By Bob Irvine
Forcing Efficacy is related to how quickly a related flux imbalance at the Top of the Atmosphere (TOA) is restored to equilibrium and could potentially make an enormous difference to how we calculate Equilibrium Climate sensitivity (ECS).
Is it possible that the Effective Radiative Forcing (ERF) of a small change in GHG forcing is a lot lower than the ERF for a similar change in solar forcing? This essay attempts to make this case.
I’m not saying here that the inputs to the model, Figure 1, are accurate, they almost certainly aren’t, I’m simply trying to show what is possible if ERF, as discussed below, is taken into account.
If, for example, a one W/M^2 change in GHG forcing only had a quarter of the surface temperature effect in total and over time as say a one W/M^2 change in solar forcing then it is possible to create a model that tracks past temperature with surprising accuracy. This has been done in Figure 1 below.
Figure 1 – The model (Red) is compared to the NOAA measured temperature (Blue) and assumes that each W/M^2 of GHG forcing has one quarter of the temperature effect of a similar solar forcing. Lag times are allowed for, and an estimate is made for aerosols and natural internal variability. The cooling effect of volcanic activity is not included in this model and may explain some of the inconsistency in 1980 and 1990.
The model in Figure 1 compares favourably with the IPCC’s solution to the same problem as shown in Figure 2 below.
The blue line in Figure 2 is the IPCC’s expected equilibrium temperature response as stated in their reports, AR5 and AR6 for the dates as shown. The relative positions of these two lines is not important and relates to the complex area of transient temperature response as compared to equilibrium temperature response.
The important thing to notice in Figure 2 is that the IPCC’s modelled equilibrium temperature response (Blue) is diverging rapidly from the NOAA measured temperature (Orange). The IPCC’s equilibrium temperature has increased on average at a rate of 0.26C/Decade since 1945 while the HadCrut temperature series has increased at an average rate of 0.11C/Decade since 1945. Importantly, this discrepancy appears to be getting worse indicating that human influences on climate forcing are not the whole story.
Figure 2 – Forcing is taken directly from the IPCC reports and converted to Equilibrium climate response temperature (Blue) then compared to the actual HadCrut temperature as recorded. The blue line is fitted using 4 points from the IPCC reports. The IPCC’s AR5 states that in 1950 humans had added 0.57 W/M2 to the global energy balance. In 1980 1.25 W/m2. In 2011 2.29 W/M2. The AR6 states that we had added 2.72 W/M2 in 2019. These are multiplied by 0.81 (3.0/3.7) to convert to Equilibrium Climate response temperature according to the IPCC’s (most likely) ECS of 3.0C.
THE CASE FOR “ERF” BEING SIGNIFICANT
Forster , make the case that “Effective Radiative Forcing” (ERF) is a much more useful way of estimating climate sensitivity than conventional; one size fits all, Radiative Forcing (RF). They make their case succinctly in the following quote;
“Imagine, for example, that the atmosphere alone (perhaps through some cloud change unrelated to any surface temperature response) quickly responds to a large Radiative Forcing to restore the flux imbalance at the TOA (Top Of Atmosphere), yielding a small effective climate forcing. In this case the ocean would never get a chance to respond to the initial Radiative Forcing, so the resulting climate response would be small, and this would be consistent with our diagnosed “Effective Climate Forcing” rather than the conventional “Radiative Forcing.”
It follows that a shorter response time at the “Top of the Atmosphere” (TOA) produces a lower climate sensitivity. Hansen  confirm and support this by saying “On a planet with no ocean or only a mixed layer ocean, the climate response time is proportional to climate sensitivity. ………..Hansen et al (1985) show analytically, with ocean mixing approximated as a diffusive process, that the response time increases as the square of climate sensitivity.”
ERF is the metric we should be using to evaluate any warming from increased GHGs in the atmosphere and this is directly calculated from energy residence time in the earth’s climate system. Importantly this system includes the world’s oceans.
It is established physics that the world’s oceans are opaque to energy with the wavelength generally reemitted by CO^2. This energy is almost totally absorbed in the first 0.15mm skin of the oceans. The top 1.0 mm of the ocean is generally referred to as the evaporation layer, so accordingly energy reemitted by the GHGs and striking the ocean is largely returned to the atmosphere immediately as latent heat of evaporation. It is then returned to space relatively quickly.
Solar energy, on the other hand, is overwhelmingly responsible for the temperature profile of the world’s oceans. It is readily absorbed to a depth of up to 100 meters in clear water and can remain in the ocean for centuries or sometimes thousands of years.
It follows that equilibrium restoration times at the top of the atmosphere (TOA) are likely to be significantly faster, on average, for a change in GHG forcing than for a similar change in solar forcing.
Ignoring these factors, the IPCC AR4 states that ERF for solar forcing is actually lower than ERF for the GHGs. They then modified this position in later reports and now believe that ERF for solar and the GHGs is similar.
The IPCC position appears to be erroneously supported by the models.
It is clear that the latest models treat all energy that is absorbed in the first 10m grid of the ocean in the same way. The “Technical Guide to MOM 4.0, GFDL Ocean Group Technical Report 5, 2008) divides solar penetration into the water column into three exponentials. The assumption and quote from MOM Guide 8.3.2 is copied below;
“The first exponential is for wavelengths >750nm (i.e., IIR) and assumes a single attenuation of 0.267 meter…”
This assumption means that all long wave solar energy with wavelength greater than 750 nm will be modelled as being subject to significant turbulent mixing.
My understanding is that the models treat the long wave GHG energy the same way that they treat the long wave solar component, by simply including it in the first, nominally 10-meter, grid by assuming an attenuation of 0.267 meter. They simply do not distinguish or account for the fact that long wave energy reemitted by CO^2 and centred at 15 microns is almost totally absorbed in the evaporation layer and is proportionately returned to the atmosphere almost immediately by evaporation when compared to solar energy. To treat this accurately the models would need the first ocean grid to be 0.2mm thick. This is something they appear not to do for reasons of complexity.
In general, if a photon of energy is absorbed by a water molecule it increases the velocity of that molecule. This increase in velocity may cause the molecule to break the surface tension of the water body or it may not. If the surface tension of the water is broken, then evaporation occurs and both the introduced and original energy of the molecule is lost to the water body and that body is evaporatively cooled.
If, on the other hand, the molecule does not break the surface tension of the water body for any reason, including that it is too far from the surface, then the introduced energy remains in the water and warms that water.
If this is an accurate representation of what happens then it follows that down Long Wave Infrared Radiation (LWIR) could have a slight cooling effect in warmer water, and for colder water a slight warming effect.
The overall effect of this on the earth’s Ocean Heat Content (OHC) is difficult to quantify. What can be said is that LWIR from GHGs will have a different and smaller effect on OHC than a similar amount of solar radiation as the LWIR is nearly totally absorbed in the evaporation layer while nearly all short-wave solar radiation is not.
It follows that the efficacy of GHG Forcing is likely to be significantly lower than the efficacy of a similar sized solar forcing. A change in GHG forcing is likely, therefore, to have a significantly smaller effect on global temperature, in total and over time, than a similar change in solar forcing to the point where the effect of recent rises in GHG forcing may be negligible.
Forcing efficacy is discussed here;
- Forster, P.M.F., & Taylor, K.E., – Climate Forcings and Climate sensitivities Diagnosed from Climate Model Integrations Coupled. Journal of Climate, 6183, 2006.
- Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., – Earth’s Energy Imbalance & Implications. Atmos. Chem. Phys. Discuss, 11, 27031-27105, pp 19-21, 2011.