early and comprehensive policy interventions, including rapid phase-out of fossil fuels, efficiency gains, and land-use changes favoring carbon sinks. Non-CO₂ greenhouse gases, such as and , also decline sharply due to coupled reductions in , waste, and industrial activities. Overall energy demand stabilizes or decreases despite to about 9 billion by 2100, enabled by , renewables dominance (over 80% of primary energy by late century), and minimal reliance on unabated fossil sources. Aerosol emissions drop in tandem, contributing to the forcing level after accounting for cooling effects. In climate model ensembles, RCP 2.6 yields global mean surface temperature increases of 0.9–2.3°C by 2100 relative to 1986–2005 levels, with a likely below 2°C above pre-industrial, though transient overshoot occurs before stabilization. Sea-level rise projections under this scenario range from 0.19–0.61 m by 2100, reflecting lower and ice melt contributions compared to higher-forcing pathways. The scenario serves as a benchmark for compatibility, illustrating outcomes from immediate, economy-wide decarbonization starting in the . Assessments of RCP 2.6's plausibility highlight technical feasibility within modeling constraints but underscore challenges in real-world , including the need for annual decarbonization rates exceeding 6% globally—unprecedented outside wartime economies—and scaling BECCS to remove 5–10 GtCO₂ annually by , a absent at commercial levels as of 2025. Developers noted internal debates on its stringency during formulation, with post-2011 emission growth diverging from the assumed early peak, rendering alignment increasingly improbable without compensatory overshoot and recapture. Integrated assessment models confirm compatibility with 2°C limits only under optimistic assumptions of rapid technological diffusion and geopolitical coordination, contrasting with observed delays in renewable transitions and fossil fuel phase-outs.

RCP 4.5

RCP 4.5 is a representative concentration pathway designed as a stabilization scenario, in which reaches approximately 4.5 W/m² above pre-industrial levels by the year 2100 without exceeding this value thereafter. This level corresponds to an equivalent of roughly 650 ppm CO₂ in terms of total forcing, reflecting a moderate effort involving global policies such as emissions pricing and technological advancements. Developed using the GCAM integrated assessment model by the Joint Global Change Research Institute at , it serves as an input for modeling in IPCC assessments, positioned as an intermediate pathway between low- and high-emission scenarios. The forcing trajectory in RCP 4.5 assumes a gradual increase, stabilizing around 2080 through deliberate reductions in anthropogenic emissions across multiple agents, including CO₂, CH₄, N₂O, , aerosols, and precursors like CO and VOCs. Carbon dioxide emissions peak at approximately 42 GtCO₂ per year around 2040 before declining sharply to about 15 GtCO₂ per year by 2100, driven by assumed shifts toward lower-carbon sources and efficiency improvements. Atmospheric CO₂ concentration rises to 526 ppm by 2100, while other greenhouse gases follow patterns consistent with policy-induced declines, though non-CO₂ forcing from short-lived species like contributes to the overall 4.5 W/m² benchmark. Socioeconomic assumptions underlying RCP 4.5 include a global population that peaks at 9 billion in 2065 and declines slightly to 8.7 billion by 2100, alongside sustained economic growth in gross domestic product. These projections incorporate expectations of improved energy intensity, expanded bioenergy with carbon capture and storage, and land-use changes favoring reforestation over expansion of agriculture, enabled by yield enhancements. The pathway presumes widespread adoption of climate mitigation measures post-peak emissions, though its realism depends on timely policy implementation, which empirical trends in emissions decoupling from GDP growth partially support but have not yet achieved the required scale globally. In climate model applications, typically projects global mean surface temperature increases of 1.7–2.6°C above pre-industrial levels by 2100 across CMIP5 ensembles, with higher confidence in regional patterns like amplified warming over land and poles. Critics note that while less extreme than higher pathways, achieving stabilization requires aggressive interventions that contrast with historical in international agreements, potentially overestimating feasibility without breakthroughs in negative emissions technologies. Nonetheless, it aligns more closely with current pledges than no-mitigation baselines, serving as a benchmark for assessing partial success in emissions trajectories.

