The low-down on aerosols

Updated on Jan 26, 2013 by Bishop Hill

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:

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Lewis on Schmidt on climate sensitivity

This is a guest posting by Nic Lewis. Nic has cross posted this to the comments at RC, with the normal style of response from Schmidt.

Gavin Schmidt

I am glad to see that my input into the Wall Street Journal op-ed pages has prompted a piece on climate sensitivity at RealClimate. I think that some comment on my energy balance based climate sensitivity estimate of 1.6–1.7°C (details at http://www.webcitation.org/6DNLRIeJH), which underpinned Matt Ridley's WSJ op-ed, would have been relevant and of interest.

You refer to the recent papers examining the transient constraint, and say "The most thorough is Aldrin et al (2012). … Aldrin et al produce a number of (explicitly Bayesian) estimates, their ‘main’ one with a range of 1.2°C to 3.5°C (mean 2.0°C) which assumes exactly zero indirect aerosol effects, and possibly a more realistic sensitivity test including a small Aerosol Indirect Effect of 1.2–4.8°C (mean 2.5°C)."

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Ridley response to Romm

This is a guest post by Matt Ridley.

Joe Romm of ThinkProgress described my Wall Street Journal op-ed as:

riddled with basic math and science errors

Yet he fails to find a single basic math or science error in my piece.

He says I :

can’t do simple math

and then fails to produce a single example of my failing to do simple math.

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Questions and non-rebuttals

The antics of the upholders of the climate orthodoxy are becoming truly hilarious. In response to Nic Lewis's findings about the contradictions and failures of the draft IPCC report, we have now had no fewer than three "rebuttals" (Desmog, Media Matters, Think Progress) none of which link to the actual article. In fact none of them even mentions Lewis, preferring instead to concentrate on Matt Ridley's WSJ op-ed.

It does rather tell a story.

I think we can say that there is a consensus that the IPCC's models don't include their latest best estimates of aerosol forcings and that the empirical evidence suggests that that climate sensitivity is low.

There have been some scientific objections to Lewis's paper however and it's worth pointing these out.

BBD, commenting at Keith Kloor's blog, is querying the claim that the IPCC places little weight on LGM studies because of the huge uncertainties.

Lewis references this claim to AR4 WG1 Box 10.2, and I can't find anything like that in the referenced text.

And on Twitter, we have the observation that Aldrin et al also considered a scenario in which the cloud lifetime effect was considered and that this raised the sensitivity. The tweet says it raised it to 3.3, but this is the mean rather than the most-likely value, which is still around the 2 mark. The cloud lifetime effect is probably worthy of a blog post in its own right.

That said, these last two observations seem at least to be addressing Lewis's arguments and are worthy of follow-up.

Not waving but drowning

The Climate Science Rapid Rebuttal Unit has finally issued its response to the Climate Sensitivity is Low articles - or at least to Matt Ridley's Op-Ed. Nic Lewis's article barely gets a mention. 

At first glance, they are struggling to keep their heads above water.

[Updated to direct link to main site rather than mobile version]

Update on Dec 20, 2012 by Bishop Hill

From Nic Lewis's article:

Table 8.7 shows that the best estimate for total aerosol RF (RF_ari+aci) has fallen from −1.2 W/m² to −0.7 W/m² since AR4, largely due to a reduction in RF_aci, the uncertainty band for which has also been hugely reduced. It gives a higher figure, −0.9 W/m², for AF_ari+aci. However, −0.9 W/m² is not what the observations indicate: it is a composite of observational, GCM-simulation/aerosol model derived, and inverse estimates.

From the Media Matters article.

With regard to aerosols: Abraham told Media Matters: "it is very clear [they] have a cooling impact," adding, "I don't know of any reputable scientist that would dispute that." Boston University's Robert Kaufmann, lead author of a 2011 sulfur emissions study, agreed:

I know of no evidence that would suggest that the temperature effect of sulfur emissions are small.  This conclusion is totally at odds with my peer reviewed publication in the area, which indicate that sulfur emissions have a significant effect on temperature.

