169
crossed when ΣCO 2
EMISS
reaches 943 Gt C. This back of the envelope calculation is
close only to the EM-GC values of ΣCO 2
EMISS
given in Table 4.2 for 50 % and 66 %
probability.
30
Indeed, we can use another line of reasoning to suggest the CMIP5 GCM based
values of ΣCO 2
EMISS
associated with crossing the Paris target are too low. As noted
in the introduction to this section, IPCC (2013) stated the likely range for TCRE is
0.8–2.5 °C warming per 1000 Gt C of CO 2 emissions. Our probabilistic projection
of ΔT shown in Fig. 4.11, for the point where ΣCO 2
EMISS
= 1000 Gt C, is bounded
by 0.8 and 2.4 °C, in near perfect agreement with the range stated by (IPCC 2013).
Conversely, the CMIP5 GCM estimate that the 1.5 °C threshold will be crossed
when ΣCO 2
EMISS
= 633 Gt CO 2 implies a warming of 2.4 °C per 1000 Gt
C. Simulations conducted in the EM-GC framework suggest this value is possible
but highly unlikely.
Science is driven by reproducibility of results. As stated at the end of Chap. 2, we
urge that more effort be devoted to assessing GCM-based forecasts of global warming using energy balance approaches such as our EM-GC framework. It is our sincere hope that others will evaluate and publish values of ΣCO 2
EMISS
and ΣCO 2 -eq
EMISS
,
such as those in Tables 4.2 and 4.3, using various model frameworks. Time will tell
whether our estimates of ΣCO 2
EMISS
and ΣCO 2 -eq
EMISS
survive the scrutiny of others.
In the interim, we urge policy makers to tentatively consider that achieving the target of the Paris Climate Agreement, via the existing INDC pledges, may indeed be
a realistic goal.
4.4.2 CH 4
One final complication must be addressed: the potential rise of atmospheric CH 4 .
The present globally averaged mixing ratio of CH 4 , the second most important
anthropogenic GHG, is 1.84 ppm.
31
Projected future values of CH 4 diverge by an
enormous amount among the four RCP scenarios (Fig. 2.1).
The RCP projections of CH 4 reflect the large uncertainty in future emissions. The
RCP 2.6 scenario (van Vuuren et al. 2011) projects a CH 4 mixing ratio of 1.37 ppm
in 2060 (Table 4.1; see also Fig. 2.1). Atmospheric CH 4 has numerous humanrelated sources (Fig. 1.9). The RCP 2.6 design projects a 26 % decline of CH 4 by
2060, due to stringent controls on human release from all sources other than agriculture. Their projection considers the climate benefit of diet, particularly global con30 The fact this back of the envelope estimate for ΣCO 2
EMISS lies closer to our 66 % probability value
for keeping warming below the 1.5 °C, rather than the 50 % outcome, is due to the small non-linearity in ΔT versus ΣCO 2
EMISS manifest in the EM-GC framework that is shown in Fig. 4.10a.
31 Those keeping score are encouraged to visit http://www.esrl.noaa.gov/gmd/ccgg/trends_ch4;
this site continually updates the global mean CH 4 , albeit with a delay of a few months.
4.4 Emission Metrics
crossed when ΣCO 2
EMISS
reaches 943 Gt C. This back of the envelope calculation is
close only to the EM-GC values of ΣCO 2
EMISS
given in Table 4.2 for 50 % and 66 %
probability.
30
Indeed, we can use another line of reasoning to suggest the CMIP5 GCM based
values of ΣCO 2
EMISS
associated with crossing the Paris target are too low. As noted
in the introduction to this section, IPCC (2013) stated the likely range for TCRE is
0.8–2.5 °C warming per 1000 Gt C of CO 2 emissions. Our probabilistic projection
of ΔT shown in Fig. 4.11, for the point where ΣCO 2
EMISS
= 1000 Gt C, is bounded
by 0.8 and 2.4 °C, in near perfect agreement with the range stated by (IPCC 2013).
Conversely, the CMIP5 GCM estimate that the 1.5 °C threshold will be crossed
when ΣCO 2
EMISS
= 633 Gt CO 2 implies a warming of 2.4 °C per 1000 Gt
C. Simulations conducted in the EM-GC framework suggest this value is possible
but highly unlikely.
Science is driven by reproducibility of results. As stated at the end of Chap. 2, we
urge that more effort be devoted to assessing GCM-based forecasts of global warming using energy balance approaches such as our EM-GC framework. It is our sincere hope that others will evaluate and publish values of ΣCO 2
EMISS
and ΣCO 2 -eq
EMISS
,
such as those in Tables 4.2 and 4.3, using various model frameworks. Time will tell
whether our estimates of ΣCO 2
EMISS
and ΣCO 2 -eq
EMISS
survive the scrutiny of others.
In the interim, we urge policy makers to tentatively consider that achieving the target of the Paris Climate Agreement, via the existing INDC pledges, may indeed be
a realistic goal.
4.4.2 CH 4
One final complication must be addressed: the potential rise of atmospheric CH 4 .
The present globally averaged mixing ratio of CH 4 , the second most important
anthropogenic GHG, is 1.84 ppm.
31
Projected future values of CH 4 diverge by an
enormous amount among the four RCP scenarios (Fig. 2.1).
The RCP projections of CH 4 reflect the large uncertainty in future emissions. The
RCP 2.6 scenario (van Vuuren et al. 2011) projects a CH 4 mixing ratio of 1.37 ppm
in 2060 (Table 4.1; see also Fig. 2.1). Atmospheric CH 4 has numerous humanrelated sources (Fig. 1.9). The RCP 2.6 design projects a 26 % decline of CH 4 by
2060, due to stringent controls on human release from all sources other than agriculture. Their projection considers the climate benefit of diet, particularly global con30 The fact this back of the envelope estimate for ΣCO 2
EMISS lies closer to our 66 % probability value
for keeping warming below the 1.5 °C, rather than the 50 % outcome, is due to the small non-linearity in ΔT versus ΣCO 2
EMISS manifest in the EM-GC framework that is shown in Fig. 4.10a.
31 Those keeping score are encouraged to visit http://www.esrl.noaa.gov/gmd/ccgg/trends_ch4;
this site continually updates the global mean CH 4 , albeit with a delay of a few months.
4.4 Emission Metrics