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The low-down on aerosols

The leak of the draft of the IPCC's Fifth Assessment Report before Christmas has had perhaps rather less of an impact than might have been expected, but the rock that Alec Rawls' chucked in the IPCC's pond was a large one and not all of the ripples have reached the shore yet. Here's a slightly unexpected one.

One of the immediate responses to the leak came from New Scientist, a bastion of climatological rectitude and stern upholder of the IPCC consensus. The article, by Michael Marshall and Fred Pearce, discussed what the IPCC report "really said", much of it being a lengthy attempt to rebut Rawls' case that the IPCC was acknowledging a much greater role for solar activity. However, the close of the article shifted to other aspects of the report:

Other conclusions are also more sobering. The IPCC is predicting greater sea level rise than it did in 2007, as it now includes models of ice sheet movements. And we now have a gloomier picture of the extent to which smogs and other human-made aerosols in the atmosphere shade us from the worst of global warming. This is still a big uncertainty in temperature forecasting. The draft says their cooling effect is 40 per cent less than thought in 2007, suggesting this positive side effect of air pollution has been overstated.

As BH readers no doubt know, the relatively slow pace of warming that the earth has exhibited since the start of the industrial revolution could have one of two explanations. The higher climate sensitivity argument is that the Earth tends to warm quickly in response to greenhouse gas emissions but that this warming has been partially countered by so-called aerosols - soot and chemical droplets released into the atmosphere by industrial activity. It is said that as industry cleans its act up, levels of aerosols in the atmosphere will fall and their damping down of the greenhouse warming will decline too, leading to rapid temperature rises. The alternative explanation - the lower sensitivity case - is that a relatively modest warming in response to greenhouse gases is countered by only a modest aerosol effect. The effect of a cleaner atmosphere in this latter case would be only a relatively small warming.

Perceptive readers might now be able to see that Pearce and Marshall have got their arguments completely back to front, as Nic Lewis tried to explain in a letter to the editor at the start of the year:


Michael Marshall and Fred Pearce (22/29 December issue, page 8, “Sceptics misuse leaked IPCC report”) make several fair points, but they have got completely the wrong end of the stick in saying that we now have a gloomier picture of the extent to which human-made aerosols reduce global warming, being 40% less than thought in 2007. This is in fact excellent news.

Smoothed global temperatures have been virtually unchanged since 2007, while greenhouse gas warming net of the latest, well constrained, estimates of the offsetting effects of aerosols and ocean heat uptake is over 60% higher that it was thought to be in 2007. This implies that the climate system must be much less sensitive to greenhouse gas warming than previously thought: a doubling of carbon dioxide concentration can be expected to cause the mean global temperature ultimately to rise by 1.5-2°C (a detailed calculation is available at That is much less than the central estimate of climate sensitivity per the 2007 IPCC report of 3°C.

While lower climate sensitivity doesn’t mean a continuing unchecked rise in carbon dioxide emissions is safe, it is certainly good news."

In other words, the finding that aerosols have a smaller-than-expected shading effect makes the higher-sensitivity/higher-aerosol explanation for the slow warming much less plausible than the lower-sensitivity/lower-aerosol shading case. With the continuing rise in greenhouse gases producing no apparent rise in temperatures in recent years, the case for low climate sensitivity only looks stronger. This is unequivocally good news.


Perhaps predictably, New Scientist did not publish Lewis's letter, which was a pity, since the view that we should be welcoming the new assessment of aerosols is not restricted to sceptics: Piers Forster of Leeds University has corresponded recently with Lewis and in one of his emails suggested that he was generally supportive of Lewis's position on aerosols (I am quoting with permission):

Your letter seems very apt, so I hope it is published. I agree that evidence is pointing towards small aerosol forcing and low sensitivity and that this is good news.

I don't think Forster would want to suggest that low sensitivity is a done deal yet, but it seems that there is at least a measure of cross-party agreement that this is the way the evidence is pointing at present. Good news indeed.

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Reader Comments (36)

climate sensitivity, if it actually exists outside the agenda driven world of the IPCC, is most likely not a constant.

Jan 24, 2013 at 8:55 AM | Unregistered CommenterN.Tropywins

Bish -

I suggest that your post will be upsetting to viewers of a sensitive nature :-/

Jan 24, 2013 at 9:00 AM | Unregistered Commenterssat

Yes, it is quite extraordinary that the implications of this are taking so long to sink in in the mainstream media.

