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Radiative forcing by aerosol used as a wild card: NIPCC vs Lindzen

Posted on 22 February 2011 by Bart Verheggen

The greatest source of uncertainty in understanding climate change is arguably due to the role of aerosols and clouds. This uncertainty offers fertile ground for contrarians to imply that future global warming will be much less than commonly thought. However, some (e.g. Lindzen) do so by claiming that aerosol forcing is overestimated, while others (e.g. the NIPCC) by claiming that aerosol forcing is underestimated. Even so, they still arrive at the same conclusion…

Let’s have a look at their respective arguments. Below is a figure showing the radiative forcing from greenhouse gases and from aerosols, as compared to pre-industrial times. The solid red curve gives the net forcing from these two factors. The wide range of possible values is primarily due to the uncertainty in aerosol forcing (blue dotted line).

The greenhouse gas forcing (dashed red curve) is relatively well known, but the aerosol forcing (dashed blue curve) is not. The resulting net anthropogenic forcing (red solid curve) is not well constrained. The height of the curve gives the relative probability of the associated value, i.e. the net climate forcing is probably between 1 and 2 W/m2, but could be anywhere between 0 and 3 W/m2. (From IPCC, 2007, Fig 2.20)

NIPCC’s argument

The NIPCC report (a skeptical document, edited by Craig Idso and Fred Singer, made to resemble the IPCC report) says:

“The IPCC dramatically underestimates the total cooling effect of aerosols.”

They hypothesize that natural emissions of aerosol precursors will increase in a warming climate, causing a negative feedback so as to dampen the warming. Examples of such gaseous aerosol precursors are dimethyl sulfide (DMS) emitted by plankton or iodocompounds created by marine algae. They use these putative negative feedbacks to claim that

“model-derived sensitivity is too large and feedbacks in the climate system reduce it to values that are an order of magnitude smaller.”


These are intriguing processes (the “CLAW hypothesis” first got me interested in aerosols, when I assisted with DMS measurements on some remote Scottish islands), but their significance on a global scale is ambiguous and highly uncertain. As a review article about the DMS-climate link says:

“Determining the strength and even the direction, positive or negative, of the feedbacks in the CLAW hypothesis has proved one of the most challenging aspects of research into the role of the sulfur cycle on climate modification.”

The NIPCC report exaggerates the uncertainty in climate science, but seems to put a lot of faith in elusive and hardly quantified processes such as natural aerosol feedbacks coming to our rescue.

Lindzen’s argument

On to Lindzen:

“The greenhouse forcing from man made greenhouse gases is already about 86% of what one expects from a doubling of CO2 (…) which implies that we should already have seen much more warming than we have seen thus far (…).”

Lindzen again (E&E 2007):

“How then, can it be claimed that models are replicating the observed warming? Two matters are invoked.”

The “two matters” he refers to are aerosol cooling and thermal inertia from the oceans (a.k.a. “warming in the pipeline”). He then proceeds to argue that both of these factors are much smaller than generally thought, perhaps even zero. E.g. on aerosols, Lindzen writes:

“a recent paper by Ramanathan et al (2007) suggests that the warming effect of aerosols may dominate – implying that the sign of the aerosol effect is in question.”

By neglecting the importance of these two factors, Lindzen argues that the observed warming implies a small climate sensitivity.


While it is true that aerosols can warm and cool the climate (by absorption and reflection of solar radiation, respectively, besides influencing cloud properties), most evidence suggests that globally, cooling is dominant. Whereas Ramanthan et al (2007) don’t quantify the net aerosol effect (in contrast to Lindzen’s implicit claim), Ramanathan and Carmichael (2008) do. They estimate both the warming ánd cooling effects to be stronger than most other estimates, but the net forcing (-1.4 W/m2) is right in line with (even a little stronger than) the IPCC estimate (see the above figure). Taking into account realistic estimates of aerosol forcing and ocean thermal inertia, the earth has warmed as much as expected, within the admittedly rather large uncertainties. Ironically, it is exactly because aerosol forcing is so uncertain and because the climate hasn’t equilibrated yet that the observed warming since pre-industrial times is only a very weak constraint on climate sensitivity. Lindzen seems very certain of something that most scientists would readily admit is very uncertain.


So we have the peculiar situation that both of these approaches try to claim that climate sensitivity is small, but the NIPCC approach is to claim that aerosol forcing is very large (thus providing a negative feedback to warming), whereas the Lindzen approach is to claim that aerosol forcing is very small (thus necessitating a small sensitivity to explain the observed warming so far). Of course they can’t both be right, and probably neither of them are. Looking back at the figure above, both approaches are based on assuming that aerosol forcing is at the edge of the probability spectrum (as if it were some fudge factor), whereas the most likely value is somewhere in the mid range. Both approaches also ignore the other lines of evidence that point to climate sensitivity likely being in the range of 2 to 4.5 degrees. E.g. a value as small as suggested by the NIPCC (0.3 degrees) is entirely inconsistent with the paleo-climate record of substantial climate changes in the earth’ history. And finally, both approaches implicitly assign high confidence to some of the most uncertain aspects of climate science, even though they routinely mock climate science as if nothing is known at all.

Whereas the existing uncertainty in the science is sometimes put forward as an excuse to continue business as usual, such an approach invariably suffers from viewing this uncertainty going in one direction only: making the problem seem smaller. What about the other direction? What if the risks actually increase faster (in the latter direction) than they decrease (in the former direction)? Combined with the basic picture of where we know we're heading, this paints a rather different picture of what can be considered "prudent" (even if that's ultimately a subjective judgment).