Update on Dec 20, 2012 by Bishop Hill

It's now up on Think Progress. No mention of Lewis.

Stranger and stranger.

Reactions to low climate sensitivity

Apart from the Judith Curry piece, which I linked to yesterday, there have been a few others interested in what Lewis has to say.

Tim Worstall covered the story twice - once at his own blog, where the comments thread was quickly overwhelmed by the rantings and ravings of a well-known green, and once at Forbes.

Meanwhile, at Reason magazine, Ronald Bailey gave the story another airing.

My impression is that people are still trying to work out what to say. I think we will hear more in the coming days.

Update on Dec 20, 2012 by Bishop Hill

James Annan, a specialist in climate sensitivity, says this:

I think a lot of what Nic Lewis says seems reasonable, though I also suspect that some of his choices will have served to underestimate sensitivity somewhat. Don't forget, "the ipcc" does no research to estimate sensitivity, they only summarise the literature which generally lags the latest evidence.

Why doesn't the AR5 SOD's climate sensitivity range reflect its new aerosol estimates?

Updated on Dec 19, 2012 by Bishop Hill

This is a guest post by Nic Lewis. Please note that comments will be tightly moderated for tone and relevance.

There has been much discussion on climate blogs of the leaked IPCC AR5 Working Group 1 Second Order Draft (SOD). Now that the SOD is freely available, I can refer to the contents of the leaked documents without breaching confidentiality restrictions.

I consider the most significant – but largely overlooked – revelation to be the substantial reduction since AR4 in estimates of aerosol forcing and uncertainty therein. This reduction has major implications for equilibrium climate sensitivity (ECS). ECS can be estimated using a heat balance approach – comparing the change in global temperature between two periods with the corresponding change in forcing, net of the change in global radiative imbalance. That imbalance is very largely represented by ocean heat uptake (OHU).

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+++Climate sensitivity is low+++

Matt Ridley has an article in the Wall Street Journal which is of incalculable importance.

Mr. Lewis tells me that the latest observational estimates of the effect of aerosols (such as sulfurous particles from coal smoke) find that they have much less cooling effect than thought when the last IPCC report was written. The rate at which the ocean is absorbing greenhouse-gas-induced warming is also now known to be fairly modest. In other words, the two excuses used to explain away the slow, mild warming we have actually experienced—culminating in a standstill in which global temperatures are no higher than they were 16 years ago—no longer work.

In short: We can now estimate, based on observations, how sensitive the temperature is to carbon dioxide. We do not need to rely heavily on ­unproven models. Comparing the trend in global temperature over the past 100-150 years with the change in “radiative forcing” (heating or cooling power) from carbon dioxide, aerosols and other sources, minus ocean heat uptake, can now give a good estimate of climate sensitivity.

The conclusion—taking the best observational estimates of the change in decadal-average global temperature between 1871-80 and 2002-11, and of the corresponding changes in forcing and ocean heat uptake—is this: A doubling of CO2 will lead to a warming of 1.6°-1.7°C (2.9°-3.1°F).

Warming at a snail's pace is a small problem, not a big one.

The Wall Street Journal is paywalled, unless you go via Google. Click here and then click on the first entry on the search results.

Murphy's paper

Updated on Dec 17, 2012 by Bishop Hill

One of the posts I've been pleased with in recent months was the one on Climate sensitivity and the Stern report. This was based on the premise that economic assessments of the cost of carbon ought to be based on the existing empirical measurements of climate sensitivity rather than the hypothetical estimates from climate models.

I reviewed the climate sensitivity estimates of the Fourth Assessment Report and noted that only Forster and Gregory 2006 was free of the influence of models. I then observed that once one had replaced the IPCC's fiddled figure for the Forster and Gregory estimate with the value those authors had actually calculated one could see that the range of estimates used in the the PAGE model (the one used by Stern) did not even include the range of Forster and Gregory.