RCP 6.0

RCP 6.0 is one of four representative concentration pathways developed for the Intergovernmental Panel on Climate Change's Fifth Assessment Report (AR5), characterized by a trajectory that reaches approximately 6 watts per square meter (W/m²) by 2100 relative to pre-industrial levels and stabilizes thereafter without exceeding this value. This pathway assumes a continuation of relatively high through much of the , with total anthropogenic forcing peaking mid-century before declining due to eventual deployment of technologies, though stabilization occurs only after 2100. The scenario was generated using the Integrated Model (AIM), an integrated assessment model focused on energy systems, , and emissions pathways, emphasizing delayed but eventual climate policy interventions later in the century. Under RCP 6.0, (CO₂) concentrations rise to around 660–670 parts per million (ppm) by 2100, driven by emissions that peak around 2060–2080 before gradual reductions, reflecting assumptions of moderate socioeconomic development with limited early mitigation but increased adoption of low-carbon technologies post-2050. This trajectory aligns closely with the earlier (SRES) A1B pathway, particularly after 2100, but incorporates updated representations of aerosols, , and non-CO₂ gases for a fuller suite of forcing agents. In climate modeling applications, such as those in the Phase 5 (CMIP5), RCP 6.0 projects continued global warming of about 2–3°C above pre-industrial levels by 2100, with regional variations in and responses depending on model specifics, though it implies less aggressive warming than RCP 8.5 due to the post-2100 stabilization. The pathway does not prescribe unique socioeconomic narratives, allowing pairing with various (SSPs) in later assessments, but its construction highlights the role of delayed decarbonization in achieving mid-range forcing outcomes. Data for RCP 6.0, including detailed of concentrations and forcings up to 2300 for extended experiments, are archived in the IIASA RCP Database.

RCP 8.5

RCP 8.5 projects a level of 8.5 W/m² by 2100 relative to pre-industrial conditions, resulting from sustained high without significant efforts. This scenario, developed using integrated assessment models, assumes increasing atmospheric concentrations of CO₂ and other long-lived greenhouse gases throughout the century, with CO₂ reaching approximately 936 ppm by 2100 and total continuing to rise beyond that date. It serves as an upper-bound reference for experiments in the IPCC's Fifth Assessment Report, spanning the 90th to 98th percentile of unmitigated emissions pathways from prior literature. The underlying assumptions emphasize socioeconomic factors conducive to high emissions: a global population peaking near 12 billion by 2100, relatively slow growth especially in low-income regions, modest improvements in (energy use per unit of GDP), and persistently high demand met largely by fossil fuels. Energy production in RCP 8.5 relies heavily on unabated coal expansion, projecting a 6.5-fold increase over 2010 levels by century's end, alongside limited adoption of renewables and carbon capture technologies. Land-use changes contribute additional emissions from deforestation and agriculture, exacerbating the forcing trajectory. These inputs were derived from models like , prioritizing high-end outcomes over median projections to explore plausible extremes. In projections, RCP 8.5 yields the most severe outcomes among the core RCPs, with global mean surface increases of 3.7–4.8°C by 2100 (likely range, relative to 1986–2005), sea-level rise of 0.52–0.98 m, and substantial sea ice loss by mid-century. However, its plausibility has been questioned due to discrepancies with empirical trends: global coal consumption peaked in 2013 and has since declined in many regions amid cost-competitive renewables and shifts, rendering the assumed coal surge incompatible with observed decarbonization. Studies indicate RCP 8.5 exceeds realistic fossil fuel extraction limits and overestimates near-term emissions by factors aligning more closely with modest-mitigation scenarios like RCP 4.5 or 6.0 when benchmarked against data. Critics, including analyses in peer-reviewed literature, argue it misrepresents baseline futures by conflating high-end forcing with "business-as-usual," potentially inflating projected risks despite lower-probability assumptions. Subsequent frameworks like have de-emphasized such extremes, favoring narratives with faster technological and economic convergence.

Additional Pathways (RCP 1.9, 3.4, 7.0)

