The IPCC CMIP5 scenarios are the Representative Concentration Pathways (RCPs) used in the Coupled Model Intercomparison Project Phase 5 (CMIP5), which underpinned the IPCC’s Fifth Assessment Report (AR5, 2013–2014).
They describe possible future greenhouse‑gas concentration pathways, named after radiative forcing in 2100 (measured in W/m²), rather than by detailed socioeconomic narratives.
More detailed information on HIRDS and the IPCC CMIP5 scenarios climate change projection it is based on can be found here.
The Four CMIP5 / RCP Scenarios
RCP2.6 (Very low emissions)
· Radiative forcing: ~2.6 W/m² by 2100
· Emissions trend: Sharp reductions; emissions peak early (around 2020) and decline rapidly
· Warming by 2100: ~1.0–1.8 °C (likely below 2 °C)
· Interpretation: Strong climate policy, large-scale mitigation, some net negative CO₂ late-century
RCP4.5 (Intermediate stabilization)
· Radiative forcing: ~4.5 W/m² by 2100
· Emissions trend: Peak around mid-century, then decline
· Warming by 2100: ~1.8–2.7 °C
· Interpretation: Moderate mitigation and technology deployment
RCP6.0 (High stabilization)
· Radiative forcing: ~6.0 W/m² by 2100
· Emissions trend: Peak late in the century
· Warming by 2100: ~2.2–3.7 °C
· Interpretation: Delayed mitigation, continued fossil fuel reliance
RCP8.5 (Very high emissions)
· Radiative forcing: ~8.5 W/m² by 2100
· Emissions trend: Continued growth throughout the 21st century
· Warming by 2100: ~3.7–5.0 °C
· Interpretation: Minimal climate policy; often described as a “high‑end” or worst‑case pathway
Key Characteristics of CMIP5 RCPs
· Concentration-driven: Specified by greenhouse gas concentrations, not directly by population or economic assumptions
· Used globally: Basis for climate projections in AR5 (temperature, rainfall, sea level, extremes)
· Still widely used: Many impact and adaptation studies continue to rely on CMIP5/RCP outputs
Quick Summary Table
|
RCP |
Forcing (2100) |
Emissions Path |
Typical Use |
|
RCP2.6 |
2.6 W/m² |
Strong reductions |
Paris‑aligned futures |
|
RCP4.5 |
4.5 W/m² |
Peak ~2050 |
Moderate mitigation |
|
RCP6.0 |
6.0 W/m² |
Late peak |
Delayed action |
|
RCP8.5 |
8.5 W/m² |
Continual growth |
High‑end risk analysis |
Figure: Mullan, B., A. Sood, and S. Stuart (2016), Climate Change Projections for New Zealand: Atmosphere
Projections Based on Simulations from the IPCC Fifth Assessment, edited, Ministry for the
Environment, Wellington.