6.1.3 Representative concentration pathways (RCPs)

A set of four RCPs were selected. The most extreme one, RCP8.5 displays a continuous rise in radiative forcing during the 21^st century, leading to a value of about 8.5 Wm^–2 in 2100. RCP6.0 and RCP4.5 are characterised by a steady rise during the 21^st century, up to a radiative forcing of about 6 and 4.5 Wm^–2 respectively, and a stabilisation after 2100. Finally, in RCP3-PD (peak and decline), the radiative forcing peaks before 2100 at about 3 Wm^–2 and then declines. Emissions and atmospheric concentrations of CO₂ corresponding to those RCPs are shown in Figure 6.4. As expected, CO₂ being the largest contributor to radiative forcing (see Fig. 4.2), the time series of atmospheric CO₂ concentration have the same shape as the time series for radiative forcing. By construction, the emissions cover a wide range of possibilities, with, for instance, nearly no emission of CO₂ after 2080 in RCP3-PD while in RCP8.5 the emissions are more than 25 PgC per year (i.e. more than 3 times greater than in 2000).

The RCP also includes estimates of emissions of a large number of greenhouse gases and atmospheric pollutants (CH₄, N₂O, chlorofluorocarbons, SO₂, black carbon, etc.) as well as estimates of future changes in land use. For instance, in all the RCPs, the decrease in SO₂ emissions (Fig. 6.5) is even larger than in the SRES (Fig. 6.3).

Furthermore, the RCPs have been extended to 2300 and even 2500 for studies of long-term climate change (Fig. 6.6). Because of the very large uncertainties in the driving forces influencing the emissions, the long-term scenarios are kept as simple as possible and thus highly idealised. Nevertheless, they provide a reasonable range for the possible changes, give time developments compatible with the RCPs over the 21^st century, and display a common framework in which the results of different models can be displayed. Among the various possible extrapolations, a forcing stabilisation path emerges for RCP4.5 and RCP6.0. For RCP3-PD, the forcing is assumed to continue to decrease after 2100. For RCP8.5, the extension suggests an increase in forcing until at least 2200, although the emissions growth slows in the second part of the 21^st century, leading to a nearly flat profile after 2100.

---