## The TCR-to-ECS ratio

Something I’ve been interested in is whether or not the ratio between the Transient Climate Response (TCR) and the Equilibrium Climate Sensitivity (ECS) best estimates from most of Nic Lewis’s work makes sense. Typically his TCR best estimate is around 1.35oC, while his ECS best estimate is around 1.65oC; giving a TCR-to-ECS ratio bigger than 0.8. For climate models, the ratio is typically below 0.8 and, most likely, below 0.7. I did ask Nic Lewis if he could explain his rather high TCR-to-ECS ratio, and he responded in his normal charming fashion. His argument, I think, is simply that you take the energy balance forms of the equations that describe TCR and ECS

$TCR = \dfrac{F_{2\times} \Delta T}{\Delta F},$

and

$ECS = \dfrac{F_{2\times} \Delta T}{\Delta F - \Delta N},$

and you simply plug in the values for the change in temperature ($\Delta T$), change in forcing ($\Delta F$), and change in system heat uptake rate ($\Delta N$) and out pops an answer. My question, however, was trying to delve slightly deeper into this issue, which I’ll try to explain below.

As discussed in Section 4 of this paper (Earth’s energy imbalance and implications by Hansen et al.) for a given climate sensitivity, the climate response function (defined as the fraction of the fast-feedback equilibrium response to a climate forcing) depends on the rate at which energy is mixed into the deep ocean. If energy is mixed into the deep ocean slowly, then most of the energy will heat the upper ocean, land and atmosphere, and the climate response will be fast. If, however, it is rapidly mixed into the deep ocean, then the climate response will be slow (the upper ocean, land and atmosphere will warm more slowly). This will manifest itself as a difference in the magnitude of the planetary energy imbalance that can be sustained; if the climate response is fast, we’d expect the typical planetary energy imbalance to be small; if it is slow, we’d expect it to be larger.

This is illustrated in the figure below, which shows examples of 3 different climate response functions, all of which can match the historical surface temperature record, but which produce different planetary energy imbalances. If the climate response is slow, we might expect a planetary energy imbalance today of around 1Wm-2. If it is fast, we might expect it to be less than 0.5Wm-2. Given that, today, it is probably somewhere between 0.6 and 0.8Wm-2, Hansen et al. (2011) suggest that an intermediate response is more realistic and that most climate models possibly mix energy into the deep ocean too rapidly (they tend to have slow climate response functions). However, is this sufficient to explain the discrepancy between a TCR-to-ECS ratio of 0.6 to 0.7 – suggested by most climate models – and a value in excess of 0.8, as suggested by Nic Lewis’s work?

Credit : Hansen et al. (2011)

Credit: Miller et al. (2015)

In trying to understand this better, I came across a paper called Model structure in observational constraints on transient climate response, by Miller et al. It includes the figure on the right that shows the TCR-to-ECS ratios for Perturbed Physics Ensembles (PPE), CMIP5 models, and also includes the AR4 and AR5 ranges for TCR and ECS. Judging by this figure, it would seem that a TCR-to-ECS ratio between 0.8 and 0.9 is possible, but would seem to require that the TCR be quite low; less than 1.5oC, which is indeed consistent with Nic Lewis’s best estimate. However, this still doesn’t really indicate that a TCR-to-ECS ratio greater than 0.8 is consistent with the planetary energy imbalance being greater than 0.6Wm-2.

I’m not quite sure where to go with this. This post is really just some thoughts on this. It seems that a TCR-to-ECS ratio above 0.8 is possible if the TCR is sufficiently low. Given how much we’ve now warmed, however, a low TCR is becoming less likely. Also, if the system can sustain a planetary energy imbalance of between 0.6Wm-2 and 0.8Wm-2 might also suggest that a TCR-to-ECS ratio above 0.8 is unlikely, because the potential amount of committed warming could then be larger than 20% of a low TCR.

I should say that I think Nic Lewis’ work is very interesting and makes a valuable contribution to our understanding of climate sensitivity. However, it does suggest some things that are at odds with other estimates; such as a much larger probability of the ECS being below 2oC, and a higher TCR-to-ECS ratio. Also, there is the possibility that feedbacks are time-dependent in such a way that observationally-based estimates of climate sensitivity may suggest that climate sensitivity is lower than it actually is.

