To be certain sensitivity is still uncertain...

This is a follow-on from my earlier article about a new paper on climate sensitivity. It isn't a critique or analysis of the paper itself. Maybe other people will look at the work and how it differs from other sensitivity analysis. This is more about the reaction in the deniosphere with some of my meandering thoughts rolled in. I've been in two minds about whether to publish this because there are no firm conclusions. It's rather long-winded and a bit repetitive. But I don't feel like spending any more time editing it down. You can make of it what you will.

As you may know, the denialati are all over a new paper in Climate Dynamics by Nic Lewis and Judith Curry, who have taken a shot at estimating climate sensitivity. You can download a pre-publication pdf version here, together with supplementary information zipped here, courtesy Nic Lewis.

It's not at all odd that deniers would latch on to it when you think about it, even though the WUWT crowd variously "believe":

• Climate change isn't happening
• Climate change is always happening
• If the world is warming, it's not because of CO2 (ie climate sensitivity to CO2 is zero)
• There is no such thing as a greenhouse effect. The world is warmed by magic.
• The world is cooling
• We're heading for an ice age
• The increase in CO2 is not caused by humans (ie burning hydrocarbons doesn't release CO2)
• Atmospheric CO2 hasn't increased

One reason I say this isn't surprising is because even though the paper suggests the world is about to get a lot hotter, the authors argue a likelihood, or at least a possibility, that the surface may not get quite as hot, with a doubling of CO2, as a lot of other research suggests. This paper suggests that when CO2 hits 560 ppm later this century, Earth will at first be somewhere between 0.9°C and 2.5°C hotter than it was in the mid-1800s. And even if we stopped adding any more CO2 to the atmosphere at that point, temperatures would continue to rise, settling at between 1°C and 4°C hotter as the system approaches medium term equilibrium. None of which is by any means comforting. While these numbers extend almost to the same range as stated in the latest IPCC report (1.5°C to 4.5°C), it goes below the lower end of the IPCC most like range and doesn't hit the high end.

That's not quite an apples to apples comparison though. Lewis and Curry were looking at "effective" climate sensitivity which is not quite the same as equilibrium climate sensitivity. The difference as far as I can make out is as follows. "Effective" is on a centennial time scale whereas equilibrium is on a millenial time scale. "Equilibrium" refers to after the ice sheets stop melting and maybe even after the ocean circulation has stopped responding to climate change. The deep ocean circulation has a cycle of around a thousand years. Ice sheets can take several millenia to settle down after a climate forcing.

So all in all, the difference between Lewis and Curry and other estimates is one of a few tenths of a degree at most.

The authors say that they pit their results against research based on climate model studies and (though they don't say it explicitly) against most research based on knowledge of past climates (paleo-climate research). Lewis and Curry argue that the median effective climate sensitivity (before ice stops melting, for example) is 1.64°C, with a range of around 1°C to 4°C. (The IPCC AR5 WG1 report didn't provide any best estimate, it provided a range of 1.5°C to 4.5°C.)  The Lewis and Curry paper doesn't attempt to quantify equilibrium climate sensitivity (after the system has reached longer term equilibrium). I've written the Lewis/Curry estimates in °C, but the paper is in K.

Another reason it's not surprising that deniers have latched onto this paper is because deniers have a tendency to pick out one study or only similar studies while ignoring the rest. This is what WUWT has done. But one thing that isn't discussed much at WUWT, is the high end of the low end estimates. Many of the low end estimates listed at WUWT have a range that extends to 3°C or beyond. That's for ECS (variously defined). That's beyond what is considered "safe". It also means that if CO2 is quadrupled - gets up to 1120 ppm, they are talking about a possible rise in global surface temperature of six to eight degrees or more. And since deniers are not wanting to reduce CO2 emissions, if they had their way this would happen sooner rather than later.

On the other hand, if deniers have their way and the world warms rapidly over the next few decades then almost certainly there would be food shortages, civil unrest and probably regional and global wars. Economies would stagnate or go backwards. Therefore CO2 emissions would stop accelerating and would most likely start to fall. On top of that, there'd be lots of smog and other aerosols to temporarily cool the earth. In other words, humans will probably bring about a reduction in CO2 emissions no matter what. Most of us would prefer to plan to reduce CO2 emissions by shifting to clean energy so as to maintain social harmony, viable economies and protect democracy. Deniers would send us spinning into a downward ecological, economic and social spiral. Their route would result in a rapid disintegration of civilised society. Which would probably result in a drop in CO2 emissions, but at a huge cost.

Just how certain are deniers?

One thing I noticed is all the mixed messages about uncertainty, at the various denier blogs.

