Is climate sensitivity low or high?

Question

Several AGW climate skeptics claims that climate sensitivity is low. These include climate scientists Dr. Roy Spencer and Dr. Richard Lindzen. AGW proponents on the other hand claim that climate sensitivity is high. Their argument is summed up at skepticalscience.com here: http://www.skepticalscience.com/climate-sensitivity-advanced.htm

The following is an analysis of that skeptical science page (source: http://joannenova.com.au/2011/03/unskeptical-science-uses-unitless-fudge-factors/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+JoNova+%28JoNova%29"

It’s important to note that the surface temperature change is proportional to the sensitivity and radiative forcing (in W m-2), regardless of the source of the energy imbalance. The climate sensitivity to different radiative forcings differs depending on the efficacy of the forcing, but the climate is not significantly more sensitive to other radiative forcings besides greenhouse gases.

So sensitivity is all the same regardless of the forcing, but at the same time, it might be different thanks to an “efficacy” which means whatever you want it to mean, in order for everything to have the same sensitivity. Hmmmm, right I think I’ve got it. So what are the units of “efficacy”? Oh, it doesn’t have any units, it is unitless because all factors are scaled relative to CO2 forcing… except we have some of those forcings being solar radiation, others being a gas, and others as particulate matter. What sort of unitless scaling factor can relate particulate counts to solar radiation? Why, CO2 of course! Is that the LOG of CO2 vs the LOG of solar radiation (presumably giving an answer in decibels)? Maybe it is the small signal gain based on the first derivative about some arbitrary operating point? Who knows, who cares, just blurt some numbers on the table, no one is about to check any of this.

In other words, if you argue that the Earth has a low climate sensitivity to CO2, you are also arguing for a low climate sensitivity to other influences such as solar irradiance, orbital changes, and volcanic emissions.

Unless you happen to argue for different “efficacy” factors, in which case you get any result you feel like getting.

In fact, as shown in Figure 1, the climate is less sensitive to changes in solar activity than greenhouse gases.

So some forcings are more equal than others, makes sense. Solidarnosk comrade, we will outlast them.

Thus when arguing for low climate sensitivity, it becomes difficult to explain past climate changes. For example, between glacial and interglacial periods, the planet’s average temperature changes on the order of 6°C (more like 8-10°C in the Antarctic). If the climate sensitivity is low, for example due to increasing low-lying cloud cover reflecting more sunlight as a response to global warming, then how can these large past climate changes be explained?

Well, the easiest way to explain it would be that the system is nonlinear so there’s no reason to presume sensitivity is the same as it was during the last glaciation. But with all of these excellent “efficacy” fudge factors (all of which probably are also nonlinear) we could comfortably explain anything at all. Really. So let’s go over those units again :-)

People write learned papers about this $#!+ such as the following:

We use a global climate model to compare the effectiveness of many climate forcing agents for producing climate change. We find a substantial range in the “efficacy” of different forcings, where the efficacy is the global temperature response per unit forcing relative to the response to CO2 forcing.

You heard it, “per unit of forcing”. Under the IPCC system of units, forcing is a fundamental unit and well established property of all matter (a bit like mass, but only special people can measure it). The units of forcing are CO2’s. By gum, what I can’t understand is why I ever sat through high school science.


It seems to me that this use of the term "efficacy" is really meaningless and all that is being said here is "I believe climate sensitivity is high and you can't prove me wrong." Does this seem like good science to you?

Answer 1

>>>So sensitivity is all the same regardless of the forcing, but at the same time, it might be different



No, that is a straw man. What was said was that the response is proportional to the forcing from the mechanism, no matter what type of forcing you have. That does not equal "temperature response is the same to matter the forcing" - "same" and "proportional" are not synonyms, as Nova tries to imply is said.



T_eq = [lambda]∆F



This is not disputed amongst anyone. However, the lambda value is NOT CONSISTENT BETWEEN MECHANISMS. This is what she does not understand, for whatever reason. If we accept:



[T_x] = [lambda_x]∆F

[T_y] = [lambda_y]∆F



Then:



[T_x] / [lambda_x] = [T_y] / [lambda_y]

[T_x] / [T_y] = [lambda_x] / [lambda_y]

[T_x] / [T_y] = A

[T_x] = A[T_y]



Since the lambdas have the same units, they can be related by a unitless scaling factor A. This is the efficacy.