Its like we are trying to figure out the flow rate of a pipe by looking at how fast a container is filling. Suddenly we realise there is another big pipe feeding it, there are two pipes, not one, so the conclusion is the first one's flow must actually be rather small.

Takes a while I guess.

Jan 24, 2013 at 9:20 AM | Unregistered Commentermichel

Good analogy michel; and we also discover that there is an outlet pipe (or two) at the bottom of the container.

Jan 24, 2013 at 9:26 AM | Registered CommenterPhillip Bratby

A case of "I agree with Nic"?

Jan 24, 2013 at 9:47 AM | Registered CommenterJonathan Jones

Good old Fred. Can always be relied on to stir things up. Wittingly or otherwise. :-)

Jan 24, 2013 at 9:51 AM | Unregistered Commentertallbloke

I have been using a similar analogy, of filling a bath, for years, to explain in simple terms to the uninitiated why catastrophilia is essentially nonsense.

CO2 emissions reduction is analogous to observing a bath into which the tap is running. You have no idea what effect this will have because you don't know the rate of flow in, whether the plug is in, where the overflow is situated, or even what the current depth of water is; but you just assume the bath will eventually overflow, so you start baling water from it, by the teaspoon.

Jan 24, 2013 at 9:53 AM | Unregistered CommenterJustice4Rinka

[Snip - let's keep this thread calm please]

Jan 24, 2013 at 10:06 AM | Unregistered CommenterDon Keiller

J4R, don’t forget that you don't know what the bath is made of either, so it is quite possible that the sides can deform with extra water pressure without the surface level increasing significantly.

Jan 24, 2013 at 10:11 AM | Registered Commentersteve ta

The scare about CO2 has obviously suited so many people for personal, political, and financial reasons, e.g. BBC environment reporters, and many more seem now to hold this dramatic view of CO2 as part and parcel of being progressive and caring, e.g. the BBC management, that the reduction of their mountain into a molehill will not be at all welcome. We rightly look with disdain at the ill-informed but verbose psychoanalysts on the previous thread, but there is more substantial work they could be doing if only they would take a deeper look into the astonishing rate at which a flimsy hypothesis took the world by storm. So to speak.

Jan 24, 2013 at 10:17 AM | Unregistered CommenterJohn Shade

It occurred to me that the same argument applied to the "it's all going into the oceans" argument.
This must have implications for "climate sensitivity".
Unless you believe (as SS does) that if it all resurfaced it would raise the earth's temperature to 35 degrees.
But even they have the grace to add that "this is impossible"

Jan 24, 2013 at 10:18 AM | Unregistered CommenterGeoff Cruickshank

Why is the editor of the New Scientist hell-bent on damaging the reputation of the magazine? (I won't refer to it as a "journal" because that word tends to be associated with academic journals whereas publications like the New Scientist and Scientific American are, quite rightly, meant to be popular as well as informative).

Jan 24, 2013 at 10:30 AM | Unregistered CommenterRoy

We should be fair to Marshall & Pearce for acknowledging that shortly after the AR5 leak they drew attention toward 'much more interesting and surprising conclusions,' including a retreat from alarmism over droughts, over cyclones and over the collapse of the gulf stream.

And we should be fair to Pearce that right from the beginning he has not been afraid of revealing the dark side of this controversy and in ways that embarrass the alarmists.

On the first of April 1995, he broke the Price of Life controversy, which exacerbate the Rich/Poor divide at the first CoP in Berlin (and embroiling a young Richard Tol). After Climategate he did the hard work in analyzing the controversy and issuing reports, blogs and a book that were not entirely one-sided. This even included a retrospective criticism of Ben Santer involvement in report doctoring and exclusion of dissent -- which was so strong it elicited a powerful defence on Real Climate from the otherwise reticent Santer.

However, Pearce has his blind spots...and this one over aerosols is most curious. Listen to him:
"The draft says their cooling effect is 40 per cent less than thought in 2007, suggesting this positive side effect of air pollution has been overstated."

His interpretation of this adjustment is all the more curious given the history...

Back in the early 1990s Pearce went into bat with a whole league of communicators (including Bolin) to prepare the governments and the public for the introduction of an aerosol dampener into the model modelling of the past...and then in the modeling of the business as usual future scenarios.

In order to include aerosols in the models so as to replicate the temperature record they had absurdly pathetic aerosol data to go by - all they had was rough estimates of emissions and their locations, but no actually concentrations. And they had very little evidence of the magnitude (or sign!) of the impact.

