Clarify what RCP means. This stands for Representative Concentration Pathway. [Alan]
During earlier IPCC reports, the concept of scenarios was used in projections. These referred to patterns of energy use, population size and economic changes. For the 2013/2014 Assessment Report (AR) 5, this was changed to a system referring to the Representative Concentration Pathways of greenhouse gas emissions identified in term of the overall radiative forcing (warming) they would induce by 2100. The values are identified in the increased average watts per meter squared (wm2) of heating compared to a value set at 0 before the industrial revolution. The current warming is 2.75 wm2. The RCP values used are:
RCP2.5 which essentially means that by the end of the century we have returned to an atmospheric concentration of gases about equivalent to now.
RCP 4.5 and 6.5 represent increases beyond that such that warming is greater by 2100
RCP8.5 represents the most extreme future and seems to be the trajectory which we are following – hence it is often call the Business As Usual ‘scenario’ although it isn’t so much a scenario as an atmospheric GHG concentration future.