Appraising Marvel et al.: Implications of forcing efficacies for climate sensitivity estimates

Guest essay by Nicholas Lewis

Summary –

Different agents may have effects on global temperature (GMST) different to those which would be expected simply by reference to the radiative forcing they exert. This difference is encapsulated in the term “forcing efficacy”. In their recent paper, Marvel et al. estimate efficacies for various forcings from climate simulations of the GISS-E2-model over the period from 1850 to 2005 (the historical period). They then use data pertaining to three recent observational climate sensitivity studies, incorporating the efficacy figures and calculating new estimates for the transient climate response (TCR) and effective climate sensitivity (a proxy for equilibrium climate sensitivity: both are designated ECS). Taking the average for the three studies, these new estimates imply an increase in TCR from 1.3°C to 1.8°C and in ECS from 1.9°C to 3.0°C. The increases are due to the efficacy-adjusted sum of forcings over the historical period being substantially less than the unadjusted sum of forcings.

Marvel et al. conclude that “GISS ModelE2 is more sensitive to CO2 alone than it is to the sum of the forcings that were important over the past century” and that “Climate sensitivities estimated from recent observations will therefore be biased low in comparison with CO2-only simulations owing to an accident of history: when the efficacies of the forcings in the recent historical record are properly taken into account, estimates of TCR and ECS must be revised upwards.” The second statement would not be scientifically valid even if Marvel et al.’s findings were correct. Results from any single-model model study reflect the characteristics of the particular model involved, which may well behave differently from the real climate system – and from other models. Moreover, due to multiple methodological, data and computational errors and deficiencies in their study, Marvel et al. fail to establish that their first assertion is true either. When these problems are corrected, GISS ModelE2 does not appear to be materially, if at all, more sensitive to CO2 alone than it is to the sum of the forcings acting over the historical period.

Marvel et al.’s revised observationally-based TCR and ECS figures substantially exceed the GISS-E2-R model’s TCR of 1.4°C and effective climate sensitivity of 1.9–2.0°C.[1] However, GISS-E2-R already exhibits warming that is greater than in the real climate system: the simulated GMST increase and ocean heat uptake rate are both higher than observations at the end of the historical period, which implies that its TCR and effective climate sensitivity are probably excessive. That their new estimates of TCR and ECS are higher still is therefore paradoxical and suggests that there is something seriously wrong with their work.

Fig1_Fig1aMarvelFigure 1: Reproduction of Figure 1a of Marvel et al.

Figure 2 reproduces Figure 1b of Marvel et al. It differs from Figure 1 only in that the x-axis shows ΔF−ΔQ rather than ΔF, as here equilibrium rather than transient sensitivity (and hence efficacy) is being estimated.

Fig2-Fig1bMarvelFigure 2: Reproduction of Figure 1b of Marvel et al.

The transient efficacy estimates in the paper disagree with estimates from more detailed work by James Hansen using the earlier GISS Model E, and with other work using different models. The equilibrium efficacy estimates use the same GISS-E2-R forcing data as do the transient estimates, and are therefore also very questionable. Moreover, Marvel et al.’s use of ocean heat uptake values rather than radiative imbalance data, which is what should have been used, biases down its estimates of equilibrium efficacies and of ECS.

Marvel et al. estimate forcing efficacies from simulations in which the climate is forced by just a single forcing at a time. Efficacies that are derived in this way may be different to those that apply when all forcings are applied simultaneously in the same model. The forcing produced by a forcing agent may vary substantially with climate state. Previous studies show that to be the case in GISS-E2-R for both aerosol and ozone forcing. Moreover, efficacies estimated from a climate simulation may be substantially different to those that apply in the real climate system.

The efficacy estimates Marvel et al. made using instantaneous radiative forcing (iRF) are largely irrelevant, since few if any observational studies use that measure of forcing. IPCC AR5 does not provide estimates of iRF either, preferring the effective radiative forcing (ERF) measure. Moreover, Marvel et al.’s iRF efficacy estimates use a regression model under which a zero forcing may, unphysically, have a materially non-zero effect on temperature. In some cases, requiring zero forcing to have no effect on GMST radically changes the estimated efficacies.

The efficacy estimates scale with the forcing arising from a doubling of CO2 concentration (F2xCO2). Marvel et al. use the RF value of F2xCO2, 4.1 W/m2, as its iRF value and hence in calculating iRF efficacies, and they imply that they use the same value for the ERF F2xCO2 and for ERF efficacies. No value appears to have been published for either iRF or ERF F2xCO2 in GISS-E2. In GISS-E, iRF F2xCO2 was 10% higher than the RF F2xCO2 value. Were the same true in GISS-E2, all the iRF efficacy, TCR and ECS estimates calculated from Marvel et al.’s data would need to be increased pro rata. Likewise, there are grounds for thinking that the true ERF F2xCO2 value is ~10% higher than the one they used. Without accurately established iRF and ERF values for F2xCO2, efficacy estimates can have little credibility.

On the basis of the indicated ERF F2xCO2 of 4.1 W/m2, all ERF efficacy estimates given in the Marvel et al. paper disagree with those I calculate using their data. Moreover, the climate sensitivity (TCR and ECS) estimates that they give using ERF appear to be inconsistent with both their data and their ERF efficacy estimates.