Yet this move was a marvelous success for the models as they now accounted for the 1970s pause in the warming as itself anthropogenic -- caused by aerosol emissions before pollution controls in the west. Moreover, Santer used the regional aerosol impact to explain why mid-latitude north had not warmed, and to thereby find the fingerprint of man. The trouble was that to be consistent they had to also modify the predictions. And the predictive part of SAR was slow to change.

The trouble was that in the drafting of SAR it was like the forecast guys were not told or not listening that they had to pull their heads in a bit in order to for the detection and attribution guys reach a hard won beach-head..the famous 'discernible human influence.'

In the first round of USA official comments on the SAR draft and summary, Rob Watson gently encouraged that this be sorted out. In the second round of comments (supposedly on the Summary only) he gives a real blasting (restated in the off-quoted state dept cover letter) says that the Report should be changed to sort this out - even though it was clearly against the rules to do so. Predictions of future warming needed to be dampened, and charts of model results that did not include the aerosol dampening should be removed. This problem was not entirely fixed and contradictions remain in the published Report and Summary.

Now, 20 years since the aerosol fix was first introduced into the model results (in 1993-4), we have the opportunity of 2 more decades of empirical data -- triple the time they had since the warming recommenced after the 1970s pause. And this is 20 years to validate the models against empirical results. And all the talk now is about acknowledging another new (more or less) decade-long pause in the warming. And all Pearce can see is that aerosols cant save us...

...forgetting that it was aerosol cooling that had been saving the warming alarm all along.

Jan 24, 2013 at 10:32 AM | Unregistered CommenterBernieL

I had a nice email from Fred Pearce, who agrees that it's wrong.

Jan 24, 2013 at 10:34 AM | Registered CommenterBishop Hill

'The IPCC is predicting greater sea level rise than it did in 2007, as it now includes models of ice sheet movements.'

and the facts on the ground are ?

As ever model claims are given greater weight than reality because those claims support 'the cause ' while reality does not . Is climate 'science' an actual physical science at all , or is it just make up and guess game ?

Jan 24, 2013 at 10:59 AM | Unregistered CommenterKnR

Roy on 24, 2013 at 10:30 AM

Why is the editor of the New Scientist hell-bent on damaging the reputation of the magazine? (I won't refer to it as a "journal" because that word tends to be associated with academic journals whereas publications like the New Scientist and Scientific American are, quite rightly, meant to be popular as well as informative).

I read NS for decades -- the first ones were in the school library with four colour covers on pulp paper. Every Mess I belonged to either had it already or ordered it at my recommendation. When I retired my daughter gave me an annual subscription.

Then about four years ago I could take no more and, at my instruction, she cancelled the order. AGW was on every page, no critical thinking, no science, just propaganda on and on. The magazine was so poor that I couldn't, wouldn't read it even though it was free. I've been to look at John Cook's Skeptical Science blog and I get the same impression there, closed minds, true believers, damn fools.

It was so bad that they couldn't even get me to read it when it cost me nothing. I did write to them, but I doubt it made any impression -- closed minds, damn fools, true believers all. I've seen a few copies since and I can see no improvement.

Very sad. Nearly as sad as what has happened to the RS under Sir Paul Nurse's watch. And May's. And that astronomer chap.


Jan 24, 2013 at 11:26 AM | Unregistered CommenterJulian Flood

The ability of people such as Fred Pearce to acknowledge error is worthy of respect and to be welcomed.
It is certainly preferable to the Doctrine of Mannian Infallibility

Jan 24, 2013 at 11:27 AM | Unregistered CommenterAndyL

While lower climate sensitivity doesn’t mean a continuing unchecked rise in carbon dioxide emissions is safe, ... "

This is like buying into your opponent's argument so they allow you to argue about the number of angels dancing on the head of the pin.

Jan 24, 2013 at 11:41 AM | Registered Commentershub


I've just posted a similar comment regarding New Scientist in Unthreaded - their main sales pitch, according to this morning's email is:

Whether you're worried about increasing energy bills, rising sea levels or extreme weather and superstorms, New Scientist explores what you need to know about climate and our changing world.

No, [snip - raise the tone please], I'm a scientist because I want to find stuff out, [snip - venting]

Jan 24, 2013 at 11:44 AM | Registered Commenterflaxdoctor

They've been spraying the atmosphere with 'aerosols' for years. You just have to go outside on most days and look up. But no one seems to care.

