Climate Sensitivity and What Lewis and Curry 2014 Has to Say About It 12

[rconnor]

Ah yes, it’s that time again folks. A paper is released, in this case Lewis and Curry 2014, that says climate sensitivity is on the low end of the spectrum and the “skeptic” community starts banging pots and pans claiming the ACC theory is dead. Well, like most things in the field of climate science, it's not nearly that simple. Let's look at the entire story.

Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR)
Equilibrium Climate Sensitivity (ECS) – the amount the planet will warm in response to a doubling of atmospheric CO2 concentration (the base is usually taken from preindustrial levels of 280 ppm). ECS includes both fast and slow feedbacks, so ECS is not fully realized for decades or centuries after CO2 is held constant.

Transient Climate Response (TCR) – similar to ECS but only includes fast responding feedbacks. In other words, TCR is the temperature rise at the time atmospheric concentrations hit 2x the baseline, not where it will settle out to. As slow responding feedbacks tend to be positive, TCR is smaller than ECS.

These two are not the same and should not be confused. Many “skeptic” arguments prey on this confusion, be careful.

The Body of Knowledge on Climate Sensitivity
First, here’s a good list of the spectrum of peer reviewed literature addressing climate sensitivity. If you actually want to understand the topic (instead of cherry picking things that fit your viewpoint), it’s import to look at the body of work, that’s kinda how science works. Here’s a graphical representation, from AR5 WG1 Fig Box 12.2-1:
[image

http://www.climatechange2013.org/images/figures/WGI_AR5_FigBox12.2-1.jpg

]

To claim that a single paper can definitely set climate sensitivity, is false. While on the low side, Lewis and Curry 2014 does sit within the spectrum of other estimates.

Lewis and Curry 2014
Now to the paper itself. Lewis and Curry 2014 (LC14) is very similar to Otto et al 2013 (they both take the energy balance model approach), just with different heat uptake rates and reference periods.

LC14 has a heat uptake rate (0.36 Wm^-2) that is almost half of Otto et al 2013 (0.65 Wm^-2). The uptake rate used in LC14 comes from a single model, not an ensemble mean, and is, surprise, surprise, a very low value (which leads to lower ECS).

The ending reference period (1995-2011) was selected to “avoid major volcanic activity”. Although this seems odd considering Vernier et al. 2011 found that volcanic activity greatly affected the 2000’s. Furthermore, it is well known that the last decade has been a La Nina dominated period which would further add a cooling bias to their ending reference period, and thus artificially lower their ECS and TCR estimates.

Now new evidence (Durack et al 2014) suggests that “observed estimates of 0-700 dbar global warming since 1970 are likely biased low. This underestimation is attributed to poor sampling of the Southern Hemisphere”. Using the results of Durack et al 2014, the ECS would rise (15% according to a tweet from Gavin Schmidt).

The paper makes no mention of Cowtan & Way 2013 which demonstrates and corrects the cooling bias in HadCRUT caused by a lack of coverage in the heavily warming Arctic. Therefore, much of the recent warming which is occurring in the Arctic is unaccounted for in this paper. This would cause an artificially lower value of ECS and TCR.

The paper also ignores Shindell 2014 and Kummer & Dessler 2014 (most likely because they are too recent). Both of these papers highlight the inhomogeneities in aerosol forcing which may cause energy balance models to underestimate ECS and TCR.

Finally, the rather simplistic technique used in LC14 (and Otto et al 2013 as well) ignores all non-linearities in feedbacks and inhomogeneities in forcings. The exclusion of these elements leads to a lowering bias in TCR and ECS. Due to the fact the sample period and technique used introduce lowering biases into the results, LC14 may be useful in establishing the lower bound of sensitivity but in no way offers a conclusive value for the median or best estimate.

It should be noted that the results of Lewis and Curry 2104 implicitly accept and endorse the core of the Anthropogenic Climate Change theory; namely that increases in atmospheric CO2 will result in increases in global temperatures and that feedbacks will amplify the effect. For example, if you feel that the recent rise in global temperatures is due to land use changes and not CO2, then the TCR and ECS to a doubling of CO2 should be near zero. Or, if you feel that "it's the sun" and not CO2 then the TCR and ECS to a doubling of CO2 should be near zero. The recent change in climate is "just natural" and not CO2 you say? Well then TCR and ECS should, again, be near zero. So, if you've found yourself claiming any of the preceding and now find yourself trumpeting the results of LC14 as proof for your side, then you, unfortunately, are deeply confused. If you want to accept LC14's value for TCR of 1.33 K as THE value for TCR (which it isn't), then you also accept that majority of global warming is due to anthropogenic CO2 emissions.

