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


(This is also featured as a guest post on Skeptical Science)

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 downplaying the importance of these two factors, Lindzen argues that the observed warming implies a small climate sensitivity. The same line of argument is also used by Dutch journalist Marcel Crok, who writes in his recent book (in Dutch, my translation):

aerosols probably cool much less than commonly thought


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) (quoted by Crok) 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.

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.

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22 Responses to “Radiative forcing by aerosol used as a wild card: NIPCC vs Lindzen”

  1. sharper00 Says:

    An important distinction with Fred Singer is that he’s repeatedly argued there’s no actual warming but rather temperatures have declined and been covered up. From the NIPCC section on urban heat islands

    “A second important reason comes from the realization that it would be extremely easy for a spurious warming of 0.12°C per decade to be introduced into the surface air temperature trend as a consequence of the worldwide intensification of the urban heat island effect that was likely driven by the world population increase that occurred in most of the
    places where surface air temperature measurements were made over the last two decades of the twentieth century”

  2. sharper00 Says:

    Oops clicked post by mistake!

    Also on the section on satelite data

    “The IPCC claims that data collected by satellitemounted microwave sounding units (MSU) and advanced MSU measurements since 1979 reveal a warming trend of 0.12º C to 0.19º C per decade, which it says “is broadly consistent with surface temperature trends” (IPCC, 2007-I, p. 237). This would be surprising, since we indicated in the previous section that the surface-based temperature record is unreliable and biased toward a spurious warming trend. In this section we investigate the truth
    of the IPCC’s claim in this regard and report other findings based on satellite data.”

    Hence the NIPCC report is actually trying to explain why the world is cooling (and at the same time why the data says its warming).

    Of course none of that will stop people referencing both Lindzen and Singer as if they agreed with each other and indeed it’s common to simultaneously attack the entire concept of consensus but then try and put together every single skeptical scientist as if they all agree with each other.

  3. Tom Fuller Says:

    Really, really good. Glad you’re finally writing about this type of subject.

    Small typo (in case this is to be published elsewhere): should read ‘probably neither of them are.’

  4. MapleLeaf Says:

    Great post Bart, exposes the logic (and scientific) flaws of arguments used by contrarians.

    In addition to the issues pointed out by Bart, this Nature paper exposes another major problem with the argument/hypothesis proposed by Idso and Singer et al. According to the NIPCC document:

    “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.”

    However, the paper linked to above finds that:

    “We observe declines [in phytoplankton] in eight out of ten ocean regions, and estimate a global rate of decline of ~1% of the global median per year.

    and their research suggests that:

    “whereas long-term declining trends are related to increasing sea surface temperatures. “

  5. Eli Rabett Says:

    It’s lawyer science.

    One of the hard things to convey is how important consistency is for science.

  6. Bart Says:

    Thanks, fixed the typo.

  7. Paul Kelly Says:


    “to see two important “spokespeople” for climate contrarians take such mutually inconsistent approaches is peculiar.”

    Why so? Aren’t both attempting to falsify the notion that the only thing left to explain recent temperatures is the increase in CO2 concentrations? It shouldn’t be surprising that researchers from different backgrounds and perspective choose different avenues to find that falsification.

  8. matt Says:

    I hadn’t encountered the NIPCC argument before, but Lindzen at least has half a point in that the aerosol feedback is highly uncertain. The climate models have a fairly large spread in sensitivity (See http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-6-2-3.html#table-8-2) which is explained in AR4 as being mainly due to different treatments of cloud feedback. However, AR4 also indicates that the models use very different values of aerosol forcing (http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-4-4-1.html#table-2-4). This SAP report attributes the variation in model climate sensitivity to variations in assumed sulphate forcing rather than clouds, and Lindzen doesn’t seem to be associated with it: http://downloads.climatescience.gov/sap/sap2-3/sap2-3-final-report-all.pdf

    From page 4 section ES 3.1

    “Despite a wide range of climate sensitivity (i.e. the amount of surface temperature increase due to a change in radiative forcing, such as an increase of CO2) exhibited by the models, they all yield a global average temperature change very similar to that observed over the past century. This agreement across models appears to be a consequence of the use of very different aerosol forcing values, which compensates for the range of climate sensitivity.”