Jan 24, 2013 at 12:02 PM | Unregistered Commentergeorge

John Shade "... the BBC management, that the reduction of their mountain into a molehill will not be at all welcome."

Yes, John. Perhaps they have reason to think that the BBC pension fund has invested in the carbon-trading market?

Jan 24, 2013 at 12:26 PM | Unregistered Commentermichael hart

Julian Flood's experience of the decline of New Scientist is similar to mine. It fell below the quality line of being worth the subscription some years ago (and the delivery time was too slow compared with appearance in the shops.)

I noticed the greatest decline into lower quality sensationalism just after the magazine was given a makeover/revamp. Maybe poor circulation figures triggered that, which is equally sad. The days of Daedalus and DREADCO are long gone, yet supposedly(?) serious articles suggest a high level of gullibility in authors or headline-writers.

On a slightly happier note, the letters page indicates that some quality people still read it. What fraction that is, I can only guess.

Jan 24, 2013 at 1:02 PM | Unregistered Commentermichael hart

BH says, "... the relatively slow pace of warming ... could have one of two explanations." Is it really a binary choice? Surely there are many possibilities in between your high and low sensitivity extremes?

Also, Lewis says, "Smoothed global temperatures have been virtually unchanged since 2007,...". He's talking of observable climate trends in a 5 year period - and you take him seriously?

Jan 24, 2013 at 1:14 PM | Unregistered CommenterBitBucket

Regarding the bathtub metaphor mentioned above...I've used a similar analogy of a lake which is being fed by a large river, with an equally large river emanating from it. [Natural energy flows.] Houses are built on the shore whose drains also enter the lake, raising the water level. [That's the GHG warming.] Some of the residents water their gardens, which naturally tends to reduce the water level. [Aerosols] Now try to estimate how fast the lake is rising over the years, given measurements from a single place, and irregular variations such as annual runoff rate from the surrounding mountains. Not to mention that we don't know how the downstream rate is affected by lake level -- in general, one expects a higher flow associated with higher levels, but that might be mitigated if erosion allows greater flow, or increased if silt restricts it. [Analogous to climate sensitivity feedback being zero, negative, or positive.]
The situation is that we first observed the water level closely during years of high runoff, when the lake level would have risen naturally, resulting in an over-estimate of the effect of the houses. When the lake didn't continue to rise (when inflow wasn't as high), rather than revising the house-effect, there were guesses that gardening increased to offset it. But eventually we will settle on the true state of affairs.

Jan 24, 2013 at 1:30 PM | Registered CommenterHaroldW

His Grace said:

I had a nice email from Fred Pearce, who agrees that it's wrong.

What is "it" ?

Jan 24, 2013 at 2:38 PM | Registered Commenterjferguson


The New Scientist article re aerosols.

Jan 24, 2013 at 3:17 PM | Registered CommenterBishop Hill

The rise in drug resistant infections is comparable to the threat of global warming, according to the chief medical officer for England.

Now at first glance, this appears to be standard journalistic scare tactics. But later in the same article we get this:

Prof Davies said: "It is clear that we might not ever see global warming, the apocalyptic scenario is that when I need a new hip in 20 years I'll die from a routine infection because we've run out of antibiotics."


Jan 24, 2013 at 3:26 PM | Unregistered Commentersteveta

So Pearce agrees that his article, or part of it, is wrong. What does he intend to do about it?

The big headline followed by quiet admission that "Oops, no, that's wrong" is so common in the CAGW buisness that I suspect it to be a deliberate tactic.

It's certainly a very old tactic of unscrupulous journalists; every reader sees the front page story but only a few see the tiny correction in the back pages of a later issue in among the adverts.

[BH adds: No, he has been the model of integrity here]

Jan 24, 2013 at 5:19 PM | Unregistered CommenterNW

Good to see that Fred Pearce has acknowledged the error. I think he's an excellent writer. He wrote a piece on the so-called "population bomb" a while ago for the Mail, which was particularly good, I thought. His stuff tends to be smart and fair, I'd say.

Jan 24, 2013 at 5:34 PM | Unregistered CommenterJames Evans

"The big headline followed by quiet admission that "Oops, no, that's wrong" is so common in the CAGW business that I suspect it to be a deliberate tactic."

It's also called hedging your bets. Some Climate-changeologists are increasingly realising that this might be a sensible policy.

They already know that, given the will, and despite many efforts at suppression, they can find contradictions in the primary scientific literature of their previously noisily-espoused views.