Over at ClimateAudit (archived here), Nic Lewis claims that "Considerable care was taken to allow for all relevant uncertainties". One may be forgiven for interpreting this as a claim that he regards the probability ranges the paper comes up with as robust if not the very last word on the subject.

In contrast, Nic's co-author, Judith Curry writes about their very same paper that:

Is this paper the last word on climate sensitivity estimates? No. The uncertainty analysis in the Lewis and Curry paper relates only to the uncertainty in external forcing, surface temperature and ocean heat uptake. There remains considerable meta uncertainty in the determination of climate sensitivity, including how the problem is even framed.

And at WUWT Pat'n Chip wrote:

The estimate is not perfect, as there are plenty of uncertainties, some of which may never be completely resolved. But, nevertheless, Lewis and Curry have generated  a very robust observation-based estimate of the equilibrium climate sensitivity. 

Going back to Judith's quote above, what she means by "meta uncertainty" is apparently this:

In particular, the energy balance approach does not account for factors that do not directly relate to the energy balance, e.g. solar indirect effects and natural internal variability that affects forcing (although an attempt has been made in the Lewis and Curry paper to make some allowance for uncertainty associated with these factors) . Further, there was ‘something else’ going on in the latter 19th and early-mid 20th century that was causing warming, that does not seem to relate directly to external forcing. The paper does attempt to factor out the impact of the Atlantic Multidecadal Oscillation through the selection of base and final periods, but this is by no means a complete account for the effects of multi-decadal and century scale internal variability, and how this confounds the energy balance estimate of climate sensitivity.

I'm not sure quite what Judith was getting at there. Maybe someone can take a stab at this in the comments. Is Judith talking about cosmic rays or Force X when she refers to "solar indirect effects"/:D And what is she referring to when she talks about "natural internal variability that affects forcing"? All this could be explained in the paper itself - I haven't gone back to check the detail.

As Judith suggests, their paper doesn't attempt to attribute cause to warming in the late 19th and early 20th century. It's not an attribution study. Others have looked at this, for example, Tett et al (2002) wrote:

Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability.

I'd be interested in more recent attribution papers about early twentieth century warming if anyone knows of any. Thanks to Anonymous in the comments, click here for some more papers on the subject.

Judith concludes by writing:

Resolving the reasons for differences between observational/energy balance estimates and GCM estimates of climate sensitivity is an issue of substantial importance. At this point, I find the estimates in the Lewis and Curry paper to be the most convincing estimates available to date.

Well, I'd hope she would find her and Nic's paper convincing. Otherwise why would Judith add her name to it. That doesn't mean it will convince everyone. But as I showed earlier, it's just another paper to add to the mix to give people something to think about. Its result is not all that different to other estimates of climate sensitivity. It's not a "bombshell" paper. It doesn't come up with anything earth shattering. It has sliced and diced the problem a different way to some other papers but came to not that different a result. All of which shows how difficult it is to get a precise number for climate sensitivity. Even when CO2 is doubled we won't know for sure what TCR is to one decimal place. That would require being able to separate the noise of natural variation, seasons, long and medium term oscillations etc from the signal of global warming.

So there you have it. Lots of mixed messages about certainty, uncertainty and "robustness" of climate sensitivity estimates.

A new direction for Judith Curry

AFAIK, this is the first time Judith has tackled the subject of climate sensitivity. That she would do so with an amateur scientist (not trained in climate science and never having worked as a climate scientist) could be considered consistent with her shift to Lindzen-style contrarianism, which blog-watchers have observed over the past couple of years. Perhaps a distrust of mainstream scientists, or maybe it's as simple as Nic approaching Judith so that he could add the name of a professional scientist to the paper while at the same time sending a message to the deniosphere. For example, it would be difficult for Nic to find any other co-author who would be comfortable being promoted by the GWPF.  Judith Curry has already shown she's quite comfortable being associated with the GWPF, writing a foreword to one of their other publications, and is more than comfortable being associated with other denier lobby groups. If Nic had co-authored with one of his previous co-authors, he might have been considered to be going "mainstream" rather than contrarian. (It's not clear what Judith contributed to the paper. None of the blogs, nor the published version indicates who did what.)

I don't view the Lewis/Curry paper as a contrarian paper. Reading it, the only hints that it is intended as such could be statements like this: "This approach avoids to a substantial extent the dependence on AOGCM simulations in previous energy budget studies", which is consistent with Judith Curry's well-known antipathy toward climate models. (Or coupled climate models. She probably uses weather forecasting models in her consultancy business, which would not be dissimilar to climate models.) But that's only if you go looking for evidence of contrarianism. Quote-mining of that sort will allow a person to interpret stuff however they want to. It's an example of how confirmation bias works. I prefer to take the paper on its merits rather than read into it what may or may not be there. All it's really saying is that this paper is based more on observations than on modeled climate.