I should also add that focusing only on climate sensitivity can be a bit misleading, as how much we will actually warm will depend on climate sensitivity and on carbon cycle feedbacks. These can be combined into a single quantity called the transient response to cumulative emissions, which is thought to be between 0.8oC and 2.5oC per 1000GtC. Since we’ve warmed by almost a degree after emitting just under 600GtC might suggest that the lower end of this range is rather unlikely. I’ll stop there. Comments welcome.

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### 76 Responses to The TCR-to-ECS ratio

1. bobcobbblog says:

Hi Physics,
Considering the standard consensus for ECS is 3C, even in spite of that recent cloud study, what is your thought on China and the US pushing for early ratification of Paris?
Also, I’ve heard from Bob Berwyn that scientists he’s talked to say the range for the pledges could be 2.5-2.7C. Any idea where the likely range for Paris from 2.7-3.5C might fall, feedbacks included?

2. John Hartz says:

ATTP: You wrote:

I should also add that focusing only on climate sensitivity can be a bit misleading, as how much we will actually warm will depend on climate sensitivity and on carbon cycle feedbacks.

Correct me if I am wrong, but isn’t climate sensitivity merely an overarching ratio created by climate sientists for comparing the relative performance of climate model outputs? If so, it no more exists in the physical world than does the the gobal average mean temperature of the lower troposphere.”

3. bob,
I don’t know that I have a good answer to that. My preference is to think in terms of TCRE (the transient response to cumulative emissions). The range is 0.8 to 2.5oC per 1000 GtC. We emitted, in total, just under 600GtC and warmed by just under 1oC. This warming may not be all forced, but my guess would be that something just under 2oC per 1000GtC would be a reasonable estimate. We’re emitting around 10GtC per year, so unless these comittments can result in us not emitting anything more than about another 400GtC (40 years at the current rate) we’re going to find it difficult to stay below 2oC. From the Paris deal alone, my understanding is that the expectation is that we’ll warm to about 3.5oC, essentially because the Paris deal doesn’t really lead to the emission reductions that would be required to achieve the 2oC target. Joe Romm’s post lays it out quite clearly.

4. JH,
That’s essentially right. Climate sensitivity is really a model metric that indicates how sensitive that model is to changes in external forcing. Clearly it can be related to the real world, since that is what the models are trying to represent, but there is no real world scenario exactly equivalent to a model run in which climate sensitivity is determined (increasing atmospheric CO2 at 1% per year for 70 years).

5. bobcobbblog says:

Do you think getting under 3C is a reasonable possibility?

6. bob,
If our current understanding is correct, then it’s not – IMO – going to be easy. On the other hand, we seem to have the ability to make quite substantial changes on relatively short timescales. I think the biggest hurdles are societal and political, not technological.

7. bobcobbblog says:

I tend to think we’ll probably end up between 2.5 and 3C. As you said, it’s not easy for sure. It’s also not a great place to end up, but it’s better than higher up. There’s also the question of SRM, which I believe research funding was allocated for in a recent US Senate bill. That’ll be an interesting situation to follow.
I find your post about the ability to make substantial changes in short timescales revealing. It’s part of the contrast I see with Kevin Anderson. I agree with his data, but his interpretation of it is up for debate, especially compared to Michael Mann, who didn’t have very positive things to say about him in an email exchange I had with him. It is important for Anderson to highlight the severity of the challenge we face, but it is quite another to consistently downplay every good piece if news about emissions and renewables. His message is in essence that no matter what we do with emissions and renewables, it won’t matter. I don’t think that’s true and it’s certainly not helpful. He also doesn’t ground his own solutions in feasible reality.

8. I actually have quite a lot of time for Kevin Anderson. I realise that his message is a bit pessimistic, but he backs it up with numbers and is at least both illustrating the severity of what we face and trying to get us to recognise that we have to take it seriously (at least, that’s how I interpret what he says). I also, on the other hand, have sympathy for those who argue that we can do something about this and that we should remain optimistic. At the end of the day, I don’t think anyone really knows how best to be effective and I have trouble being too critical of those who are at least trying to get people to take this seriously.

9. bobcobbblog says:

His numbers can be a bit misleading at times, though. For instance, he tweeted that the first review cycle for Paris is in 2023, which is not true. It’s from 2018-20. I also fail to see how saying “I think we will fail”, and emissions reductions and renewables progress as of little consequence as a good way to get people to take the issue seriously. That kind of language turns people off, which is part of the reason why Michael Mann has a much bigger following than Anderson. My point is that he can spread his message without being so doomerish.