Jan 24, 2013 at 6:01 PM | Unregistered Commentermichael hart

As I haven’t yet seen a simple computation with the new data, I tried it by myself. My result is very close to Nic Lewis’ estimate. I also included the new black carbon forcing estimate which will further reduce CO2 sensitivity by about 20%. All computations are based on IPCC data.

The estimates including aerosols + black carbon are
Transient climate sensitivity : 0.83 K
Equilibrium climate sensitivity : 1.35 K

Transient sensitivity below AR4 “very likely” range.
Equilibrium sensitivity in AR4 “very unlikely” range.


CO2 Climate Sensitivity from Instrumental Temperature Record and based on IPCC Data

What is the best time span for an estimate ?


Why ?

1. CO2 increase from 1750-1945 was only 30 ppm (280-310 ppm) but took off after 1945.
2. Minimizes influence of PDO/AMO decadal oszillations, as they were in very similar phases at 1945 and 2005.

CO2 @ 1945: 310 ppm
CO2 @ 2005: 380 ppm

380/310 = 1.226

1.226^3.4 = 2.

Due to logarithmic temperature increase, temperature difference has then to be multiplied with 3.4 to compute sensitivity for CO2 doubling.

HadCrut global temperature increase 1945 –2005: 0.4 Kelvin

How to compute equilibrium sensitivity from transient climate sensitivity ?

Quote IPCC AR4
“Equilibrium climate sensitivity is likely to be in the range 2°C to 4.5°C with a most likely value of about 3°C, based upon multiple observational and modelling constraints. It is very unlikely to be less than 1.5°C. {8.6, 9.6, Box 10.2}
The transient climate response is better constrained than the equilibrium climate sensitivity. It is very likely larger than 1°C and very unlikely greater than 3°C. {10.5}“
Take quotient of mean values:
S(equilibrium) / S(transient) = 0.5*(4.5+2) / 0.5*(3+1) = 1.62

Sensitivity Estimates

IPCC AR4 Estimate

IPCC AR4 total net forcing was assumed to be about equal to CO2 forcing.

AR4 transient climate sensitivity : 0.4 K * 3.4 = 1.36 K
AR4 equilibrium climate sensitivity : 1.36 K * 1.62 = 2.2 K

IPCC AR5 Estimate

CO2 forcing only about 0.75 of total forcing due to reduced aerosol cooling.

AR5 transient climate sensitivity : 0.4 K * 3.4 * 0.75 = 1.02 K
AR5 equilibrium climate sensitivity : 1.02 K * 1.62 * 0.75 = 1.65 K

Remark: transient sensitivity at the edge of AR4 “very likely” range.

State of the Art Estimate

Due to black carbon forcing doubled, CO2 forcing is only about 61% of total forcing
(Black carbon forcing about doubled to about 1.1, add 0.55 to AR5 total forcing)

Transient climate sensitivity : 0.4 K * 3.4 * 0.61 = 0.83 K
Equilibrium climate sensitivity : 0.83 K * 1.62 * 0.61 = 1.35 K

Transient sensitivity below AR4 “very likely” range.
Equilibrium sensitivity in AR4 “very unlikely” range.

Possible further corrections:

Temperature increase 1945 –2005: 0.25 K instead of 0.4 K


Overwriting of SST raw data without proper reason after 1941 (increased SST warming from about 0.2 K to 0.3 K)
See “The new HadSST3 dataset still contains some seemingly arbitrary assumptions…” and subsequent text.

UHI warming not accounted for (about half of land warming since 1979 due to UHI / non-climatic contamination):

Best Transient climate sensitivity : 0.25 K * 3.4 * 0.61 = 0.52 K
Best Equilibrium climate sensitivity : 0.52 K * 1.62 * 0.61 = 0.84 K

Best estimate probably still too high due to missing solar amplifier consideration. IPCC forcing universe cannot explain Medieval, Roman, Minoan and other Warm Periods.

Jan 24, 2013 at 6:09 PM | Unregistered CommenterManfred

"My considered estimate for 2100 is at most one degree F --- based not on climate models but on the observational evidence."

Letter submitted to Raleigh (NC) News Observer (Dec. 24, 2004) by S Fred Singer.

(1 degF approx= 0.6 degC)

My money has always been on Singer's estimate being about right. He knows his atmospheric physics.

Jan 24, 2013 at 6:37 PM | Registered Commenterthinkings