What are the major differences between the Special Report on Emissions Scenarios (SRES) and the Representative Concentration Pathways (RCP)?

• Special Report on Emissions Scenarios (SRES)
  • Group of 40 scenarios developed by the International Panel on Climate Change (IPCC) in 2000
    • Each scenario is divided into one of four "families" (A1, A2, B1, B2), each with common themes (e.g: Population changes, energy sources, economic development, etc.).
    • In the case of the A1 model, there are 4 scenario "groups" based on the possibility of a certain energy source becoming more dominant (e.g: A1C scenarios are based on a coal-driven society). 
  • ​SRES use a sequential approach to determine their scenarios.
    • Political or legislative actions have no effect on the development of these scenarios.
    • The scenarios are based on socio-economic futures; this setup requires each individual scenario (emission scenario, radiative forcing scenario, climate model scenario) to be formed in sequence with the previous scenario. If there is a change in any previous scenario, the whole sequence must be restarted. This makes the sequential approach time-consuming.
• Representative Concentration Pathways (RCPs)
  • Group of 4 individual scenarios developed by the IPCC in 2014 to supersede SRES
    • Each scenario consists of a specific radiative forcing projection and makes assumptions about future population, GDP, energy use, etc. based on the radiative forcing. 
  • RCPs use a parallel approach in the development of its scenarios.
    • The parallel approach allows for policy changes to be implemented since the socio-economic scenarios are not the starting point for the RCPs.
    • Scenarios are based off of radiative forcing projections; this allows for socio-economic, emissions and climate scenarios to be developed in parallel with each other. This way, changes can be made to one individual scenario without having to restart the whole sequence.

Need to add:

1. definitions of "emission scenario, radiative forcing scenario, climate model scenario" - minimally please make each of these a new glossary term and link to those pages
2. clarify on SRES whether or not there are a limited number of sequences (I assume there are since there are a limited number of SRES - include how many were developed (I think it's near 40 even though there is a smaller set of popular ones, so please check))
3. I think we need to develop some text to explain how SRES are based on "driving forces from demographic to social and economic developments, and they encompass a wide range of future greenhouse gas (GHG) emissions" (http://www.ipcc.ch/ipccreports/sres/emission/index.php?idp=90) which are then translated into emissions of GHGs, SO₂ , and other radiatively important gases versus RCPs that are driven by a specific radiative forcing amount in year 2100 (derived from 4 plausible radiative forcing pathways) which are based on emission, concentration and land-use trajectories and independent of specific socio-economic storylines.