    The GLORY satellite that was supposed to be launched yesterday will hopefully yield some more precise aerosol data. Launch was rescheduled for today: hope all goes well!

  9. Bart Says:


    The situation as I see it is opposite: Mainstream science acknowledges there is a large uncertainty in the net aerosol forcing, whereas Lindzen picks onevalue at the outer edge of the probability distribution function and builds his entire argument on that (rather improbable) value as if it’s highly certain: His argument is implicitly built on high confidence/certainty that aerosol forcing is very low.

  10. Bart Says:


    As I also wrote, there is no must for all critics to agree, but I find it peculiar that they arrive at the same conclusion via diametrically opposed (and mutually inconsistent) arguments. The only consistent part in their viewpoints is that the conclusion is always the same: “not the IPCC”.

    Plus of course the fact that Lindzen, by signing the “prudent path” letter, presumably would agree with its contents. That’s more than peculiar; that’s patently inconsistent.

  11. Steve Fitzpatrick Says:

    In about ten years both ocean heat accumulation and atmospheric aerosol effects should be much better defined (assuming that the Glory Mission finally launches successfully). That would seem a good point to make an estimate of the credible range of climate sensitivity. Until then, values for climate sensitivity strike me mainly as products of fear, personal values, and politics. Not very interesting, and impossible to narrow at present.

  12. Bart Says:


    Your statement reminds me of a Dutch saying hanging above the piano in many Dutch houses: “Tomorrow we’ll start saving” (morgen gaan we sparen). What a nice resolution, every day again!

    I’m sure someone made the exact same argument as you did ten years ago: “Let’s wait ten years before we start acting on our incomplete current knowledge; we’ll know much more then.”

    However, our current understanding of the range of likely climate sensitivity values isn’t much better than it was decades ago. Perhaps the estimate will be slightly better bounded in ten years; perhaps it isn’t. But why would that preclude going by what we know now? We know something after all, albeit imperfectly and surrounded with large uncertainties. Going by that seems to me a better idea than picking a highly improbable value and building one’s whole argument on that (as Lindzen and NIPCC do).

    And before you paint a scientific sub-field as being merely based on “fear, personal values, and politics”, perhaps you ought to make yourself familiar with said field before making ridiculous and insulting pronouncements. And if you are familiar with the field, then give your arguments without the smear. People may actually listen if you do.

    I’m open to hearing different opinions but not to insults. Enough of that already.

  13. Steve Fitzpatrick Says:

    No insult was intended. I do think that the NIPCC and maybe even Lindzen’s estimates of climate sensitivity are the result of “fear, personal values, and politics”. Don’t you? According to the graph you show above for net forcing, the credible sensitivity range is from extremely high (approaching infinite) to quite low. Without better data, what anyone thinks is the true value doesn’t make much difference to me.
    My associates in Holland all seem to be very frugal with their money. Perhaps I have not seen a representative sample.

  14. Bart Says:


    The different estimates of aerosol forcing don’t all have the same probability of being correct. Values of 0 or -3 W/m2 can perhaps not be excluded with 100% certainty, but they’re extremely unlikely nevertheless.

    Uncertainty is not the same as knowing nothing. (why does it always seem to come down to that?)

  15. Steve Fitzpatrick Says:

    “Values of 0 or -3 W/m2 can perhaps not be excluded with 100% certainty, but they’re extremely unlikely nevertheless.”

    For sure, but eye-ball estimating from your graph above, the 95% confidence range looks like ~0.6 watt/M^ to ~2.4 watts/M^2. That is a huge range, and only includes values that are not very unlikely. From “frightening” to “so what”.

