53of climate, is it called a negative feedback. The total effect of all responses to the
prescribed perturbation to RF of climate by GHGs and aerosols is called climate
feedback, which can vary quite a bit between GCMs, mainly due to the treatment of
clouds (Bony et al. 2006 ; Vial et al. 2013 ). GCMs also provide estimates of the future
evolution of precipitation, drought indices, sea-level rise, as well as variations in
oceanic and atmospheric temperature and circulation (IPCC 2013 ).
Our focus is on analysis of projections of ΔT for the RCP 4.5 (Thomson et al.
2011 ) and RCP 8.5 scenarios (Riahi et al. 2011 ). Atmospheric abundances of the
three most important anthropogenic GHGs given by the RCP 4.5 and RCP 8.5 sce-
narios are shown in Fig. 2.1. Under RCP 8.5, the abundances of these GHGs rise to
alarmingly high levels by end of century. On the other hand, for RCP 4.5, CO 2 sta-
bilizes at 540 parts per million by volume (ppm) (~35 % higher than contemporary
level) and methane (CH 4 ) reaches 1.6 ppm (~10 % lower than today) in 2100. The
atmospheric abundance of nitrous oxide (N 2 O) continues to rise under RCP 4.5,
reaching 0.37 ppm by end of century (~15 % higher than today).
The ΔRF of climate associated with RCP 4.5 and RCP 8.5 are shown in Fig. 2.2,
using the grouping of GHGs defined in Chap. 1. The contrast between these two
scenarios is dramatic. For RCP 4.5, ΔRF of climate levels off at mid-century,
reaching 4.5 W m−^2 at end-century. For RCP 8.5, ΔRF rises throughout the century,
hitting 8.5 W m−^2 near 2100. Both behaviors are by design (Thomson et al. 2011 ;
Riahi et al. 2011 ). While CO 2 remains the most important anthropogenic GHG for
both projections, other GHGs exert considerable influence.
The RCPs are meant to provide a mechanism whereby GCMs are able to simulate
the response of climate for various prescribed ΔRF scenarios, in a manner that allows
differences in model behavior to be assessed. Evaluation of GCM output has been
greatly facilitated by the Climate Model Intercomparison Project Phase 5 (CMIP5)
(Taylor et al. 2012 ), which maintains a computer archive of model output freely avail-
able following a simple registration procedure,^5 as well as the prior CMIP phases.
(^5) CMIP5 GCM output is at http://cmip-pcmdi.llnl.gov/cmip5/data_getting_started.html
Fig. 2.1 GHG abundance, 1950–2100. Time series of the atmospheric CO 2 , CH 4 , and N 2 O from
RCP 2.6 (van Vuuren et al. 2011b), RCP 4.5 (Thomson et al. 2011 ), RCP 6.0 (Masui et al. 2011 ),
RCP 8.5 (Riahi et al. 2011 ), and observations (black) (Ballantyne et al. 2012 ; Dlugokencky et al.
2009 ; Montzka et al. 2011 ). Values of GHG mixing ratios from RCP extend back to 1860, but this
figure starts in 1950 since most of the rise in these GHGs has occurred since that time. See Methods
for further information
2.1 Introduction