The Intergovernmental Panel on Climate Change (IPCC) has replaced their older climate change scenarios with ‘Representative Concentration Pathways’ (RCPs) developed for IPCC AR5 report, scheduled to be published in 2014.
Unlike the scenarios they replace (A1, B1, A1F1, etc.), the RCPs are not based on social, technological, and economic storylines. Instead, they are simply plausible trends in atmospheric CO2 (and other greenhouse gases) concentration and are named for the corresponding additional heat retained by 2100 in W m-2.
RCP 2.6 and 8.6, are the minimum and maximum emissions scenarios, respectively. RCP 2.6 is considered quite optimistic as it assumes that emissions peak roughly ten years from now and drop to zero about 50 years after that.