What About Other Papers that Claim Lower Sensitivity?
As I stated from the outset, Lewis and Curry 2014 is hardly the only paper to address climate sensitivity. Beyond that, it’s hardly the only paper to suggest that climate sensitivity is on the lower end of the IPCC spectrum. I’ve addressed a few already but there are more (Lindzen 2001, Spencer & Braswell 2008, etc.). However, almost all of these papers have been found to have some significant flaws that cast doubt on their conclusions. Various peer reviewed rebuttals to these papers are listed below. I’d welcome readers to review the rebuttals and the original authors response to them.
[image

http://static.guim.co.uk/sys-images.../11/1397174224604/lowSensitivityRebuttals.jpg

]

...But What if Climate Sensitivity WAS Lower Than Expected
Let’s ignore all this for a second and pretend that, with Lewis and Curry, we can definitively say that climate sensitivity is lower than expected. Then what? Does this completely debunk the ACC theory? Does this mean rising CO2 levels really aren’t a concern? Well, many “skeptics” would say “YES!” but they do so without ever actually examining the issue.

According to Myles Allen, head of the Climate Dynamics group at Oxford:

A 25 per cent reduction in TCR would mean the changes we expect between now and 2050 might take until early 2060s instead…So, even if correct, it is hardly a game-changer…any revision in the lower bound on climate sensitivity does not affect the urgency of mitigation

.

The issue is that, with atmospheric CO2 levels rising as quickly as they are, a lower TCR does not mean anything significant. It just means that the effects will be delayed slightly. So even if “skeptics” were correct in saying that climate sensitivity is definitely at the lower end of the IPCC range (which they’re not), it would have no substantial impact on future global temperatures or the need to control CO2 emissions.

So, Lewis and Curry 2014 is:
1) Inconclusive to definitely say that climate sensitivity is on the low end of the IPCC spectrum
2) The results are suspect and appear to include numerous biases that would lead to lower TCR and ECS
3) Even if it were conclusive and accurate, it would still not suggest that reductions in CO2 emissions are unnecessary. In fact, it adds to the scientific body of knowledge that temperatures will continue to rise to unsafe levels if we continue with the status-quo, just maybe a decade later than other estimates.

(Note: I’ve started this new thread to discuss climate sensitivity specifically. It is an important topic that popped up in another thread and I felt it merited its own discussion. I would, as much as possible, like to keep the conversation on this subject…although this is likely wishful thinking)

 

[rb1957]

but does it matter ? if the lower estimate delays the world's doom by 10 years then i'd say the difference is pretty negligible.

but, personally, a average a bunch of estimates is not a very "scientific" thing to do. they are all determined by well meaning and humanly biased scientists (rconnor has suggested the L&C mayhave "nudged" their numbers to suit their opinion; having introduced that concept, is it unreasonable that other's may have also ?). each makes assumptions on a whole host of factors. none are "right" ('cept possibly one by superior intuition on the part of it's maker).

but if the range of the projected "doom" varies only by a few years then it really doesn't matter, IMHO. The accuracy on any estimate of "doom" can't by much less than a couple of decades. If the low end of the ramge suggested that "doom" might be postponed almost indefinitely then you'd have something to argue over.

in any case, it quickly won't matter what the US or Europe does. I expect CO2 output from China, India, and Brazil will dominate the US's and we won't a a rat's chance to affect their behaviour. Particularly if we've already committed economic suicide.

another day in paradise, or is paradise one day closer ?

 

[GrandpaDave]

Not too worry... WWIII will trump all concerns about climate change (aka global warming) and other extinction events... IMHO.
G-Pa Dave

 

[GregLocock]

I said I'd repeat the analysis for a variety of start years. Just for fun I've increased the projected CO2 content for 2100 to 740 ppm, approx, which is perhaps towards the high end of what the IPCC projects, and on the low end of what a capitalist would predict looking at SE Asia.

The major outcome is that a curve fitting technique based on 1850 onwards is remarkably insensitive to the precise fitting date, from 1960 to 2000, and the resulting anomaly varies only between 2.5 to 2.9 K at 2100 AD.

