Atlas scenarios
The future climate depends on how much we change the composition of the atmosphere (CO2, other gases, aerosols) and land properties. This in turn depends on economical growth, technological progress, and choices that are made whether to limit emissions or not. Possible futures are described by scenarios, plausible ways that the world could develop that are constructed so that the consequences of climate change and choices to limit it can be investigated.

The most recent scenarios used for the CMIP5 experiments run in support of the IPCC AR5 report are the Representative Concentration Pathways or RCPs. The four scenarios have been chosen to span a wide range of possible futures:

Emissions of main greenhouse gases across the RCPs. Grey area indicates the 98th and 90th percentiles (light/dark grey) of the literature. The dotted lines indicate four of the SRES marker scenarios (from van Vuuren et.al. 2011).
emissions in the four RCPs and other scenarios
Concentrations of main greenhouse gases across the RCPs. Grey area indicates the 98th and 90th percentiles (light/dark grey) of the literature. The dotted lines indicate four of the SRES marker scenarios (from van Vuuren et.al. 2011).
concentrations in the four RCPs and other scenarios

The two previous IPCC report (TAR and AR4) used the SRES scenarios. One of the major differences between these two sets is that the SRES scenarios are based on socio-economic storylines giving rise to emissions. These are then converted to concentrations including feedbacks that affect how much greenhouse gases are absorbed by land and ocean. In contrast, the RCPs are specified in terms of concentrations, so that these carbon-cycle feedbacks enter in the connection from the concentrations to the storylines, but not in the physical climate models.