Paris Climate Agreement Beacon of Hope

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Figure 2.6 shows total ΔRF (black line) due to tropospheric aerosols that was used
as EM-GC input (i.e., the term Aerosol ΔRFi in Eq. 2.2) for the calculations shown in
Figs. 2.4 and 2.5, as well as the contribution to aerosol ΔRF from the six classes of
aerosols. This particular time series, based on RCP 4.5, has been designed to match the
IPCC ( 2013 ) best estimate of AerRF 2011 (aerosol ΔRF in year 2011) of −0.9 W m−^2.
As detailed in Canty et al. ( 2013 ), a specific value of AerRF 2011 can be found
using a variety of combinations of scaling parameters that account for the aerosol
indirect effect. Figure 2.7a shows time series of aerosol ΔRF for RCP 4.5 designed
to match five rather disparate estimates of AerRF 2011 from IPCC ( 2013 ):

• −0.9 W m−^2 (best estimate)


• −0.4 and −1.5 W m−^2 (upper and lower limits of the likely range, denoted by the
  upper and lower edges of rectangle marked “Expert Judgement” in Fig. 7.19b of
  IPCC ( 2013 ), which are the 17th and 83d percentiles of the estimated distribution)


• −0.1 and −1.9 W m−^2 (upper and lower limits of the possible range, denoted by
  the error bars on the “Expert Judgement” rectangle in Fig. 7.19b, which are the
  5th and 95th percentiles of the estimated distribution)

Figure 2.7b shows aerosol ΔRF designed to match these same five values of
AerRF 2011 , except for the RCP 8.5 emission of aerosol precursors. Three estimates
of Aerosol ΔRF are shown for each value of AerRF 2011 , found using scaling param-
eters described in Methods.
Variations in the RF of climate due to the land use change (LUC) is the final
anthropogenic term considered in our EM-GC. Numerical values of LUC ΔRFi in
Eq. 2.2 are based on Table AII.1.2 of IPCC ( 2013 ). This term, which has an
extremely minor effect on computed ΔT and is included for completeness, repre-
sents changes in the reflectivity of Earth’s surface caused, for example, by conversion
of forest to concrete. The release of carbon and other GHGs due to LUC is not
represented by this term, but rather by the GHG ΔRFi term.

RCP 4.5

Fig. 2.6 Aerosol ΔRF versus time, RCP 4.5, for AerRF 2011 = −0.9 W m−^2 (open square), The
figure shows ΔRF for six aerosol components (as indicated), the sum ΔRF for all aerosols that
warm (red), the sum of ΔRF for all aerosols that cool (blue), and the net ΔRF of aerosols (black).
See Methods for further information

2.2 Empirical Model of Global Climate

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