RCPs quantify future greenhouse gas concentrations and the radiative forcing (additional energy taken up by the Earth system), due to increases in climate change pollution. The number (i.e. 2.6) by the scenario describes the number of Watts per square meter of Radiative Forcing resulting from the greenhouse gas emissions .
RCP 2.6 (very low future emissions)
CO2 emissions increase slightly until around year 2020, then decline, becoming negative by the end of the century. This scenario assumes a sharp decline in the use of fossil fuel, increased use of cropland for biofuel production, and reduction of methane emissions by 40%.
RCP 4.5 (low to moderate future emissions)
There is a slight increase in CO2 emissions until mid-century, then declines. Energy use sharply declines and there is large scale reforestation. The size of agricultural land is reduced due to increased yield and much lower meat consumption. Strict new climate policies will be introduced and methane emissions stabilize.
RCP 8.5 (very high future emissions)
By the end of the century, CO2 emissions will be three times higher than present. There is a large increase in methane emission. Energy use will further increase, mostly using fossil fuels. Uptake of renewables is very limited, and there is hardly any implementation of climate policies.
Due to the different future greenhouse gas concentrations, RCP2.6 will result in the least amount of global warming. RCP 8.5 will result in more rapid warming and larger effects on global and local climate.
When selecting the RCPs it is important to realize that until mid-century the differences in outcomes between the RCPs is often very small. The reason for this is that the climate system responds relatively slowly to changes in greenhouse gas concentration. Therefore, the choice of RCP is not important until mid-century. For analyses after mid-century, it is important to distinguish between different RCPs. RCP8.5 gives a much more rapid warming and more pronounced changes in important indicators such as river flow, water temperature and precipitation. The difference between RCP 2.6 and 4.5 remain relatively small until the end of the century.