10. bob,

My point is that he can spread his message without being so doomerish.

Yes, I do tend to agree.

11. bobcobbblog says:

Even so, I respect him for his activism.We need more of it.
On another point, I can’t overstate the importance of the 2016 election here in the states. The Dems have to win this to keep Paris going and strengthen our commitments. Fortunately, I think that will happen. I realize some people are skeptical about Clinton, but I honestly see her expanding Obama’s policies, in no small part due to Sanders’ influence.

12. Even so, I respect him for his activism.We need more of it.

Indeed, and he also seems to be one of the more outspoken UK-based climate scientists.

13. BBD says:

ATTP

My preference is to think in terms of TCRE (the transient response to cumulative emissions). The range is 0.8 to 2.5oC per 1000 GtC. We emitted, in total, just under 600GtC and warmed by just under 1oC. This warming may not be all forced, but my guess would be that something just under 2oC per 1000GtC would be a reasonable estimate.

And from the back of a very old envelope, we have rough estimates for the PETM:

~3000GtC

~6C increase in GAT

So very much in the ballpark.

14. pete best says:

Personally I like Kevin Anderson and his message. He does ask people to do something about their demand side as supply side technologies can’t cut demand in the short term. Sure it’s 2c is unlikely but we can all try. I doubt personally that just pushing all technology onto a revamped electricity grid that’s GHG free at source and has the capacity to run everything is likely. Lifestyle changes will be required also.

15. bobcobbblog says:

It depend what type of lifestyle changes you’re talking about. If you’re talking about major ones in developed countries in favor of letting developing ones emit willy-nilly, that’s not going to happen. And I don’t necessarily agree that major lifestyles changes are needed.

16. > And I don’t necessarily agree that major lifestyles changes are needed.

Contingently agreeing ought to be enough.

17. bobcobbblog says:

If you’re talking about eating less meat and stuff like that, then I do agree. What I don’t agree with is developing countries being let off the hook for emissions reductions, which is where Anderson’s argument starts to fall apart. India is doing a great job with solar and reducing agricultural emissions by upgrading their pumps. Now that’s something I can get behind 🙂 We in the West have to provide some aid to help them out, which Obama is starting to do

18. pete,best says:

The only lifestyle choice that matters is reducing your GHG emissions individually. How you do it is up to you. If we think that all can be achieved without any individualistic reductions than fine but I personally am not looking forward to seeing the UK covered in wind turbines and solar farms which is what is required.

19. bobcobbblog says:

Of course, Pete. Completely agree with you.

20. Eli Rabett says:

Much depends on time. If it takes a long time to reach E, then the ratio is low because the extra energy diffuses into the deep ocean reservoir.

21. wheelism says:

“What I don’t agree with is developing countries being let off the hook for emissions reductions”

Yet it would seem to be the obvious response to all those insisting, “You’re killing MILLIONS of poor people!!”

22. anoilman says:

I’m not certain the developing world is entirely getting on the hook for emissions. In many cases, they are going directly to renewables for many obvious reasons.

23. bobcobbblog says:

It’s definitely not an easy quandary to solve, wheelism. I wish I had the answers.
Anoilman, you are right when it comes to India. And I would say you’re probably right in general.
I have to say, first time being on this blog, I like talking with you guys.

24. anoilman says:

Bobcobbblog: Nothing going on is ‘easy’. The first 20-30% conversion to renewables is easy and doable. After that, we need storage and bigger grids, neither are there, and the grids are pricey.

If you want a dose of OMG.. I recommend you read the articles on renewables over at the Science of Doom. It really is a good and pessimistic look at it all.
https://scienceofdoom.com/2015/07/30/renewable-energy-i/

With all the doom and gloom out there you need to look at the fine print. Sure China is building new coal power plants, but all those plants were built with Western environmental gear to clean up their air. Those sales dropped off ages ago (trust me… I know). And as they are opening new plants, they are closing old ones and moving factories onto the grid.