RCP 1.9, corresponding to the SSP1-1.9 in CMIP6, outlines a pathway with stabilizing at 1.9 W/m² above pre-industrial levels by 2100, requiring aggressive global mitigation to limit warming to approximately 1.5°C. This involves very low starting from 2015, with CO₂ emissions declining to net zero around or shortly after 2050, driven by rapid adoption of low-carbon technologies and under the SSP1 narrative of global sustainability and equity. Unlike the original RCPs, this pathway was developed post-Paris Agreement to evaluate ambitious 1.5°C-compatible outcomes and was not part of CMIP5 experiments. RCP 3.4, aligned with SSP4-3.4, projects of 3.4 W/m² by 2100 under a narrative of persistent inequality (SSP4), where uneven development exacerbates regional disparities and limits effective despite some technological progress. As a Tier 2 scenario in the Scenario Model Intercomparison Project (ScenarioMIP), it explores lower-forcing outcomes in fragmented governance contexts, bridging gaps between stringent and intermediate pathways like RCP 2.6 and RCP 4.5. This scenario assumes moderate emissions growth due to unequal access to and resources. RCP 7.0, linked to SSP3-7.0, describes a medium-to-high trajectory reaching 7.0 W/m² by 2100, stemming from SSP3's "regional rivalry" storyline, where nationalism, conflicts, and slow economic convergence result in higher reliance and . It fills the forcing range gap between original RCP 6.0 and RCP 8.5, serving as a central baseline for medium-high emissions in CMIP6, with projections indicating substantial warming and challenges to development goals. This pathway was included to better represent plausible no-policy-change futures amid geopolitical fragmentation.

Model Projections Using RCPs

21st-Century Climate Outcomes

Climate models from the CMIP5 ensemble, driven by Representative Concentration Pathway (RCP) forcings, project global mean surface air temperature () increases by the end of the that scale with levels. Relative to the 1986–2005 baseline, the multi-model mean GSAT rise for the 2081–2100 period is 1.0°C under RCP2.6 (likely range 0.4–1.6°C), 1.8°C under RCP4.5 (1.1–2.6°C), 2.0°C under RCP6.0 (1.3–3.0°C), and 3.7°C under RCP8.5 (2.6–4.8°C). These projections reflect equilibrium estimates of 3.0°C (likely range 1.5–4.5°C per CO2 doubling) embedded in the models, with greater warming in higher-forcing scenarios due to compounded and effects.
RCP ScenarioGSAT Increase (°C, mean [likely range])Global Mean Sea Level Rise (m, mean [likely range])
RCP2.61.0 [0.4–1.6]0.44 [0.28–0.61]
RCP4.51.8 [1.1–2.6]0.53 [0.36–0.71]
RCP6.02.0 [1.3–3.0]0.55 [0.38–0.74]
RCP8.53.7 [2.6–4.8]0.74 [0.52–0.98]
Global mean sea level rise (SLR) projections under the same periods and baseline show increases driven mainly by ocean thermal expansion (40–60% of total) and glacier mass loss, with minor contributions from ice sheets and land water storage changes. RCP2.6 yields a mean rise of 0.44 m (likely 0.28–0.61 m), escalating to 0.74 m (0.52–0.98 m) under RCP8.5, as higher warming accelerates melt but Antarctic ice sheet dynamics contribute little dynamically within the century. Regional SLR variations arise from gravitational and , with greater rises projected in the and northern high latitudes. Precipitation projections indicate a global mean increase of approximately 1–3% per °C of GSAT warming across RCPs, amplifying the hydrological cycle and leading to wetter conditions in the deep tropics and high latitudes but drier . Regional patterns vary widely, with medium confidence in decreased summer over large Mediterranean regions and increased intensities under higher RCPs like 8.5. Uncertainty in precipitation extremes is substantial, but models consistently project more frequent and intense heavy precipitation events over most land areas in all scenarios, linked to increased atmospheric moisture capacity via the Clausius-Clapeyron relation (about 7% more moisture per °C warming). Heat extremes intensify across all RCPs, with hot days and nights becoming more frequent and severe; for instance, under RCP8.5, a once-in-10-years hot event in the late baseline becomes a once-in-2-years event by mid-century over much of the globe. Cold extremes decrease similarly. Drought risks rise in regions like the Mediterranean and under RCP4.5–8.5 due to enhanced outpacing changes, though model spread limits confidence. Ocean outcomes include upper 700 m heat content increases of 0.6–1.6 × 10^{23} J by 2081–2100 across RCPs, and surface ocean declines of 0.14–0.35 units relative to preindustrial, with greater acidification under RCP8.5 from elevated CO2 absorption. Arctic sea ice extent diminishes nearly year-round under higher forcings, with September means projected near zero by mid-to-late century in RCP8.5.