Using better justified estimation methods, and the GISS-E2-R effective rather than equilibrium climate sensitivity, the Historical iRF and ERF data are both found to produce efficacies within 10% or so of unity, both when using Marvel et al.’s estimates of the forcing from a doubling of CO2 and with them adjusted up by 10%. This indicates no material bias in climate sensitivity estimation as a result of forcings that were important over the last 100–150 years having differing efficacies from CO2.

Marvel et al.’s calculations of TCR and ECS estimates for the three observational studies cited contain multiple errors. They are also conceptually wrong in the case of Otto et al. 2013, since the underlying forcing estimates used in that study already reflect efficacies.

The methodological deficiencies in and multiple errors made by Marvel et al., the disagreements of some of its forcing estimates with those given elsewhere for the same model, and the conflicts between the Marvel et al. findings and those by others – most notably by James Hansen using the previous GISS model, mean that its conclusions have no credibility.



[1] The ECS estimate also substantially exceeds the GISS-E2-R model’s equilibrium climate sensitivity figure of 2.3°C.

Read more in the full article at Climate Audit

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January 10, 2016 7:52 am

Different agents may have effects on global temperature (GMST) different to those which would be expected simply by reference to the………
…totally fake made up temperature history
The computer games will constantly predict a fake future…….when they are fed a fake pastcomment image

Reply to  Latitude
January 10, 2016 8:45 am

Interesting, what two versions of USHCN were you using?

Reply to  tomcourt
January 10, 2016 9:03 am

take the spaces out of the last word…. r a u d/

george e. smith
Reply to  Latitude
January 11, 2016 10:03 am

Sounds like something Captain Marvel would come up with !!

January 10, 2016 8:21 am

It still amazes me that we are all living inside the experiment, yet can’t measure the temperature.

Reply to  u.k(us)
January 11, 2016 4:54 am

Oh, we can, but when we point out what’s really going on with the world they scream that we’re “denying the science.”

Curious George
January 10, 2016 8:25 am

I thought the science had been settled long ago. RICO on any dissenter!

Gunga Din
Reply to  Curious George
January 10, 2016 12:27 pm

Pride and politics say the “science is settled”.
Ma’ Gaia and genuine and honest scientist know it never is.
(At best, they know they can only get a better grip on the handle of something that is bigger than they are.)

Antti Naali
January 10, 2016 8:38 am

IPCC still uses the same estimate for CS than 20 years ago. What else could that mean than they have found no scientific evidence to make ECS more accurate. All this despite hundreds of billons of dollars invested in climate science. And ECS is for climate science like speed of light is for physics. There is absolutely no way of making any relevant prediction of future climate state if the most important factor ECS is not known.

January 10, 2016 9:03 am

The commonly admitted value for the primary forcing F2xCO2 as estimated by Myhre (1998) is 3.71 W m-2.
Why general circulation models amplify the temperature response to this forcing while all indicators of the system feedbacks (λ) are negative, thus attenuating the primary effect.
And why is IPCC describing ECS =F2xCO2/α (page 920 of WG1 AR5), where alpha (define as α = -λ the sum of all feedbacks) can well be close to zero? An impossible model is therefore used, that leads to impossible results!
Why so ? I never got an answer?

Jeff Alberts
January 10, 2016 9:21 am

“Different agents may have effects on global temperature (GMST) ”
Only problem with that statement, there is no global temperature.

Reply to  Jeff Alberts
January 10, 2016 10:51 am

Unlike Venus.

DD More
January 10, 2016 6:57 pm

So now we know what Marvel says, what does DC Comics have as a reply? They both get to draw anything they want.

Reply to  DD More
January 10, 2016 7:23 pm

I’m so glad you went there DD More. I was going to….

January 10, 2016 10:40 pm

“Marvel et al. estimate forcing efficacies from simulations in which the climate is forced by just a single forcing at a time.”
Most of the adsorption bands of CO2 are already saturated by water vapor. If they removed all the other forcings, did they remove the water vapor as well? If so, no wonder CO2 became a much more effective greenhouse gas in isolation…
If they didn’t remove the water vapor, what’s stopping them from doing so in the next paper and jacking the CO2 effect through the roof, just to grab a headline?

January 11, 2016 3:41 am

Marvel comics – a nonsensical study – TCS and ECS are much lower, if they exist at all
CO2 LAGS atmospheric temperature at all measured time scales.
Regards, Allan

george e. smith
Reply to  Allan MacRae
January 11, 2016 10:10 am

When atmospheric CO2 abundance and global surface temperature go in opposite directions, that is definitive incontrovertible proof that they are NOT logarithmically related.

Reply to  george e. smith
January 11, 2016 10:16 am

Or really, related in any measurable way at all.

johann wundersamer
January 20, 2016 6:30 pm

(the book) 1815:
many soldiers of the defeated napoleon army fleeing dragged with them the spoils
from the noble houses in Moscow. Silverware, golden
chandeliers. They died of exhaustion, hairline cracks in the bones, frozen, starved. They could not leave the prey, their effort would have been futile.
These researchers have lied so much. The respective final model shall seem true by clearly standing out from its predecessors.
Regards – Hans

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