The estimated CO2 sensitivity is higher as more of the later data is incorporated into the model, curve fitting from 1850 to 1960 gives 2.1 K/doubling CO2, whereas curve fitting from 1850 to 2000 gives 2.3 K/ doubling. If the ENSO/PDO theory is right (not currently a good look, NASA says the upper half isn't warming enough, and the lower not at all) this will decrease as the 50/60 year ocean cycle takes effect over the next 30 years, we shall see. (More annoying data from NASA, average cloud cover has increased over temperate regions in the last 40 years, albedo effects tba)

It's interesting that the long term cycle seems to settle on 75 years, that is not a number I've come across in the literature. That may be a bias in my model, which explicitly assumes a K/doubling +sinusoid with a longish period .

I'll point out yet again, this is an exercise in curve fitting, not physics. Would that the the knob twiddlers would admit the same.



Cheers

Greg Locock


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[rb1957]

isn't this how engineers approach a problem ? how accurately do we need to know various data ? which data have a strong impact on the result ?

from greg's graphs it looks like the sensivity can vary between 2.3 and 2.06 without much change in the output ... yes?

another day in paradise, or is paradise one day closer ?

 

[rconnor]

but does it matter ? if the lower estimate delays the world's doom by 10 years then i'd say the difference is pretty negligible.

Replacing “world’s doom” with “negative impacts of climate change”, this is exactly my point, and one that skeptics ignore when talking about low sensitivity papers. Without mitigation, lower TCR estimates means very little in the grand scheme of things – we still have substantial increases in temperature and substantial impacts on the biosphere.

but, personally, a average a bunch of estimates is not a very "scientific" thing to do

You’re correct, that’s why the IPCC doesn’t do that. The “best estimate” from the IPCC (in AR4) was NOT the mean value between the various studies and techniques. It was established after evaluating the various studies and techniques. It’s also important to note that, outside of very simple climate models, sensitivity is an output, not an input.

If the low end of the range suggested that "doom" might be postponed almost indefinitely then you'd have something to argue over.

Yes, exactly (again, replacing “doom” with “negative impacts of climate change”).

The estimated CO2 sensitivity is higher as more of the later data is incorporated into the model, curve fitting from 1850 to 1960 gives 2.1 K/doubling CO2, whereas curve fitting from 1850 to 2000 gives 2.3 K/ doubling.

You are attempting to calculate TCR, not ECS, with this simplistic (yet appreciated) technique. Values of 2.1 K or 2.3 K would be substantially higher than the usual “consensus” value of 1.8 K. Should I call up the Huffington Post and get them to write an article stating “Skeptic Analysis Shows Warming Worse Than We Thought!”? It would be fitting after the ceremonial parade LC14 got on skeptic blogs and right wing tabloids certain media outlets. However, we both know that this technique is far too simplistic to establish anything close to a “true” sensitivity value. That aside, I appreciate the work you put in.

But GregLocock, I do have to ask again, especially after your own analysis showed a TCR higher than the expected value, what evidence do you have to support your assertion that sensitivity is likely at or below the IPCC range?

 

[GregLocock]

Evidence? Your data reanalysed in 12 Oct 14 23:11

Cheers

Greg Locock


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[rconnor]

You're going to hang your hat on that correlation? R^2 of 0.08 (and ignoring the fact you changed the start period)? That's your evidence?

Even if we ignore the incredibly weak correlation, what does this correlation actually say about the "best estimate" of sensitivity?

Even if we ignore the jump in logic that an incredibly weak correlation means sensitivity estimates are trending towards a more valid, lower sensitivity, why are they more valid than other estimates and techniques that lead to higher sensitivity?

Forget more valid, are energy budget model techniques, on their own, valid at establishing a "best estimate"? How do you resolve the issues with the technique demonstrated in the original post?

How do energy budget model TCR estimates (1.3 K) match with your TCR estimates (2.1 K or 2.3 K)?

All-in-all, if your argument hinges on an R^2 of 0.08 (found AFTER you made your assertion, by the way) and offers nothing to link an incredible weak correlation to any form of explanation, I'm going to remain skeptical on the validity of your assertion.

But let's say I banged my head against my desk and woke up believing that LC14 contains the "best estimate" for TCR and agree with your assertion. What would change then? Nothing, really. Using a TCR of 1.3 K instead of 1.8 K, impacts would be pushed out by ~10 years. We'd still require mitigation efforts to reduce future temperature rises and negative impacts on the biosphere.

 

[GregLocock]

Good. Be skeptical. Be very skeptical. That's what this all about.