Long-Term (Post-2100) Projections

Long-term projections beyond 2100 under the Representative Concentration Pathways (RCPs) highlight committed changes driven by thermal inertia in the oceans, slow responses of ice sheets and , and the persistence of long-lived gases, even if emissions cease. These extensions, known as Extended Concentration Pathways (ECPs), assume stabilization of at or near 2100 levels for RCP2.6, 4.5, and 6.0, while RCP8.5 continues rising due to unabated emissions. Global mean surface air temperature () rises toward equilibrium levels calibrated to the (ECS) of 1.5–4.5°C per doubling of atmospheric CO₂, with full equilibration requiring centuries to millennia. Projections from emulated climate models (e.g., MAGICC calibrated to CMIP5 and Earth system models of intermediate complexity) indicate divergent outcomes across scenarios. In RCP2.6, which posits strong and net negative emissions post-2100, GSAT peaks mid-century and stabilizes or slightly declines by 2300. Higher-forcing pathways show unabated warming, with RCP8.5 yielding substantial increases due to amplified feedbacks and sustained forcing exceeding 12 W/m² by 2300.
RCP ScenarioGSAT Change by 2281–2300 (°C, relative to 1986–2005)5–95% Range (°C)
RCP2.60.6 ± 0.30.0–1.2
RCP4.52.5 ± 0.61.5–3.5
RCP6.04.2 ± 1.0—
RCP8.57.8 ± 2.93.0–12.6
These estimates reflect multi-model ensembles; ranges account for structural uncertainties in and carbon uptake. Global mean (GMSL) rise persists for millennia across all RCPs due to ongoing and irreversible ice sheet mass loss, with contributions from and scaling with warming. By 2300, projected GMSL rise (relative to 1986–2005) ranges from 0.4–1.0 m in RCP2.6 to 1.0–2.5 m in RCP8.5, with millennial-scale commitments exceeding 1 m even in low-emission cases and potentially tens of meters under RCP8.5 if ice sheet instabilities amplify. Arctic summer sea ice extent approaches zero in all scenarios by late 21st century, with near-permanent ice-free conditions by 2200–2300 in RCP4.5–8.5; recovery is possible within decades in RCP2.6 if forcing declines sharply, though committed loss from feedbacks limits reversibility. thaw, affecting up to 81% of near-surface volume by 2100 in RCP8.5, triggers sustained carbon releases (e.g., 50–100 GtC by 2300), amplifying warming on centennial to millennial timescales via and CO₂ emissions, even after anthropogenic forcing stabilizes.

Criticisms and Debates

Unrealistic Assumptions in High-Forcing Scenarios

High-forcing Representative Concentration Pathways (RCPs), such as RCP 6.0 and RCP 8.5, incorporate assumptions of sustained high driven by rapid , sluggish in developing regions, high demand, and limited technological improvements in energy efficiency. RCP 8.5, in particular, projects global reaching approximately 12 billion by 2100, relatively slow growth with high , and a dramatic expansion of use, including consumption quadrupling from 2010 levels to over 20 gigatons of coal equivalent annually by century's end. These scenarios were constructed as upper-bound explorations without aggressive policies, but critics argue they diverge from empirical trends in demographics, energy markets, and constraints. A core unrealistic element in RCP 8.5 is its reliance on a late-century resurgence in dominance, assuming coal's share of global energy rises sharply after mid-century despite competition from cheaper via the shale revolution and renewables. Global consumption peaked around 2013-2014 and has since plateaued or declined in major economies like the and , with —the world's largest coal user—showing signs of peaking emissions by 2020 due to air quality policies and shifting to gas and renewables. Projections in RCP 8.5 imply coal use exceeding estimates of economically recoverable reserves, rendering the physically implausible under current geological and market realities. Pielke Jr. and colleagues have quantified this implausibility, noting that plausible emissions pathways aligned with observed trends limit warming to 2-3°C by 2100, far below the 4-5°C implied by RCP 8.5. RCP 6.0 shares similar flaws but at a moderated scale, assuming moderate to about 10 billion and continued reliance on unabated fuels without the extreme coal boom of RCP 8.5; however, it still overestimates energy demand growth relative to decoupling trends in high-income nations, where GDP growth has outpaced emissions since the . Empirical data from the indicate that improvements have accelerated beyond RCP assumptions, driven by efficiency gains and , making sustained high-forcing trajectories unlikely without deliberate policy reversals. These discrepancies highlight how high-forcing RCPs, while useful for sensitivity testing, misrepresent baseline futures by ignoring adaptive economic and technological feedbacks observed post-2010.

