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Two other scenarios, RCP 6.0 (Masui et al. 2011 ) and RCP 2.6 (van Vuuren et al.
2011b), were considered by IPCC ( 2013 ). The mixing ratio of CO 2 peaks at about
670 ppm at end-century for RCP 6.0 (Fig. 2.1); the climate consequences for this
scenario clearly lie between those of RCP 4.5 and RCP 8.5. For RCP 2.6, CO 2 peaks
mid-century and slowly declines to 420 ppm at end-century.^6 According to the
authors of RCP 2.6, this scenario “is representative of the literature on mitigation
scenarios aiming to limit the increase of global mean temperature to 2 °C”. While
this is true for literal interpretation of the output of the GCMs that contributed to the
(^6) Globally averaged CO 2 was ~404 ppm during summer 2016. To achieve the RCP 2.6 scenario,
CO 2 at the end of the century must be comparable to the present day value.
RCP 4.5
RCP 8.5
Fig. 2.2 ΔRF of climate due to GHGs, 1950–2100. Time series of ΔRF of climate, RCP 4.5 (top)
and RCP 8.5 (bottom), due to the three dominant anthropogenic GHGs (CO 2 , CH 4 , and N 2 O) plus
contributions from all ozone depleting substances (ODS), other fluorine bearing compounds such
as HFCs, PFCs, SF 6 , and NF 3 (Other F-gases), and tropospheric O 3. Shaded regions represent
contributions from specific gases or groups. See Methods for further information
2 Forecasting Global Warming