However you seemed to have swallowed holus bolus the idea that an increase in average global temperature is a bad thing. Apply some skepticism to that.




Cheers

Greg Locock


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[GregLocock]

Incidentally wittering on about TCR and ECS in the context of this one line equation model is meaningless, there is no dynamic component in the long term model at all, so TCR=ECS.

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Greg Locock


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[rconnor]

"Remain skeptical on the validity of your assertion" was a euphemism for "you have absolutely nothing to support your assertion".

(1) A correlation that is at best ridiculously weak (R^2=0.08) and at worst weakly opposite of your viewpoint (R^2=0.055 if using the original start date) is evidence of nothing. (2) It is evidence of nothing when there is no explanation to link the correlation to the assertion that sensitivity is at the lower end of the scale. (3) It is evidence of nothing when energy budget models that lead to the (ridiculously weak) negative correlation have been shown to contain significant flaws. (4) It is evidence of nothing when it is counter to your other analysis which shows that TCR is higher than the IPCC estimate (although I feel it is also flawed). (5) It is evidence of nothing when even if it were correct (which it's not) it would do nothing to support the "do nothing" position (in fact it would further emphasize the need for mitigation to limit future temperature rise).

You attempted to dodge the (five) issues with your assertion again. As it stands, you have no evidence to support your assertion. Beyond that, I have provided evidence that counters your assertion. Your assertion requires saving...or dropping. I can't stop you from dodging or ignoring the issues but it would be disingenuous to do so then repeat the same thing later on.

 

[GregLocock]

here's why the R^2 of the straight line fit is so low. This is the average CO2 sensitivity for papers published in each decade , 1967-76 etc, plotted at the centre point of that decade. the error bars show the range of results in that decade.

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Greg Locock


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[rconnor]

And how does this salvage your assertion or address any of the 5 issues with it? If anything it reinforces the fact that the correlation is far too weak, far too dependent on the start period and heavily influenced by flawed energy budget models at the end of the plot to establish anything credible.

 

[GregLocock]

It explains why a straight line fit was a stupid idea.



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Greg Locock


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[rconnor]

And when that stupid (and incredibly weak) straight line fit is apparently all the evidence you have to support your assertion, where does that leave you?

 

[GregLocock]

If the 'accuracy' of the models is getting worse then why are we bothering with developing new ones?




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Greg Locock


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[rconnor]

And how does this salvage your assertion or address any of the 5 issues with it?

Another attempt to steer the topic away in order to avoid addressing criticism of your assertion.

 

[GregLocock]

Are you arguing with yourself now?



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Greg Locock


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[rconnor]

Allow me to rephrase:

Another attempt to steer the topic away in order to avoid addressing criticism of your assertion. As I said before...

how does this salvage your assertion or address any of the 5 issues with it?

I should note that GregLocock needn't be the only one on the hook to attempt to salvage the assertion that "sensitivity estimates are likely lower than the IPCC range". Numerous people made this claim, none have offered a defense.

I'll outline what aspects need defending:

[ol 1][li]GregLocock's own analysis leads to TCR values higher (2.1-2.3 K) than the IPCC best estimate of 1.8 K and much higher than the LC14 value of 1.3 K. So what are you arguing? That TCR is higher or lower than the IPCC estimate? (Should I assume "lower" and that you'll just forget about the other analysis?)[/li]
[li]There is no strong evidence to support a statement that "sensitivity estimates are dropping over time" (a R^2 of 0.08 is evidence of nothing...especially when the start date was adjusted to give a negative correlation)[/li]
[li]Even if a strong correlation existed, it does not logically follow that sensitivity estimates are trending towards a more accurate and lower value. Evidence is required to demonstrate how these lower estimates are "more valid" than other estimates and techniques. Nothing has been brought forward. Just because you like lower values more because they fit your ideological preferences doesn't make them more valid.[/li]
[li]Building on #3, evidence has been brought forward that demonstrates that LC14 (and other energy budget model techniques) are actually less valid. They are over simplistic models that contain numerous biases that lead to artificially low sensitivity values. While these methods may be applicable to establishing a lower bound, they are not valid as a "best estimate".[/li]
[li]Even if TCR was definitively 1.3 K (which it's not), it would mean that IPCC estimates for temperature rise would still be correct but would just be ~10 years early (i.e. impacts expected to happen in 2050 would, instead, happen in 2060). This is not a deal breaker and would still require mitigation to avoid future temperature rise.[/li]
[/ol]

Note that all of these issues are independent from the previous, so a defense of the assertion that "sensitivity estimates are likely lower than the IPCC range" needs to address all 5 points.