Of course it is not mandatory for all those who dismiss mainstream climate science to agree, but to see two important “spokespeople” for climate contrarians take such mutually inconsistent approaches is peculiar. Even more so when you realize that Lindzen signed the recent "Prudent Path" letter to US Congress, in which the NIPCC report was approvingly cited… Most people can’t have it both ways, but apparently climate contrarians can.

See also a more detailed critique of the NIPCC and of Lindzen’s argument.

NOTE: This guest post was written by Bart Verheggen, who runs the 'My View on Climate Change' blog

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Comments 51 to 66 out of 66:

  1. No problem. I will post the links and 18 flaws tomorrow
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    Moderator Response: [DB] Please ensure that you post them on the most appropriate thread here at Skeptical Science and that you accompany them with the appropriate rationale as to why you think they support whatever it is position you are taking. Thanks!
  2. Chemist1... "The hockey stick has been debunked..." Well, now I know you're not serious. Good to know.
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  3. Chemist1... " the 3 rd law of thermodyamics has been ignored due to ignorance of the first and second" Doubly not serious.
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  4. Yes, the Hockey stick was a big mistake, which is why the IPCC discretely dropped it. When random data is fed into the process, and still comes up with a Hockey Stick, you have to be concerned. To be more specific, Mann's standardisation and short centring methodology produced a hockey stick bias.
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    Moderator Response: [Daniel Bailey] Incorrect. Or better to say: you've been misled. Whether PCA or non-PCA (discussed here), regardless of the dataset or proxy source used, what comes out is still a hockey stick, because that is what is in the data itself (the signal of the warming planet we live on). This has been discussed many times here at Skeptical Science, notable examples are here and here. The search function in the upper left corner of every page will show you many more posts on that matter, or almost any other climate-related subject you can think of. If you have questions along the way, please post them on the most appropriate thread for them & someone will get back to you fairly quickly. Thanks!
  5. Chemist, "No problem. I will post the links and 18 flaws tomorrow" Not on this thread please, unless they pertain to the post at hand. But I think that we have a very good idea where you are coming from and it doesn't reflect well on you or your credibility i'm afraid.
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  6. Dr. Verheggen, Do you have any thoughts on this paper by Knutti (2008) titled "Why are climate models reproducing the observed global surface warming so well? Knutti talks about aerosols quite a bit....
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  7. Can all comments about the HS be deleted please and reposted on the appropriate thread? Including this one, but i just wanted people to see this image from the 2007 IPCC Assessment.
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  8. chemist1 (#51), Laws of Thermodynamics
    The third law of thermodynamics states that if all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would occur.
    What relevance does a entropy-less state known as absolute zero have to temperature reconstructions, much less radiative forcing by aerosols?
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  9. Biblio to explain that would take a long time. I will merely explain why the first and second law are violated at a later date, but as promised I will provide the references regarding the IPCC flaws, and why the hockey stick is false. One step at a time folks.
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    Moderator Response: [DB] Sigh. All of those topics are covered by other threads. Use-the-search-function-to-find-the-most-appropriate-thread-and-post-on-them-there. Comments posted on those topics here will be deleted. Thanks in advance for your helping SkS keep a clean house!
  10. Chemist @60, You really do not have to feel like you have to talk posters through the science. I'm expecting G&T...... This thread is about Linden and others' getting the science of aerosol forcing wrong.
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  11. Albatross, 62: I an saying a Lindzen gets aerosols mostly right The claim I am making is the science supports Lindzen's main arguments. I am also stating some additional information, potentially expanding upon the aerosol discussion.
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  12. Sigh, moderator, sorry if I was not clear. I will post in the appropriate thread. I will say this though, these topics are interconnected and not so compartmentalized.
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    Moderator Response: [DB] You are welcome to then post a stub comment here with a link inviting interested readers to continue the discussion on those other threads. Most commenters here are well experienced at following thread jumps such as that.
  13. Chemist1, if you want to take things one step at a time, then why you keep posting new claims when you haven't still answered the responses to your old claims?
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  14. Albatross, The anti-correlation between climate sensitivity and aerosol forcing in climate models, as discussed by e.g. Knutti (2008) is interesting. His argument is that it makes sense to see such a correlation, as the observed temps are a major quantity that the models are supposed to simulate, so naturally, the models are optimized for such a simulation. It relates a bit to the extent which, and how models are "tuned" though. I'm not an expert on climate modeling, and find this a difficult topic to opine about. In the process modeling that I've done it makes sense to try to optimze the simulation within physically realistic boundaries of the tunable parameters. In a physics based model the similation is heavily constrained anyway (at least in my experience). You then chose the set of parameters that gives the best simulation. If different sets of paramaters give equally good results, the uncertainty of which parameter values are best increases. Note though that the indirect effect was missing from most modesl used in this study. An important point (as I also tried to make in this post) is that climate sensitivity is more constrained by other data than by the instrumental temp record, so this correlation is no reason to distrust the climate sensitivity estimates. Within the uncertainties of both cliamte sensitivity and aerosol forcing, the picture together with observed temp increase is coherent (see Dana's "case study" post on Lindzen as linked from this post).
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  15. Ari 64: To simply give some foreshadowing of where I am headed. Each piece, as you will soon see, is connected the previous piece. I will though make no more claims until these are discussed in the appropriate threads with the links as the mod suggested here, later today.
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  16. Thank you for your response @65 Dr. Verheggen.
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