Last I heard, Tesla presold some 500,000 new cars (was in the shop earlier today);
http://www.theweek.co.uk/tesla-model-3/70320/tesla-model-3-prices-specs-and-release

So.. before getting all worried, keep calm, be pragmatic, and do the right thing. We are dependent on fossil fuels for now, we can quit, but we just need to hit the right price points, and frankly new tech to help out. In the mean time, keep working at it.

25. Blaz Bratovic says:

Since debate has turned more to the scale of challenge and AGW mitigation, I decided to drop in.

bobcobbblog, I generally agree with what you said. Presume you meant SMR instead of SRM ( not sure what SRM stands for – storage resource management? ). Having said that, I don’t usually quote Forbes, but when a scientist researching RE and its integration tweets it and says it is completely consistent with study he co-authored and was published in Nature Climate Change, than it merits to be referenced:

This article also implicitly shows why LCOE, while a useful metric, is better to be replaced with system LCOE when talking about AGW mitigation.

Unfortunately, he also said that they were telling that for 5 years and how US is doing it the wrong way.

What will be very challenging in terms of AGW mitigation is that India is stepping in place of China and while it will probably face even bigger challenges than China did, its response potential is difficult to be estimated to surpass or match that of China.

At the end of the day, I do a bit understand sort of a rant by Jeffrey Sachs in this Guardian piece: http://www.theguardian.com/environment/2016/apr/22/paris-climate-agreement-united-nations

And, to the end this post, I believe it is good to end with a very decent overview of Nature Climate Change study I was referring to previously: http://www.theenergycollective.com/tom-plant/2373780/study-highlights-a-least-cost-energy-vision-for-the-u-s

Connecting the dots won’t be difficult now, I guess. 🙂

26. “Since we’ve warmed by almost a degree after emitting just under 600GtC”

sadly, it is now “just over” 600 GtC…

http://trillionthtonne.org/

… and, if I recall correctly, we’ve warmed by more than 1C, if baselined to, say, the 19th century?…

27. Joseph says:

Nic Lewis has a new post at Dr. Curry’s blog. I was wondering if he has said anything there to address your concerns you raise or if you had any other comments.

https://judithcurry.com/2016/04/25/updated-climate-sensitivity-estimates/

28. verytallguy says:

Current best estimates of GMST (4.0 ± 0.8°C below preindustrial)[24] and of relevant forcings (6–11 Wm−2 below preindustrial)[25] at the LGM imply an ECS estimate of 1.75°C, essentially identical to the updated instrumental-observation energy budget estimate presented here. That suggests effective and equilibrium climate sensitivities in the real world may be very close to each other, but only about half the average equilibrium sensitivity of CMIP5 models.

A claim that paleoclimate studies of the last glacial maximum produce a median ECS estimate of 1.75 per doubling would be startling if it appeared in a peer review journal.

29. Willard says:

Careful use of estimates of effective radiative forcings and planetary heat uptake. Simple but robust global energy budget approach was used, with thorough treatment of uncertainties.

30. Joseph,
Not really. Seems to be just an updated version of what he’s presented before with various arguments as to why he can ignore the various criticisms. As VTG says, would be interesting to see him publish a 1.75C ECS for the LGM.

31. Steven Mosher says:

“From multiple lines of evidence (sea level rise, ARGO measurements, updating Allan et al. 2014), imbalance for 2006-2015 appears to converge around 0.7-0.8W/m-2. Taking uncertainties into account a 1.65K ECS is probably compatible if you use HadCRUT4. I suspect it would be difficult to reconcile with the interpolated datasets.”

“Careful use of estimates of effective radiative forcings and planetary heat uptake. Simple but robust global energy budget approach was used, with thorough treatment of uncertainties.”

Thorough.

Sorry but I have to part ways with Nic WRT to his choice of surface temps.
That should come as no surprise as R Way and I questioned him about this on Climate Audit.

To be clear my suggestion would not be to substitute a series different from Hadcrut. Rather,
the final uncertainties in his TCR and ECS estimates should take a full view of the uncertainties
in observations. It seems pretty simple to report the results given alternative sources. That would be thorough. And easy. It’s not like we would all be confused. If the choice makes no difference then thats important too.

32. Steven Mosher says:

“As VTG says, would be interesting to see him publish a 1.75C ECS for the LGM.”

he is citing Jame’s papers for the Delta temp and change in forcing

hmm. gotta go back and read that.

