Climate change projections in WGI require information about future emissions or concentrations of greenhouse gases, aerosols and other climate drivers. This information is often expressed as a scenario of human activities, which are not assessed in this report.
IPCC WGI scenarios have focused on anthropogenic emissions and do not include changes in natural drivers such as solar or volcanic forcing or natural emissions, for example, of CH4 and N2O.
For the Fifth Assessment Report of IPCC, the scientific community has defined a set of four new scenarios, denoted Representative Concentration Pathways (RCPs, see Glossary).
They are identified by their approximate total radiative forcing in year 2100 relative to 1750:
- 2.6 W m-2 for RCP2.6,
- 4.5 W m-2 for RCP4.5,
- 6.0 W m-2 for RCP6.0 and
- 8.5 W m-2 for RCP8.5.
For the Coupled Model Intercomparison Project Phase 5 (CMIP5) results, these values should be understood as indicative only, as the climate forcing resulting from all drivers varies between models due to specific model characteristics and treatment of short-lived climate forcers.
These four RCPs include:
- one mitigation scenario leading to a very low forcing level (RCP2.6),
- two stabilization scenarios (RCP4.5 and RCP6), and
- one scenario with very high greenhouse gas emissions (RCP8.5).
The RCPs can thus represent a range of 21st century climate policies, as compared with the no-climate-policy of the Special Report on Emissions Scenarios (SRES) used in the Third Assessment Report and the Fourth Assessment Report.
- For RCP6.0 and RCP8.5, radiative forcing does not peak by year 2100;
- for RCP2.6 it peaks and declines;
- and for RCP4.5 it stabilizes by 2100.
Each RCP provides spatially resolved data sets of land use change and sector- based emissions of air pollutants, and it specifies annual greenhouse gas concentrations and anthropogenic emissions up to 2100.
RCPs are based on a combination of integrated assessment models, simple climate models, atmospheric chemistry and global carbon cycle models. While the RCPs span a wide range of total forcing values, they do not cover the full range of emissions in the literature, particularly for aerosols.
Most of the CMIP5 and Earth System Model (ESM) simulations were performed with prescribed CO2 concentrations reaching:
- 421 ppm (RCP2.6),
- 538 ppm (RCP4.5),
- 670 ppm (RCP6.0),
- and 936 ppm (RCP 8.5) by the year 2100.
Including also the prescribed concentrations of CH4 and N2O, the combined CO2-equivalent concentrations are:
- 475 ppm (RCP2.6),
- 630 ppm (RCP4.5),
- 800 ppm (RCP6.0),
- and 1313 ppm (RCP8.5).
For RCP8.5, additional CMIP5 ESM simulations are performed with prescribed CO2 emissions as provided by the integrated assessment models.
For all RCPs, additional calculations were made with updated atmospheric chemistry data and models (including the Atmospheric Chemistry and Climate component of CMIP5) using the RCP prescribed emissions of the chemically reactive gases (CH4, N2O, HFCs, NOx, CO, NMVOC). These simulations enable investigation of uncertainties related to carbon cycle feedbacks and atmospheric chemistry.