On the other hand, denier blogs are hyping it way beyond its likely significance. It is one of many papers about climate sensitivity. As Judith says, it won't be the last by a long way. Also, we won't have to wait long to find out what the transient climate response (TCR) is in the order of. We're very likely to find out this century. Maybe even in the first half of this century, which is quite extraordinary when you think about it. (CO2 at present is around 400 ppm, more than 40% higher than it was in the 1850s. The average increase in CO2 since 2001 has been 0.54% a year. In 2013 it went up by 0.7% and, as yet, there are no signs of us slowing emissions. If CO2 went up at 1% a year, CO2 would have doubled by 2048. If at 0.9% a year, then it would have doubled by around 2052. If at 0.8% a year, then we've got till around 2057. If we slow emissions and CO2 increases at 0.5% a year, then we've got until around 2082 to find out what TCR is.)

Certainty vs uncertainty

Two articles I have read - at Judith Curry's place and at WUWT, make a point of the fact that climate sensitivity is still very uncertain. At the same time, the authors have defined the uncertainty. So what to make of it al?

Deniers at WUWT, while being told it is all very uncertain, are also being told that it is quite certain that climate sensitivity is lower than most research suggests. For example, the CATO institute duo, Pat'n Chip, are writing how this is a "blockbuster" paper, yet at the same time they are saying that there are thirteen other recent low-sensitivity climate publications, by 42 authors from around the world. If there are fourteen papers of like result, then what is it that makes this one such a "blockbuster"? As for certainty vs uncertainty, Pat'n Chip wrote:

The estimate is not perfect, as there are plenty of uncertainties, some of which may never be completely resolved. But, nevertheless, Lewis and Curry have generated  a very robust observation-based estimate of the equilibrium climate sensitivity. 

What those "plenty of uncertainties" are, Pat'n Chip don't say. The paper itself points to various uncertainties, including those mentioned in other papers. These include the size of the forcings by black carbon and related aerosols, as well as aerosols in general. The authors attempted to minimise uncertainties associated with volcanic eruptions by selecting periods where volcanic activity was low.

The paper is not the first word and won't be the last. The paper doesn't draw heavily upon complex changes in the earth system overall. It mostly relies on analysis of instrumental surface temperature records, while giving a nod to some fluctuations in ocean sea surface temperature in various parts of the world (specifically the Atlantic Multidecadal Oscillation) at certain times, and avoiding the impact of volcanoes. It also mentions ENSO, which will no doubt be a big relief to Bob Tisdale. It doesn't attempt to reconcile its estimates with what can be deduced about climate sensitivity by studying past climate change. It's in the same ballpark.

Click here for other papers on climate sensitivity, going back quite a way, compiled by AGW Observer.

More waving stadiums?

PS You might be interested in what looks like a new stadium wave paper by Sergey Kravtsov, Marcia G. Wyatt, Judith A. Curry and Anastasios A. Tsonis that's just appeared at GRL, too.  They've seen "a pair of patterns, one of which evolved in sync with multidecadal swings of the global temperature, and the other in quadrature with them".

This is probably the paper Judith is referring to in a comment under her article about the sensitivity paper.

It's full of sciency-jargon, like "oscillatory-looking wiggle", which has since been termed the less sciency-sounding "stadium wave" :)

The purpose is said to be:

1. establish robustness of the observed 20th-century stadium-wave signal and refute the Mann et al. (2014) conjecture; and 
2. document and analyze the differences between the observed secular climate variability and the one simulated by a state-of-the-art climate model.

I haven't checked the extent to which they succeeded in 1. and 2. I'll read it later, along with Mann, Steinman and Miller (2014) but there's no guarantee I'll be sufficiently wise to know.

[Added some bits - Sou 27 Sept 14]

From the WUWT comments

The paper doesn't get a lot of support at WUWT and of those who welcome it, most don't seem to understand it. Anthony's pared his audience down so that most of what he has left are greenhouse effect deniers, the scientific illiterati and the 8% Dismissives.


Salvatore Del Prete is a greenhouse effect denier (extract)

September 25, 2014 at 3:40 pm
The problem is, there is NO climate sensitivity to CO2/GHG concentrations. The historical climatic record shows this to be quite clear then again who cares about data when it comes to AGW theory.