33. JCH says:

The press release for the new study states: “Rapid warming in the last two and a half decades of the 20th century, they proposed in an earlier study, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface.” The new study supports Tung’s previous (2013) conclusions by providing evidence from sub-surface ocean heat data, by filling in details of the cycle and by shedding light on the mechanism involved. If only half the warming over 1976-2000 (linear trend 0.18°C/decade) was indeed anthropogenic, and the IPCC AR5 best estimate of the change in anthropogenic forcing over that period (linear trend 0.33Wm-2/decade) is accurate, then the transient climate response (TCR) would be little over 1°C. That is probably going too far, but the 1.3-1.4°C estimate in my and Marcel Crok’s report A Sensitive Matter is certainly supported by Chen and Tung’s findings. …

It’s an AMO screw up.

34. Steven,

To be clear my suggestion would not be to substitute a series different from Hadcrut. Rather,
the final uncertainties in his TCR and ECS estimates should take a full view of the uncertainties
in observations.

Yes, this seems reasonable.

35. verytallguy says:

he is citing Jame’s papers for the Delta temp and change in forcing

Annan or Hargreaves views on the conclusions he draws would be interesting.

Sceptic that I am, I expect he’s making a cheery pick; it would be great if he made claims like this in the literature rather than a blog so I could be proved wrong.

His number certainly seems an outlier.

36. Peter Jacobs says:

Lewis’ claims about the LGM sensitivity are selective, which seems to be a naughty, reoccurring habit of his.

In addition to the the PALAEOSENS work, there’s also been recent reevaluation of past LGM ECS estimates’ state dependence.

http://www.clim-past-discuss.net/cp-2016-23/

I don’t think Lewis’ claims are as robust as he would like to imagine.

37. Steven Mosher says:

“Annan or Hargreaves views on the conclusions he draws would be interesting.

Sceptic that I am, I expect he’s making a cheery pick; it would be great if he made claims like this in the literature rather than a blog so I could be proved wrong.

His number certainly seems an outlier.
############
he has another comment on it.. dismissing James as not valid.

hmm.

I suppose one could say his observation “calls into question James work”, but it seems a bit presumptious
to raise a quesion about James work, and then conclude that James work is automatically
invalidated without getting input from James.

to be fair lots of folks engage in this.. I have I know.

James’ take on things would be in order.

38. Steven Mosher says:

“Steven,

To be clear my suggestion would not be to substitute a series different from Hadcrut. Rather,
the final uncertainties in his TCR and ECS estimates should take a full view of the uncertainties
in observations.

Yes, this seems reasonable.
#############

Ya its weird. I know when I was doing some work comparing to re analysis data
I just ask myself a simple question.

Do I have enough expertise in Re analysis to argue with conviction and knowledge that
one of these products is far superior than the others? If not, then take the time to do the
sensitivity.
And its even more weird, since if you asked me to pick the best temperature series, well, I’d
lean toward C&W, and ours, but I would definately check all the series to see if the choice
had an impact on the result.
what are we arguing about a few hundreths? just do the test

39. Joseph says:

Given how much we’ve now warmed, however, a low TCR is becoming less likely.

I am trying to understand this so tell me if I have this about right. The temperature has risen around 1C since pre-industrial times (1880?) and CO2 levels have gone from 280 to around approximately 400 ppm in that same time frame. This means that currently we are only around 40% of the way to doubling emissions. So if the warming since pre-industrial times is entirely or almost entirely due to CO2 emissions then you would expect TCR to be somewhere around 2C. One thing I am not sure is how changes to concentration of other greenhouse gases like methane fit into this conclusion.

40. JCH says:

Jim D has made this same basic post several times at Climate Etc.;

The more direct use of observations such as this gives an effective TCR of over 2.3 C per doubling.

41. Joshua says:

“Cheery pick” = selectively, overly optimistic choices of data?

42. Steven Mosher says:

“One thing I am not sure is how changes to concentration of other greenhouse gases like methane fit into this conclusion.”

$TCR = \dfrac{F_{2\times} \Delta T}{\Delta F},$

43. Steven Mosher says:

shit [Mod: fixed]

44. Some of this actually sounds like Real Science:

Whilst the AR5 ERF estimates are supported by multiple studies and thorough assessment, expert judgement also played a role.

