Energy and Human Ambitions on a Finite Planet, 2021a

9 Climate Change 146
change, but a hefty passenger.
No greenhouse gases
Greenhouse gases
Figure 9.9: In the absence of greenhouse
gases, infrared radiation has no difficulty
escaping to space (left). When greenhouse
gases (GHGs) are present (right), most of
the infrared radiation is absorbed by GHG
molecules. These molecules later release the
absorbed energy as new infrared radiation,
but in a r<strong>and</strong>omized direction, so that some
energy is returned to the ground, thereby
keeping the surface warmer than it would
be without greenhouse gases.
Naïvely speaking, going from 280 ppm v to 420 ppm v (an increase by a
factor of 1.5), might be expected to turn the 8.6 ◦ C greenhouse effect from
CO 2 into 12.9 ◦ C (1.5×) for a 4.3 degree human-caused warming. 24 But 24: This is not how it works.
the CO 2 absorption feature at ∼ 15 μmissaturated,soasCO 2 is added, it
becomes wider, but logarithmically rather than linearly as a function of
CO 2 concentration. Climate scientists often express the impact of various
influences as a radiative forcing, measured in W/m 2 .
Definition 9.2.1 Radiative forcing is used to describe the areal power (in
W/m 2 ) of absorbed solar energy <strong>and</strong> infrared radiation to space. Various
influences or constituents each contribute their own radiative forcing. In
equilibrium, a balance exists so that the net 25 radiative forcing is zero.
25: . . . sum of all: positive <strong>and</strong> negative
The average solar forcing is
RF ⊙ 1360 W/m 2 × 0.707/4 ≈ 240 W/m 2 , (9.3)
sharing substantial overlap with Eq. 9.2—the factors having been explained
in that context. Without adding to the pre-industrial set of
greenhouse gases (GHGs), we would solve for temperature 26 as
26: . . . adding the baseline 33◦ C GHG contribution
( ) 0.25 RF⊙
T + 33, (9.4)
σ
evaluating to 288 K, or about 15 ◦ C. If we add (or deduct) radiative forcing
from another source, it would add (subtract) in the numerator of Eq.
9.4. The addition of CO 2 over the original amount (CO 2,orig ; 280 ppm v )
generates a radiative forcing of
( )
CO 2
RF CO2 5.35 ln
W/m 2 , (9.5)
CO 2,orig
where the ln() function is the natural logarithm. 27 At our present
∼420 ppm v , the ratio of current CO 2 to CO 2,orig is 1.5 <strong>and</strong> RF CO2 ≈
2.2W/m 2 , so that the new temperature (adding the new forcing to Eq.
9.4)is
( RF⊙ + RF CO2
27: . . . capturing the logarithmic broadening
of the width of the CO 2 absorption
feature at 15 μm inFigure 9.7
T
σ
) 0.25
+ 33 ≈ 288.6, (9.6)
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.