Misuse and Overemphasis of RCP 8.5

RCP 8.5, the highest radiative forcing pathway among the Representative Concentration Pathways, has been disproportionately emphasized in climate impact assessments and media portrayals, often presented as a "business-as-usual" trajectory despite its divergence from empirical trends in energy use and emissions drivers. This scenario assumes sustained high population growth to 12 billion by 2100, sluggish technological progress, and coal consumption expanding to levels exceeding known recoverable reserves—projecting over 50 gigatons of coal annually by century's end, more than triple 2020 global production. Such assumptions have become increasingly implausible following the post-2010 shale gas revolution, rapid renewable energy deployment, and coal's declining share in global energy (from 44% in 2000 to under 35% by 2023), which have bent emissions trajectories toward lower-forcing outcomes like RCP 4.5 or below. The overemphasis stems from its utility in early model intercomparisons for generating strong signal-to-noise ratios in projections, but this has led to systemic misuse where RCP 8.5 dominates thousands of peer-reviewed studies on impacts—accounting for over half of those published between 2015 and 2018—without rigorous plausibility checks against socioeconomic data. Critics, including analyses in Energy Research & Social Science, argue this creates a distorted view of future risks by conflating a high-end exploratory with likely outcomes, amplifying catastrophic narratives in documents and assessments that prioritize extreme tails over projections. For instance, U.S. federal reports like those from the EPA and DOE have invoked RCP 8.5 for damage estimates, even after acknowledgments of its low probability, perpetuating reliance on outdated assumptions amid evidence of decoupling between GDP growth and emissions intensity. This pattern reflects a broader of self-correction in modeling communities, where legacy use in integrated assessment models (IAMs) and impact resists updates from observed trends, such as coal's stagnation despite RCP 8.5's requirement for exponential . Proponents of continued use defend it for bounding extreme risks or sensitivity testing, yet empirical discrepancies—e.g., cumulative CO2 emissions tracking below RCP 8.5's pathway since 2005—underscore how overemphasis risks misallocating resources toward improbable high-end threats rather than addressing more probable mid-range scenarios informed by current technological and policy shifts. Recent reassessments, including in IPCC AR6, have de-emphasized RCP 8.5's centrality, but its entrenched role in academic and advocacy discourse persists, potentially biasing public and policy perceptions away from data-driven probabilistic assessments. Observed global CO2 emissions from fossil fuels and have diverged from the high-end RCP8.5 trajectory, with emissions growth slowing post-2010 due to technological efficiencies, economic decoupling from carbon intensity, and adoption, resulting in cumulative emissions aligning more closely with RCP4.5 than RCP8.5 by 2020. Atmospheric CO2 concentrations reached approximately 419 ppm in 2023, consistent with moderate forcing pathways rather than the rapid escalation projected under RCP8.5, which assumes unabated high emissions leading to over 900 ppm by 2100. Global surface temperature trends since 1979, as measured by datasets like UAH, show warming of about 0.13°C per , falling below the ensemble mean projections from CMIP5 and CMIP6 models driven by RCP scenarios, which average 0.18–0.25°C per under medium-to-high forcing assumptions. This overestimation persists across 63% of Earth's surface area in recent CMIP6 evaluations, attributed in part to inflated equilibrium in many models relative to empirical estimates from paleoclimate and records. Lower tropospheric profiles from radiosondes and satellites further highlight discrepancies, with observed amplification in the tropical mid-troposphere weaker than RCP-informed model simulations predict. Regional and sectoral trends exacerbate these gaps; for instance, sea ice extent decline has decelerated since the early 2000s, contrasting with accelerated melt projected under higher RCP forcings, while global patterns show no statistically significant intensification of heavy events commensurate with modeled hydrological sensitivity under RCP6.0 or above. These empirical shortfalls suggest that RCP-based projections may embed overly pessimistic assumptions about forcing realization and response, prompting reassessments that weight observations more heavily in constraining future scenarios.