 

[GregLocock]

Your analysis only stands up if you assume that later models are no more accurate than earlier ones, and that the addition of more data does not improve the accuracy.

My analysis was not a physics based model, as I said, it was a curve fit. Unfortunately the models you are so keen on are also curve fitted.

Re you second point, yet you were quite happy to use an R^2 of 0.055 to prove you were right! I agree R^2<0.8 is dodgy. My decadal plot shows why using a straight line fit was stupid in the first place. You can't fit a straight line to a parabola like it shows.

You continue to assume that global warming would be a bad thing.

You continue to assume that keeping 6 billion people in poverty is defensible. Oddly enough they don't agree and are burning as much coal as they can.









Cheers

Greg Locock


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[rconnor]

Your analysis only stands up if you assume that later models are no more accurate than earlier ones, and that the addition of more data does not improve the accuracy.

Again, you miss the point. I’m not arguing that TCR should be 2.3 K instead of 1.8 K. I’m arguing that the assertion “sensitivity is lower than the IPCC range” is wrong. My analysis shows that there is no substantial evidence to suggest that the true sensitivity is below the IPCC range (issue #2). That's all it needs to show.

Your entire argument is predicated around energy budget models (Otto et al 2013, Lewis 2013, Lewis and Curry 2014) being right and other methods being wrong. Not only is there no evidence to support this (issue #3) but there is evidence to support the antithesis of this (issue #4), namely that energy budget models are LESS accurate than other methods due to inherent biases and over simplifications that lead to artificially low values of TCR. Again, I’ll note that energy budget estimates are not straight up wrong – they could be a good double check of the ultimate lower bound. However, they cannot establish a “best estimate”.

Furthermore, using the most up-to-date data from Cowtan and Way 2013, Shindell 2014, Kummer & Dessler 2014 and Durack et al 2014, energy budget estimates would be much higher. Using Durack et al 2014 alone would increase the LC14 TCR value to 1.5 K. Applying Cowtan and Way 2013, which captures the warming in the Arctic that is missed by the poor coverage in that area by HadCRUT, would further up LC14’s TCR value. So your “up-to-date data” argument actually works against your assertion.

And none of this even begins to deal with issue #5, that even if issues #2 to #4 are defended, and the "true" TCR value is likely 1.3 K, it STILL doesn't support skeptic's "do nothing" position.

My analysis was not a physics based model, as I said, it was a curve fit.

I agree that your curve fit to get a TCR of 2.3 K is flawed (mainly because it is overly simplistic). No argument here.

Unfortunately the models you are so keen on are also curve fitted.

Wrong and off-topic. (...but...Parameterization is done and locked prior to running the GCM. Parameterization is therefore not done as a curve fit to past temperature. Parameterization is tuned to better match the physical representation of the system. “Tuning” cannot be done such that forces the system to react in ways that is inconsistent with observations and the reviewed literature, even if it were to lead to a more accurate temperature fit.)

yet you were quite happy to use an R^2 of 0.055 to prove you were right!

Wrong. Purposefully ignoring what I’ve repeatedly said and replacing with nonsense. Here’s what I’ve said:

Regardless of the starting point or specific papers you choose, the correlation in any direction would be weak. As I said, "if nothing else, the results trend, more and more, within the IPCC range.

While one cannot definitively say that the trend is increasing (and admittedly my linear trend should never have been included), neither can one definitively say that the trend is decreasing. The correlation is far too weak.

The bulk of estimates show no strong correlation of dropping or increasing overtime.

I’ve continually stated there is no evidence to support a rising or falling trend. I don’t need to conclude the trend is rising as I’m not arguing that TCR is worse (higher) than the IPCC thinks, only that the IPCC range is accurate (which you are arguing against). All I need to show (and have shown) is that there is no evidence to suggest otherwise. But I’ve gone one step further to demonstrate that the papers that you cling to support your ideological preferences assertion are not valid as “best estimates”. Remember, it is your assertion that must be defended.

The last two lines are off-topic and just unsupported nonsense. If you are willing to admit that your assertion is wrong and sensitivity is likely in the IPCC range, then we can begin to talk about the consequences of a warmer planet. Alternatively, you could stop dodging the topic at hand and address the criticisms of your assertion.