[…]

This limited dependence on complex climate models makes energy budget estimates of ECS particularly robust with regard to dependence on model assumptions, subject to satisfactory forcing, heat uptake and surface temperature data being available.

[…]

Uncertainty in aerosol ERF is the largest contributor to imprecision in estimating ECS and TCR. Uncertainties in heat uptake and in WMGG forcing are substantially less important. Progress in reducing aerosol forcing uncertainty is therefore key to narrowing observationally-constrained estimates of climate sensitivity. Without a reduction in aerosol ERF uncertainty, additional observational data and extended time series may not lead to a major reduction in ECS and TCR estimation uncertainty.

Clearly further research with limited complexity, assumptions and expert judgement is necessary. Once that’s done, we can begin to whittle away at the error bars on projected future emissions. This is definitely require a lot of time for us to figure out.

On a more serious note, I thought clouds were the major Bogey Monster.

45. verytallguy says:

Joseph/JCH,

I don’t think criticisms of Lewis’ work on the basis that he should include other forcings to give an “effective TCR” are at all valid.

Remember that future temperature projections also include projections of all future forcings, (CO2e) not just CO2. So to include them in the estimation of TCR would be double counting.

46. VTG,
Indeed. I also think he does include them. What he’s using is the net change in anthropogenic forcing (in fact, it might be the next change in external forcing).

47. opluso says:

he has another comment on it.. dismissing James as not valid.

https://judithcurry.com/2016/04/25/updated-climate-sensitivity-estimates/#comment-780820

48. paulski0 says:

he has another comment on it.. dismissing James as not valid.

For background, many moons ago it was pointed out that there is no clear correlation between magnitude of global average LGM cooling and 2xCO2 warming from pre-industrial across multi-model ensembles (e.g. PMIP2,PMIP3). Therefore, a model matching (or not matching) LGM global average cooling estimates could not be used to constrain future CO2 warming.

Taking on board the lack of global average correlation, Hargreaves and Annan noticed that there was a correlation for tropical ocean surface temperatures when checking between the same simulations. Therefore, it may be that estimates of tropical temperatures could be used to identify more realistic models and therefore constrain future warming.

It now seems the correlation does not appear when looking at an ensemble of more recent models, although combining the older and more recent into a single ensemble still seems to return a correlation at least bordering significance. How to interpret that is not entirely clear. Hopcroft et al. 2015 suggest the lack of correlation was caused by introduction of vegetation and aerosol modules in the newer models, which add greater complexity.

One thing is clear: If there is no LGM/2xCO2 temperature change correlation across GCMs, then a simple energy balance calculation for LGM cooling doesn’t obviously have any relevance for 2xCO2 warming. So, dismissing Hargreaves and Annan’s tropical SST estimate on this basis likewise dismisses the simple energy balance calculation.

The odd development in this kind of work is the suggestion, explicitly made in one of the quotes collected by Brandon Gates’ above, that simple energy balance model results are more robust and make less assumptions than more complex models. I think that view confuses mathematical simplicity with robustness in terms of its relationship with the real world, and hides the myriad implicit assumptions which go along with using the simplified model. The reality is that these simple models can only really claim any validity in so far as they approximate GCM responses. The really bizarre thing I’ve noticed is people apparently deciding to side with simple energy balance models against GCMs where they disagree.

49. bobcobbblog says:

Are you talking about clouds being a major source of uncertainty?

50. Paul,
Thanks, very interesting.

The reality is that these simple models can only really claim any validity in so far as they approximate GCM responses. The really bizarre thing I’ve noticed is people apparently deciding to side with simple energy balance models against GCMs where they disagree.

Agreed.

Bob,
In terms of climate sensitivity, one of the big uncertainties is the cloud feedback.

51. Chubbs says:

Energy balance TCR+ECS estimates are pulled down by pre-1970 data. Since 1970 temperatures have been rising faster per % increase in GHG forcing than indicated by the energy balance method. To illustrate, using linear regression, since 1979 GHG forcing (all GHG except ozone per NOAA AGGI) has increased by .0092 of 2XCO2 equivalent per year or 0.42 of 2XCO2 over the entire 45-year period. During the same period HADCRUT has increased by 0.0178C per year or 0.79C in total. So the implied TCR over the past 45 years is roughly 1.9 (0.79/0.42) C per 2X CO2.

