*By Dave Burton, Oct. 11, 2020.*
(Adapted, in part, from a blog comment.)

*Last updated: Feb. 22, 2021.*

Synopsis: https://sealevel.info/Radiative_Forcing_synopsis.html

Effect on temperatures: https://sealevel.info/sensitivity.html

1. Calculated estimates of CO_{2} forcing

2. Measurements of CO_{2} forcing

2.1 Feldman, 2015

2.2 Rentsch, 2020

The formula for estimating radiative forcing (RF) from a change in atmospheric CO_{2} concentration is usually given as:

** Δ F** = 𝞪·ln(

where:

ERF = 𝞪·ln(2) = “Effective Radiative Forcing” per doubling of CO

and the most common estimate is:

𝞪 =

For a doubling of CO_{2} concentration that yields:

ERF = 𝞪·ln(2) = 3.7 ±0.4 W/m² per doubling of CO_{2} concentration

Those are the figures used by the IPCC (TAR & later). They are from Myhre 1998 (pdf), who also reported an uncertainty “of order 10%,” which would be about ±0.54.

That represented about a 15% reduction from an earlier IPCC estimate of

𝞪 =** 6.3 ** (which is ERF=4.4 W/m² per doubling; see SAR §6.3.2, p.320)

Prof. Will Happer (2013) reports calculating (and see also Happer 2015), based on corrected modeling of CO_{2} lineshapes, that the “5.35” coefficient is about 40% too high, which makes:

𝞪 ≈** 3.8 ±0.5 ** (which is 2.6 ±0.5 W/m² per doubling)

Happer & van Wijngaarden (2020 preprint)
report calculating CO_{2}'s ERF at the mesopause (which should be similar to TOA) to be
3.0 W/m² per doubling (see their Table 2,
rightmost column). That makes:

𝞪 = **4.33**

The IPCC's AR5 reported that the radiative forcing estimates for a doubling of CO_{2} assumed in 23 CMIP5 GCMs varies
from 2.6 to 4.3 W/m² per doubling, so that:

𝞪 ≈** 3.7 to 6.2**

Prof. Joshua Halpern reports:

𝞪 =** 4.35 ** (but I don't know what his source is):

**Feldman et al 2015** measured downwelling longwave IR “back radiation” from CO

However, radiative forcing is customarily defined at TOA, not at the surface. That is, it's specified in terms of an equivalent change in solar radiation. The effect of GHGs on radiation balance at the surface is similar, but not identical, to their effect at TOA.

The only way that energy leaves the Earth at TOA is by radiation, but at the Earth's surface the energy fluxes are much more complicated, including important mechanisms like convection and evaporative cooling. Warming the Earth causes its rate of cooling to increase by all of those mechanisms, making them negative feedbacks. Those negative feedbacks limit the temperature increase which results from an increase in any forcing.

GHGs in the atmosphere have their effect on temperature by absorbing outbound longwave IR radiation from below, which otherwise would have escaped to space. Absorbing that radiation warms the atmosphere. That, in turn, increases radiation *from* GHGs in the atmosphere, about half of which goes back down toward the surface, as “downwelling” or “back radiation,” which warms the surface. Hence, the surface and the lower troposphere warm or cool together.

Here's NASA's 2014 (latest?) version of the famous Trenberth energy flow diagram *(click to enlarge it)*:

The diagram was discussed on WUWT, here:

https://wattsupwiththat.wordpress.com/2014/01/17/nasa-revises-earths-radiation-budget-diminishing-some-of-trenberths-claims-in-the-process/

The diagram shows:

**1.** Incoming solar radiation at TOA = 340.4 W/m². That's one-fourth of 1361.6 W/m², which is their estimate for the
average value of the “Solar Constant.” (The so-called
“Solar Constant” actually varies somewhat,
and other estimates for its average vary from 1360 to 1370 W/m².) ↑

It's one-fourth because the 1361.6 W/m² figure is solar radiation intensity at TOA, where the Earth directly faces the Sun, and the 340.4 figure is averaged over the entire globe, including the back side. (The surface area of a sphere is exactly four times the area of a circle of the same radius.)

**2.** Estimated outgoing radiation at TOA is 239.9 W/m² IR + 77.0 + 22.9 = 339.8 W/m², which is 0.6 W/m² less than the
incoming solar radiation. (That figure is poorly constrained, and not directly measurable; I’ve also seen estimates of 0.7 or 0.8 W/m².)
That is their estimate of the “radiation imbalance,” which represents warming “in the pipe,” but not yet realized (roughly 0.2°C to 0.4°C,
depending on sensitivity estimates and timeframe). ↑

**3.** Downwelling “back radiation” at the surface, from GHGs in the atmosphere, is estimated as averaging 340.3 W/m².
That's about the same as the solar irradiance at TOA, but ≈29% greater than the average amount of solar radiation which makes it to the
surface. ↑

**4.** About 22.6% (77 W/m²) of the incoming solar radiation is reflected back into space,
without either reaching the surface or being absorbed. ↑

Thus, an increase of 1 W/m² in LW IR at the surface from GHGs in the atmosphere has an effect on
surface temperatures which is similar to a

That 1.29:1 ratio is approximate, because the effect of sunlight at the surface is not identical to the effect of downwelling LW IR from atmospheric
CO_{2}. On one hand, there's considerable overlap
between the spectrums of CO_{2} and water vapor, which reduces the warming effect from CO_{2}, especially in the
humid tropics. On the other hand, an estimated 22.9 W/m² (6.7%) of sunlight is reflected from the surface back into space,
which does not happen for LW IR. If that 6.7% were included in the calculation above, it would make an
an increase of 1 W/m² in downwelling LW IR at the surface similar in effect to a

Adjusting the Feldman values for having measured at the surface, rather than TOA, yields about 1.29 × (2.40 ±0.72) per doubling at TOA, and dividing by ln(2), yields the coefficient:

𝞪 =** 4.47 ±1.34 ** (which is 3.10 ±0.93 W/m² per doubling)

That figure is very close to Halpern's “4.35”, and closer to Happer's “3.8” than to Myhre's “5.35,” but the uncertainty interval is wide enough to encompass all three estimates. It does preclude the SAR's “6.3” figure.

**Rentsch 2020** (unpublished draft), analyzed orbital Atmospheric Infrared Sounder (AIRS) spectroscopy, and found that CO_{2}-induced radiative forcing at TOA from a 37 ppmv CO_{2}
concentration increase (13.65% of a doubling), under nighttime, cloud-clear conditions, caused +0.358 ±0.067 W/m² radiative forcing increase.
Dividing by 0.1365 × ln(2), yields the coefficient:

𝞪 =** 3.79 ±0.71 ** (which is 2.62 ±0.49 W/m² per doubling)

That's about 70% of the Myhre 1998 / IPCC prediction, and very close to Happer's result.