What are the RCPs (Representative Concentration Pathway) scenarios and the differences of them?

The Intergovernmental Panel on Climate Change (IPCC ) Fifth Assessment Report (AR5) is due for publication in 2013-14. Its findings will be based on a new set of scenarios that replace the Special Report on Emissions Scenarios (SRES) standards employed in two previous reports. The new scenarios are called Representative Concentration Pathways (RCPs). The RCPs describe 4 different scenarios based on different assumptions about population, economic growth, energy consumption and sources and land use over this century. There are four RCPs (Representative Concentration Pathway) scenarios which are based on multi-gas emission scenarios, namely RCP2.6, RCP4.5, RCP6, and RCP8.5, are labelled after a possible range of radiative forcing values in the year 2100 relative to pre-industrial values (+2.6, +4.5, +6.0, and +8.5 W/m2, respectively).

Radiative forcing, expressed as Watts per square metre, is the additional energy taken up by the Earth system due to the enhanced greenhouse effect. More precisely, it can be defined as the difference in the balance of energy that enters the atmosphere and the amount that is returned to space compared to the pre‐industrial situation. Total radiative forcing is determined by both positive forcing from greenhouse gases and negative forcing from aerosols. The database of the Rcp scenarios covers emissions of well-mixed greenhouse gases (GHGs) such as carbon dioxide, nitrous oxide and fluorinated gases at the level of five world regions and short-lived GHGs as well as radiatively and chemically active gases (black and organic carbon, methane, sulfur, nitrogen oxides, volatile organic compounds, carbon monoxide and ammonia) in addition to spatial patterns.

For each category of emissions, an RCP contains a set of starting values and the estimated emissions up to 2100, based on assumptions about economic activity, energy sources, population growth and other socio-economic factors. They will also facilitate and expedite future climate change assessments across the integrated assessment community.

The RCP 2.6 is developed by the IMAGE modeling team of the Netherlands Environmental Assessment Agency. The emission pathway is representative for scenarios in the literature leading to very low greenhouse gas concentration levels (Van Vuuren et al., 2011). The RCP2.6 is known as "Low emissions". The RCP2.6 is not comparable with any SRES scenario. This future would require:Low energy intensity, CO2 emissions stay at today’s level until 2020, then decline and become negative in 2100, A world population of 9 billion by year 2100, Declining use of oil, and Methane emissions reduced by 40 per cent.

The RCP 4.5 is developed by the MiniCAM modeling team at the Pacific Northwest National Laboratory's Joint Global Change Research Institute (JGCRI). RCP4.5 run assuming that radiative forcing will stabilize with an increases of about 4.5 Wm-2 after 2100 (Taylor et al. 2012). The RCP4.5 is known as "Intermediate emissions". The RCP4.5 is comparable with SRES scenario: B1. This future is consistent with: Decreasing use of croplands and grasslands due to yield increases and dietary changes, Stable methane emissions, and CO2 emissions increase only slightly before decline commences around 2040.

The RCP 6.0 is developed by the AIM modeling team at the National Institute for Environmental Studies (NIES), Japan. It is a stabilization scenario where total radiative forcing is stabilized after 2100 without overshoot by employment of a range of technologies and strategies for reducing greenhouse gas emissions. The details of the scenario are described in Fujino et al. (2006) and Hijioka et al. (2008). The RCP6 is known as "Intermediate emissions". The RCP6 is comparable with SRES scenario: B2. This future is consistent with: Heavy reliance on fossil fuels, Stable methane emissions, and CO2 emissions peak in 2060 at 75 per cent above today's levels, then decline to 25 per cent above today.

RCP8.5's radiative forcing levels by the end of 2100 are around 8.5W/m2 under our 'best-estimate' set of model parameters with forcing levels increasing further thereafter-up to 12 W/m2 by 2250, when concentrations stabilize. This RCP is consistent with a future with no policy changes to reduce emissions. The RCP8.5 is known as "High emissions".The RCP8.5 is comparable with SRES scenario A1F1. This future is consistent with: Rapid increase in methane emissions, Heavy reliance on fossil fuels, High energy intensity, A world population of 12 billion by 2100, and Increased use of croplands and grassland which is driven by an increase in population.