93The policy implication articulated above differs considerably from the consensus
in the climate modeling community that emission of GHGs must follow RCP 2.6 to
achieve even the 2 °C upper limit of Paris (Rogelj et al. 2016 ). We caution those
quick to dismiss the simplicity of our approach to consider the emerging view, dis-
cussed in Chap. 11 of IPCC ( 2013 ) and quantified in their Figs. 11.25 and TS.14, as
well as our Figs. 2.3 and 2.13, that the CMIP5 GCMs warm much quicker than has
been observed during the past three decades. In support of our approach, we empha-
size that our projections of ΔT are bounded nearly exactly by the green trapezoid of
IPCC ( 2013 ), which reflects the judgement of at least one group of experts as to how
ΔT will evolve over the next two decades. Given our present understanding of
Earth’s climate system, we contend the Paris Climate Agreement is a beacon of
hope because it places the world on a course of having a reasonable probability of
avoiding climate catastrophe.
We conclude by cautioning against over-interpretation of the numbers in Table
2.1 or the projections in Figs. 2.19 and 2.20. Perhaps the largest source of uncer-
tainty in the EM-GC estimates of ΔT is the assumption that whatever values of λ
(climate feedback) and κ (ocean heat export coefficient) have occurred in the past
will continue into the future. Should climate feedback rise, or ocean heat export fall,
the future increase of ΔT will exceed that found using our approach. On the other
hand, the past climate record can be fit exceedingly well for time invariant values of
λ and κ. The great difficulty is that the specific values of these two parameters are
not able to be ascertained from the climate record, due to large current uncertainties
in ΔRF due to aerosols and the ocean heat content record. Community-wide efforts
to reduce the uncertainties in ΔRF of aerosols and ocean heat storage are vital. We
urge that judgement of the veracity of the results of our EM-GC projections be
based on whether other research groups are able to reproduce these projections of
ΔT, based on similar types of analyses. Given these caveats, our forecasts of global
warming suggest that GHG emissions of RCP 4.5 constitute a reasonable guideline
for attempting to achieve the both the Paris target (1.5 °C) and upper limit (2.0 °C)
for global warming, relative to the pre-industrial era.
2.6 Methods
Many of the figures use data or archives of model output from publically available
sources. Here, webpage addresses of these archives, citations, and details regarding
how data and model output have been processed are provided. Only those figures
with “see methods for further information” in the caption are addressed below.
Electronic copies of the figures are available on-line at http://parisbeaconofhope.org.
Figure 2.1 shows mixing ratios of CO 2 , CH 4 , and N 2 O from RCP 2.6, RCP 4.5,
RCP 6.0, and RCP 8.5, which were obtained from files:
RCP*MIDYEAR_CONCENTRATIONS.DAT provided by the Potsdam
Institute for Climate Research (PICR) at: http://www.pik-potsdam.de/~mmalte/
rcps/data
2.6 Methods