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An error too embarrassing to correct

In the comments to the earlier thread, Nic Lewis left this comment about what happened when he discovered that the IPCC had altered the Forster and Gregory climate sensitivity findings. I thought it striking enough to warrant a posting of its own.

A recap for those unfamiliar with the story. My complaint about the alteration of the Forster & Gregory 2006 results was rejected on the grounds that it was done to put all the climate sensitivity probability density graphs on the same, uniform prior in sensitivity, basis. Justifying changing a result from a correct to an incorrect basis on the grounds that all the other results were given on that basis seems very dubious to me. But I knew that at least one of the other studies, Gregory 2002, actually had its results shown on the same basis as the original Forster & Gregory 2006 results, being a uniform prior in the climate feedback parameter - that is, a prior inversely proportional to the square of sensitivity. So my letter to Gabi Hegerl complained that the statement that the Gregory 2002 results were stated on a uniform prior in sensitivity basis was incorrect.

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Opening the can of worms

Nic Lewis is best known as the co-author of the O'Donnell paper on Antarctic temperatures, which underwent a famously concerted attempt to stifle it at birth from those on the consensus side of the global warming debate.

Nic's attentions have more recently turned to climate sensitivity, his most notable success being the uncovering of the IPCC's rewriting of the Forster and Gregory results to make them look more alarming - in my view one of the great scientific scandals of our time.

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Lucia on climate sensitivity

Lucia Liljegren's post on climate sensitivity written by Paul-K is rather technical but a must read for those that can handle such stuff. She's looking at a series of papers that have examined the effects of the Mount Pinatubo eruption on radiative fluxes at the top of the atmosphere, analyses that should lead, usually via comparison to a computer model, to an estimate of climate sensitivity.

So of these two papers, Wigley2006 offers a climate sensitivity but fails to reconcile to the measured flux data.  Soden2002 does seek to match the observed flux data but does not report on the implied climate sensitivity.

I am now sufficiently cynical about mainstream climate science to believe that the reason for the non-barking dogs in the two cases is the same.  I will show that you cannot get a reasonable match to the flux and temperature data with a high [equilibrium climate sensitivity].  Wigley would have had to show a serious mismatch with the flux data, and Soden would have had to report and explain the politically incorrect, low, climate sensitivity implied by his match of the flux data.

This seems very important to me.

Update: author name added 9.40am, 23.10.12.

The survey that wasn't

After my posting on climate sensitivity and policy attracted so much attention, I thought it would be interesting to see whether my arguments on the priority of empirical measurement over theory for policy purposes carried any weight with mainstream scientists.

I therefore set up a very short survey and emailed it around a bunch of mainstream scientists and a few journalists. The questions essentially sought a single value for climate sensitivity and a range of values that the respondents thought should inform policymakers.

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Asten 2012

A new discussion paper at Climate of the Past presents a new estimate of climate sensitivity from proxy records (H/T Bob Carter). The figure it comes up with is 1.1±0.4°C. This is in line with the Forster and Gregory estimate, and far below the IPCC's figure.

Climate sensitivity is a crucial parameter in global temperature modelling. An estimate is made at the time 33.4Ma using published high-resolution deep-sea temperature proxy obtained from foraminiferal δ¹⁸O records from DSDP site 744, combined with published 5 data for atmospheric partial pressure of CO2 (pCO2) from carbonate microfossils, where 11B provides a proxy for pCO2. The pCO2 data shows a pCO2 decrease accompanying the major cooling event of about 4 C from greenhouse conditions to icecap conditions following the Eocene-Oligocene boundary (33.7 My). During the cooling pCO2 fell from 1150 to 770 ppmv. The cooling event was followed by a rapid and huge 10 increase in pCO2 back to 1130 ppmv in the space of 50 000 yr. The large pCO2 increase was accompanied by a small deep-ocean temperature increase estimated as 0.59±0.063 C. Climate sensitivity estimated from the latter is 1.1±0.4°C (66% confidence) compared with the IPCC central value of 3 C.

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