JohnWho is still stuck in the fact that in the past there were different forcings. Warming and cooling doesn't always start with a change in CO2. Only sometimes. Like now. And probably like at least part of the Permian-Triassic extinction events.

September 25, 2014 at 6:35 pm
That bothers me about this conversation also.
Since, in the past, temp changes first and then CO2 level, then we have no history to show that either a doubling of CO2 would cause warming or a halving of CO2 would cause cooling. Simply stated, it never has before, so why are we so sure it is happening now.
Seems there might be a problem with the idea that warming increases CO2 which then increases warming which then increases CO2 some more … until??? (Or the opposite: cooling decreasing CO2 which decreases temp, etc.)

Konrad is a plain vanilla greenhouse effect denier.

September 25, 2014 at 3:54 pm
No, the “slow walk back” can never work. While CS for CO2 doubling is at best a dubious metric, unless it has a “-” before the figure it cannot possibly be correct.
No paper using the “basic physics” of the “settled science” can ever break the “0.0 barrier”. The foundation assumption of 255K being raised 33K is in grave error. Nothing of any real scientific value can be built on this foundation.
The only use these “slow walk back” papers have is political, and even here the value is limited.

Paul Schnurr decides that Judith Curry and Nic Lewis are science deniers, writing:

September 25, 2014 at 4:12 pm
Maybe the appearance of these papers will help convince the “warmists” that the “deniers” don’t deny that the climate system is affected by the build-up in CO2, just that it may not be as alarming as it’s being characterized, which, of course, has been the position of most of the “deniers” from the beginning.

Bill Illis mutters something about radiosonde measurements.

September 25, 2014 at 5:57 pm
The other issue that has not been mentioned to date is that if you just use the lower troposphere satellite temps from RSS/UAH and the weather balloon radiosonde measurements, the sensitivity drops another 0.4C or so down to 1.2C.

daveandrews723 commends Lewis and Curry on the one hand, but then effectively says that it was a waste of effort because they don't have a clue.

September 25, 2014 at 6:26 pm
I commend Lewis and Curry for their work. I am not a scientist, but it seems strange to me that there are so many “possibles,” “uncertainties,” and “estimates,” in a field of science where so many “experts” on one side claim “the science is settled.” I don’t believe anybody in climatology actually has the faintest idea of the impact of CO2 on global temperatures.

Leo Geiger asks a question of WUWT and suggests they can't just pick and choose what they like and reject the rest. Surely WUWT would never do that :(

September 25, 2014 at 8:53 pm
Yes, but are these 14 papers on sensitivity actual scientific papers or are they “Claim:” scientific papers?

That’s the problem with constantly downplaying the importance of most other published research by doing things like attaching the word “Claim” in front of them in blog posts. It becomes hard to then turn around and make a credible appeal to *any* published research for support.

Published research isn’t an ‘à la carte’ menu.

Konrad says that anything he doesn't like must be pal-reviewed. From the look of it, Konrad doesn't accept any climate science at all. Full stop.

September 25, 2014 at 9:40 pm
Just because they are pal-reviewed and passed the gatekeeping of the journals does not make them “science”.
Like it or not peer-review is not a critical component of scientific method. It is just part of our current scientific bureaucracy. Repeatable observation and experiment are what is critical to science.
Given that each of these papers are based on the 255K assumption, none can truly be considered science.

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Lewis, N. and J.A. Curry, C., 2014. "The implications for climate sensitivity of AR5 focring and heat uptake estimates". Climate Dynamics, doi:10.1007/s00382-014-2342-y

Sergey Kravtsov, Marcia G. Wyatt, Judith A. Curry and Anastasios A. Tsonis. "Two contrasting views of multidecadal climate variability in the 20th century." Geophysical Research Letters (2014).  DOI: 10.1002/2014GL061416

Mann, Michael E., Byron A. Steinman, and Sonya K. Miller. "On forced temperature changes, internal variability, and the AMO." Geophysical Research Letters 41, no. 9 (2014): 3211-3219. DOI: 10.1002/2014GL059233

PALAEOSENS Project Members. "Making sense of palaeoclimate sensitivity." Nature 491, no. 7426 (2012): 683-691. doi:10.1038/nature11574

Tett, S. F. B., G. S. Jones, P. A. Stott, D. C. Hill, J. F. B. Mitchell, M. R. Allen, W. J. Ingram, T. C. Johns, C. E. Johnson, A. Jones, D. L. Roberts, D. M. H. Sexton, and M. J. Woodage, "Estimation of natural and anthropogenic contributions to twentieth century temperature change," J. Geophys. Res., 107(D16), doi:10.1029/2000JD000028, 2002.