You can compare the sensitivities of any two mechanisms, and apparently what the IPCC did was use [lambda_CO2] as [lambda_y] so that they could all be compared with each other. If the sensitivity to one mechanism is lower than CO2, then A will be lower than 1, and the temperature response will be lower than that expected from CO2.







Here's what the IPCC has to say about why A has any variation between mechanisms:



"The efficacy primarily depends on the spatial structure of the forcings and the way they project onto the various different feedback mechanisms ... Therefore, different patterns of RF and any nonlinearities in the forcing response relationship affects the efficacy ... Many of the studies presented in Figure 2.19 find that both the geographical and vertical distribution of the forcing can have the most significant effect on efficacy"



So, geographical and vertical distribution of the radiative forcing. This is why A varies.



This is also why Nova's argument against the implication about past climate sensitivity is wrong. Barring drastic changes in atmospheric opacity and circulation (a mechanism for which and evidence for which would have to be found/shown), or a major geographical shift in our planet (say, large-scale continental drift), climate sensitivity should not vary greatly at all through time.





Also, the efficacy ratings of any of these things has absolutely NOTHING to do with the sensitivity to CO2, since A will not change in value as [lambda_CO2] does. This is why Dana said that if [lambda_CO2] is low, that implies that everything else must be low, since the equation dictates it. Her arguments do not add up to the conclusion "CO2 sensitivity is low."



Her further rantings about logs and decibels only display that she has no clue what the math is behind it. It's a simple power one monomial!



These are rather simple concepts if you read the explanations and do the math. What is so hard for her to grasp?







"It seems to me that this use of the term "efficacy" is really meaningless"



Absolutely correct. The efficacy rating of anything is completely meaningless in regards to the debate of what [lambda_CO2] is.







Rio: can you elaborate? The way I understand it is that if you want to relate two variables together then you have to eliminate the third by assuming it is constant between the equations, then solving the equations for that variable, then setting the equations equal to each other. Since we're talking about the difference between the climatic (temperature) sensitivities between mechanisms due to the same forcing, it is only natural that I cancel out the ∆F as that allows me to relate T_eq to lambda.



If we were talking about finding the sensitivity of only one variable, then



[lambda] = [T_eq] / ∆F



and we solve it experimentally.



I'm not sure if you're agreeing with me or disagreeing with me, you were somewhat unclear.

Answer 2

I'm not sure what you're arguing here -- efficacy is just a scaling factor. The IPCC link given on the SkS page describes it simply as the ratio between the CS parameter of one forcing agent to that of the CO2 CS parameter. It is calculated *after* determining the CS from various forcing agents (including CO2).



Otherwise nova seems to be tackling a few strawmen and talking nonsense.

---------

I suppose it's *possible* that CS was different during the last glaciation, but there is absolutely no evidence of this, and to assume it has changed so dramatically as to reduce the current sensitivity to <1 C is more than a little ridiculous. The fact that glacial/interglacial transitions are NOT the only observational evidence pointing towards a sensitivity in the IPCC range doesn't help the idea either.

Answer 3

Small changes can make a big difference. Chaos best describes our atmosphere and climate and when things are chaotic they are easily influenced To every Action there is a Reaction and the Result will always be change of some sort So YES the atmosphere is very sensitive to man's actions and is very EFFICIENT at changing

Answer 4

As I think I showed quite convincingly on the Skeptical Science page you linked, climate sensitivity is around 3°C for a doubling of atmospheric CO2. There are many, many lines of evidence supporting this, from empirical paleoclimate data to empirical modern data, to climate model runs. There's about a 90% certainty that it's between 1.5 and 4.5°C for 2xCO2.



That certainly can't be classified as "low". Whether you want to classify it as "high" is subjective.



I could only laugh when I read the jo nova page. *Gasp* the efficacy coefficient is unitless! OMG! That "rebuttal" was nothing more than an ignorant rant. I can't even respond to it because the author didn't say anything of substance whatsoever.

Answer 5

Well! without a benchmark representing climate as a whole its up for grabs. Its the same old argument since day one. Actually I do think its good science...but without foundation.



@ AMP yeah sure you can solve for everything but the deltas. Basic algebra.



@ Amp your ahead of me in non type cal, but I'd average the variables for distinction/extremes.