This simple calculation ignores aerosols, ozone and natural variability, but includes enough relevant factors to call into question the value of the simple energy-balance method in projecting future impacts of GHG.

52. Eli Rabett says:

Paul, AT
A simple energy balance model gives you a single number. A more complex model provides a great deal more. Indeed this was one of the first things Eli noticed from Hansen et al 1988, the patterns of response. It is a good reason NOT to get hung up on TCS or ECS or whatever S.

53. Eli,

It is a good reason NOT to get hung up on TCS or ECS or whatever S.

Agreed. I suspect that some of the focus on these simplistic metrics is to try and deflect from other relevant factors.

54. BBD says:

Agreed. I suspect that some of the focus on these simplistic metrics is to try and deflect from other relevant factors.

I was going to make just such an observation myself, but am now spared the effort.

55. John Hartz says:

Eli Rabbet wrote:

It is a good reason NOT to get hung up on TCS or ECS or whatever S.

It is also good not to get hung up on one’s own petard. 🙂

56. Steven Mosher says:

If you call something a control knob, dont get too hung up if people try to focus on the gain.
metaphors are good up to a point. then not so much.

57. Peter Jacobs says:
58. Peter Jacobs says:

And another one (or rather, a bunch of “ones”)…
http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-100815-024150

59. Peter,
Thanks. So, it does look like Nic’s suggestion that paleo evidence supports an ECS around 1.7C is not what one would find in the published literature.

60. BBD says:

As I also have been saying all along.

61. BBD says:

Steven

NL is *wrong*, again.

You are *wrong* to defend the indefensible behaviour of ‘sceptics’, again.

It’s one of life’s mysteries why you keep on doing it.

62. JCH says:

Bayesian estimation of climate sensitivity using observationally constrained simple climate models

Joseph/JCH,

I don’t think criticisms of Lewis’ work on the basis that he should include other forcings to give an “effective TCR” are at all valid.

My hunch is natural variation did the opposite of what it appears he has used in his estimate. If the AMO is responsible for half of the warming, his number would be around 1℃. I don’t think the AMO is responsible for any of it, and I think the PDO has been suppressing warming since around 1990.

63. Peter Jacobs says:

JCH,

I think people who don’t study it would be shocked to hear how little effort has actually been spent trying to understand the physical oceanographic/coupled ocean-atmospheric properties of the purported Atlantic Multidecadal Variability, relative to say ENSO. This is why as recently as a few years ago you can have a mainstream scientific journal publish a paper asserting that the AMV essentially doesn’t exist (at least as most know it), i.e. Booth et al. 2012.

Now, to be sure, that is a minority position, and several legitimate criticisms of that paper have been published. But there is still a very, very active debate on what exactly the AMV is, to what extent it is driven by changes in the AMOC, external forcing, and stochastic processes, etc.

For anyone to simply add in the linearly-detrended AMO index as a regression term- to me- suggests a surprisingly ignorant orientation to the relevant scientific literature.

64. mpainter says:

Regarding climate sensitivity, it is barely above 1K without amplification by positive feedback of increased atmospheric water as vapor/clouds.
This 1K figure as determined by measurements in the laboratory.

Thus the whole AGW issue rests on the question of whether increased atmospheric water acts as a positive feedback, amplifying the CO2 forcing.

The simplest of observations will provide the answer to the above question. Going from the dry Sahara to the humid tropics increases specific humidity from, say, 0.5 % to 3%, or a 600% increase. What do we observe concerning temperatures? We see that increasing specific humidity by 600% only diminishes the diurnal range through moderating its extremes. In other words, a lower tmax and a higher tmin.

Hence, proceeding according to sound scientific principles and basing our conclusions on observations rather than mere theoretical musings, what can we conclude about the role of increased water vapor in determining temperature?

Could be that there is no cause for alarm.

65. mpainter,

Hence, proceeding according to sound scientific principles and basing our conclusions on observations rather than mere theoretical musings, what can we conclude about the role of increased water vapor in determining temperature?

Given that we’ve warmed by about 1C and am probably only about 2/3 of the way to doubling CO2 (in terms of the change in forcing) we can probably conclude that some positive feedbacks are operating and the one of these is probably water vapour.

Could be that there is no cause for alarm.

