Energy and Human Ambitions on a Finite Planet, 2021a

9 Climate Change 148
9.3 Possible Trajectories
Launching from the data used to generate Figure 9.3, we can now play a
few games to underst<strong>and</strong> what our future might hold in terms of total
CO 2 rise <strong>and</strong> corresponding ΔT increases by the year 2100 under various
contrived scenarios. 32
First, let’s imagine that we suddenly arrest the upward climb characteristic
of fossil-fuel usage to date 33 <strong>and</strong> maintain present-day levels
of fossil fuel use from now until 2100. Figure 9.11 shows what happens.
The total added CO 2 rises to 2.75 times the current excess, to
34
339 ppm v above pre-industrial levels. The associated radiative forcing
would be 4.25 W/m 2 <strong>and</strong> result in a 3.4 ◦ C temperature increase. Table
9.5 summarizes this scenario <strong>and</strong> the three to follow.
1.50
2100: 2.6 ppm V /yr
200
180
Total: 339 ppm V
32: None of the scenarios we will fabricate
are realistic, exactly, but help us establish
boundaries of possible outcomes. Mathematical
models need not capture all the
nuances to still be useful guides to underst<strong>and</strong>ing.
33: . . . reflected in the left-h<strong>and</strong> panel of
Figure 9.3
34: . . . resulting in about 620 ppm v ;up
339 ppm v from the pre-industrial 280 ppm v
Verify yourself following a procedure
like Example 9.2.1 as good
practice.
CO2 ppmV annual contribution
1.25
1.00
0.75
0.50
0.25
coal
oil
gas
cumulative CO2 ppmV contribution
0.00
1800 1850 1900 1950 2000 2050 2100
year
160
140
120
100
80
60
40
20
coal
oil
gas
0
1800 1850 1900 1950 2000 2050 2100
year
Figure 9.11: CO 2 rise if fixing fossil fuel use
at today’s levels for the rest of the century,
following the conventions of Figure 9.3.We
would still add 2.6 ppm v per year from now
until the end of the run in 2100, <strong>and</strong> would
have accumulated a total rise of 339 ppm v ,
or 2.75 times the problematic amount already
accumulated to date. The associated
temperature rise would be 3.4 ◦ C. For this
<strong>and</strong> all subsequent scenarios, the plots show
only the half of emitted CO 2 that remains
in the atmosphere.
ΔCO 2 vs. today CO 2 RF CO2 ΔT
Scenario (ppm v ) (ppm v ) (W/m 2 ) ( ◦ C)
Arrest FF climb 339 2.75 × 620 4.25 3.4
Arrest; no coal 268 2.18 × 548 3.6 2.9
Curtail by 2100 235 1.91 × 515 3.3 2.6
Curtail by 2050 169 1.37 × 450 2.5 2.0
Table 9.5: Summary of scenario outcomes.
We’re already seeing serious problems emerging today, at about 1 ◦ C
increase, so this 3.4 ◦ C scenario is not desirable. 35 And reflect for a
moment how unlikely it is that we can even arrest the climb of fossil fuel
use so suddenly. It would seem that the rate of CO 2 emission is destined
to climb higher than it is today: we have not yet found the peak!
The second scenario focuses on eliminating coal, since it is the highest
intensity CO 2 emitter, 36 as Figure 9.3 makes clear. What if natural
gas—the fossil fuel having the lowest carbon intensity—could replace
all coal applications? This is already happening—gradually—in the
electricity generation sector. Countless advocates encourage such a
transition as rapidly as can be accomplished. The pretend world of
35: On the other h<strong>and</strong>, we may not have
enough fossil fuel resource to realize this
scenario, in which case it can be treated as
an upper limit.
36: . . . based on its lower energy density
(5–8 kcal/g vs. 13 for natural gas) <strong>and</strong> its
higher CO 2 -to-fuel mass ratio (3.67 vs. 2.75
for natural gas)
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.