Evolution and Current Status

Shift to Shared Socioeconomic Pathways (SSPs)

The (SSPs) were developed to complement and extend the RCP framework by integrating detailed narratives of future socioeconomic developments, which influence , , and climate challenges. Unlike RCPs, which specified levels without explicit socioeconomic underpinnings, SSPs provide five distinct reference scenarios describing global trends in , , , education, inequality, and technological progress up to 2100. These pathways were created through a multi-year international effort initiated in the late 2000s, with qualitative narratives finalized in 2015 and quantitative projections, including , , and emissions implications, published in 2017. The SSPs span a range of and challenges: SSP1 assumes sustainability-taking with low challenges due to rapid shifts toward and reduced inequalities; SSP2 follows a middle-of-the-road trajectory with moderate challenges; SSP3 depicts regional rivalry leading to high challenges from and slow technological diffusion; SSP4 emphasizes persistent inequality between and within regions; and SSP5 envisions fossil-fueled development with high but lower adaptation barriers due to . This shift facilitated the use of SSPs in the Phase 6 (CMIP6), which underpinned the IPCC's Sixth Assessment Report (AR6) released in 2021, superseding the RCP-based CMIP5 scenarios from AR5 in 2014. In CMIP6, scenarios are denoted as SSPx-y, where x identifies the socioeconomic narrative (e.g., SSP2) and y the approximate level in W/m² by 2100 (e.g., 4.5), allowing models to simulate both emissions-driven and concentration-driven outcomes while accounting for short-lived forcers like aerosols and land-use changes. This integration provides socioeconomic rationales for emission trajectories—such as policy assumptions, technological feasibility, and development priorities—absent in RCPs, enabling more comprehensive assessments of pathways compatible with goals like limiting warming to 1.5°C or 2°C. For instance, SSP1-1.9 represents a low-forcing scenario with aggressive aligned with trends, projecting global surface air temperature increases of 1.0–1.8°C by 2081–2100 relative to 1850–1900, while SSP5-8.5 assumes high reliance, yielding 3.3–5.7°C warming. The transition reflects recognition that biophysical forcing alone inadequately captures human-climate interactions, as SSPs incorporate integrated assessment models to link socioeconomic drivers to biophysical outcomes, including updated historical emissions data and nitrogen cycle processes not fully represented in RCPs. However, SSP-RCP combinations are not fully backward-compatible with prior RCPs due to differences in effective radiative forcing contributions, such as ~0.3°C higher warming in SSP5-8.5 versus RCP8.5 by mid-to-late century from varying aerosol and land-use effects. Quantitative data for SSPs, hosted by institutions like the International Institute for Applied Systems Analysis (IIASA), support scenario databases for impacts, adaptation, and vulnerability research, emphasizing plausible baselines before climate policies. This framework enhances scenario realism by grounding high- or low-emission paths in diverse societal evolutions, though it retains uncertainties in long-term projections tied to model assumptions about growth and policy implementation.

Recent Reassessments and Probabilistic Updates

In the years following the initial development of the Representative Concentration Pathways (RCPs) in the early , empirical observations of global energy trends have prompted reassessments of their plausibility, particularly highlighting divergences from high-forcing scenarios. Analyses of consumption, which RCP8.5 assumed would surge to unprecedented levels, reveal that actual global use has plateaued or declined in key regions due to market shifts toward natural gas, renewables, and efficiency gains, rendering that pathway increasingly inconsistent with observed data. For instance, projections from the indicate that coal's share in has fallen short of RCP8.5 assumptions, with emissions growth rates through 2023 aligning more closely with moderate scenarios like RCP4.5. Probabilistic evaluations have quantified these shifts, assigning low likelihoods to high-end RCPs based on updated socioeconomic and emissions data. A 2020 assessment estimated the probability of RCP8.5 occurring by 2100 at less than 5%, a figure reinforced by subsequent modeling showing even lower —around 3%—under business-as-usual extensions without aggressive reversals. By 2024, probabilistic ensembles of trajectories, incorporating millions of simulations, confirmed that exceeding 8.5 W/m² forcing requires implausible combinations of rapid , expansion, and limited technological adoption, with median outcomes favoring 3–6 W/m² by . These updates extend to integrated assessments linking RCPs with (SSPs), where combinations like SSP2-4.5 emerge as central estimates for current policy environments, reflecting slower-than-assumed decarbonization but avoiding the extremes of RCP8.5 or RCP2.6. A January 2025 review of policy-aligned s across multiple models found that pathways precluding RCP8.5-like emissions are now the norm in peer-reviewed literature, driven by of declining coal intensity and rising low-carbon deployment. Such reassessments underscore the need for ensembles to incorporate probabilistic weighting rather than equal treatment, as high-forcing cases serve primarily as upper-bound stress tests rather than baseline forecasts. Despite these refinements, some modeling persists with outdated high-end assumptions, potentially overstating tail risks without corresponding evidence.

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