Whether you’re alarmed, or not, is your choice. On the other hand, if you mean could the changes be relatively small and easy to deal with. Possibly. On the other hand, maybe the changes will be quite substantial and the impacts will be severe and damaging. If so, the evidence suggests that the changes will likely be irreversible on human timescales. Given that we probably need to think about whether or not we should do something to ultimately reduce our emission of CO2 into the atmosphere, or not.

66. Peter Jacobs says:

mpainter,

There is virtually no evidence for equilibrium climate sensitivity being close to 1°C for the entirety of the Phanerozoic. Quite literally our understanding of climate over geological time is incompatible with the claim that ECS is very low.

We can confirm the existence and bound the magnitudes of positive feedbacks like water vapor and ice-albedo using observations.

You’re substituting oversimplified argumentation for rigorous scientific inquiry.

67. Windchaser says:

Thus the whole AGW issue rests on the question of whether increased atmospheric water acts as a positive feedback, amplifying the CO2 forcing.

Well, the fact that warmer air holds more moisture before precipitating out is also pretty well-established from laboratory experiments. As are the radiative properties of water vapor.

So which part of the water vapor feedback do you feel to be iffy?

Re: deserts vs humid tropics, I think you’ve left out quite a few other variables that confound your analysis. E.g., oceans across the tropics both increase the humidity and mitigate temperature swings; correlation =/= causation.

And temperatures are not simply based on local variables like humidity, but also on variables hundreds of miles away. There are connections via wind currents between the tropics and the Sahara which will dry or moisten the air; their temperatures and absolute humidity don’t operate independently of each other. So you can’t even really separate them out like you’re trying to do. They aren’t so local.

68. John Hartz says:

mpainter stated:

Could be that there is no cause for alarm.

Could be that the melting cyrosphere didn’t get the memo.

69. KarSteN says:

@Peter Jacobs:

Well, Booth et al 2012 don’t argue AMV doesn’t exist (as a arbitrarily defined index it exists no matter what), all they say is that it may well be caused by external forcing (in fact 85% attribution). Clement et al 2015 showed that you don’t need more than a mixed ocean to get all things AMV. Only on longer timescales there’s a positive heat flux component from the ocean back to the atmosphere. However, in my mind the internal contribution is easily masqueraded by external forcing, let alone strongly varying anthropogenic (keyword “dimming/brightening”) and volcanic forcing.

Sure, there’s no consensus whatsoever on the issue in the scientific community, but that’s mainly because the traditional view has been that the ocean circulation is the grand master. As always, it’s extremely hard to shift such widely held views, but I’m confident it’ll happen sooner or later. For example, it is easy to demonstrate that the current “cold blop” in the North Atlantic is caused by persistent atmospheric circulation anomalies linked to teleconnections with the North Pacific (PDV positive leads to AMV negative). It is added on to the underlying cold anomaly in the same region which is likely the signal of the salinity-induced AMOC slowdown.

Hence I totally agree with your concluding paragraph. Anyone who uses the AMO/AMV index as a regression term can’t and should not be taken serious. Luckily, the recent surge in GMST provides ample evidence that such AMV-based regression is bound to fail miserably.

70. Peter Jacobs says:

KarSteN says:

>Booth et al 2012 don’t argue AMV doesn’t exist

In my defense, I did say “(at least as most know it)”. I think we are in much agreement here. I would actually love to chat with you over email about this if it’s a topic of interest to you. The “AMO” in its Enfield, linearly-detrended form has become something of a white whale for me. I would love to brainstorm with some people involved in the aerosols side of things on it.

71. I would actually love to chat with you over email about this if it’s a topic of interest to you.

Blog commentary leads to actual scientific discussion, brilliant 🙂

Your comment reminds me that I went to a talk by Booth about how the variability might be driven by aerosols (IIRC). I think it was about a paper that was to be published, but I haven’t checked to see if it’s appeared yet.

72. JCH says:

I’m just a cowboy with hunches, so I cannot participate. This link showed up today.

73. @Peter Jacobs:

Yepp, agreed that we agree 🙂 Very happy to chat about it in more detail via email.
Here it is: karsten.haustein (at) ouce.ox.ac.uk
I’m looking forward hearing from you. Perhaps we can share the results in a condensed version here later on as well so that our dear host ATTP benefits as well.

74. Perhaps we can share the results in a condensed version here later on as well so that our dear host ATTP benefits as well.

That would be great if you could. No pressure though 🙂