  16. Bart Says:


    You seem to linearly translate aerosol forcing into a fear level of AGW. It’s not as simple as that, as I’m sure you’re aware. Neither is climate sensitivity one to one dependent on what the aerosol forcing is. The instrumental temperature record doesn’t rate very high as a constraint on climate sensitivity; there are other lines of evidence pointing to a certain range in climate sensitivity with much more stringency (esp when taken into acount simultaneously).

    The likely range in climate sensitivity (2-4.5 K) is imho akin to “If it’s bad, it’s really bad. If it’s good, it’s still pretty bad.” (though as I also said at Lucia’s, “bad”, as well as “frightening” and “so what” are subjective judgments).


    As an exception, I moved your question (and my answer) to the open thread. Let’s keep the discussion here focussed.

  17. Steve Fitzpatrick Says:

    “You seem to linearly translate aerosol forcing into a fear level of AGW.”

    Not at all. I also look at the best available estimates of current ocean heat accumulation (on the order of ~0.3 – 0.35 watt /M^2 over the last 5-6 years) and the best estimate of warming since pre-industrial times (~0.85C). Then I do a simple calculation: sensitivity in degrees per watt = ~0.85C /(net radiative forcing – ocean heat accumulation). If net forcing (from your graph) is 0.6 watt/M^2, then equilibrium sensitivity looks like ~2.8C per watt, or about 10.5C per doubling. If net forcing is 2.4 watts/M^2, then equilibrium sensitivity looks more like 0.41 C per watt, or about 1.54C per doubling. >10C per doubling, if it were true, would be frightening, at least for me. Similarly, 1.54C per doubling falls solidly in the “so what” sensitivity region, at least for me; maybe something to consider, but nothing alarming.
    Now if you are suggesting that these extremes are not realistic…that they are in fact both well outside the 95% probability window (and I think that is what you are saying), then should not the range of net forcing in your graph be much more narrow? If the credible net range forcing range really is much more narrow, then why does the IPCC AR4 suggest the full 95% range, just as shown on your graph?

    James has concluded that the climate sensitivity is 3C per doubling, plus or minus a bit, and I think you sent me to read that blog post once before. But if is so obvious to James, then why is there any remaining doubt? Bart, I am only looking for a rigorous, consistent, and fair evaluation of the data. For what it is worth: I am not at all impressed by James’ arguments.

  18. matt Says:

    “Lindzen picks one value at the outer edge of the probability distribution function and builds his entire argument on that (rather improbable) value as if it’s highly certain” Okay, I see where you’re coming from. Lindzen isn’t highlighting that the large uncertainty in aerosol effects is responsible for much of the uncertainty in climate sensitivity estimates: he’s making an unjustified claim that the aerosol negative forcing is small. Which it may be, but according to the IPCC probability curve, it’s unlikely. Fair enough.

  19. Bart Says:


    You’re right that the observed warming can be explained by a combination of three factors:
    - climate sensitivity
    - net radiatiave forcing
    - remaining energy imbalance / ocean heat accumulatio
    If you assume the value of 2 of these 3, the 3rd one can be calculated. All three have a large error margin though (where the absolute error margin is greatest in the first two). Within generally accepted error margins for one (two) of these terms, the error margin of the remaining term can not be further constrained: They pretty much are each other’s mirror image. Which means that we need other means to further constrain climate sensitivity and/or aerosol forcing.

    Over at SkS, Dana looked into this, taking error margins into account.

  20. Steve Fitzpatrick Says:


    Only time will tell. I am guessing 10 years will narrow the uncertainty quite a lot. If not, then we are wasting a huge amount of money of research. Perhaps it is better if we talk about this subject then… assuming I am still around.

  21. Climate News and Blog Recap – 2011 03 05 « The Whiteboard Says:

    [...] on the one of the two greatest sources of uncertainty (aside from hand-waving and wand-waving). Radiative forcing by aerosol used as a wild card: NIPCC vs Lindzen Verheggen @ Our Changing [...]

  22. cohenite Says:

    Just read your piece; shame you will have to rewrite it given Hansen’s recent paper:


    Whose side, Singer or Lindzen’s, does Hansen come down on?

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