The Lewis and Crok exposition – Climate less sensitive to Carbon Dioxide than most models suggest

Full papers plus additional comments from co-author Nic Lewis follow. I have added some relevant diagrams and tables from the report, plus reproduced the foreword by Dr. Judith Curry as well as updated the summary Equilibrium Climate Response Graph originally by Dr. Patrick Michaels to include this new ECS value and range. – Anthony

Lewis-Crok_Figure6

Figure 6: Transient climate response distribution for CMIP5 models
Models per AR5 Table 9.5. The bar heights show how many models in Table 9.5 exhibit each level of TCR.

NEW REPORT: CLIMATE LESS SENSITIVE TO CO2 THAN MODELS SUGGEST

The GCMs overestimate future warming by 1.7–2 times relative to an estimate

based on the best observational evidence.

Press Release

A new report published by the Global Warming Policy Foundation shows that the best observational evidence indicates our climate is considerably less sensitive to greenhouse gases than climate models are estimating.

The clues for this and the relevant scientific papers are all referred to in the recently published Fifth Assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). However, this important conclusion was not drawn in the full IPCC report – it is only mentioned as a possibility – and is ignored in the IPCC’s Summary for Policymakers (SPM).

For over thirty years climate scientists have presented a range for climate sensitivity (ECS) that has hardly changed. It was 1.5-4.5°C in 1979 and this range is still the same today in AR5.

Lewis-Crok_table1

The new report suggests that the inclusion of recent evidence, reflected in AR5, justifies a lower observationally-based temperature range of 1.25–3.0°C, with a best estimate of 1.75°C, for a doubling of CO2. By contrast, the climate models used for projections in AR5 indicate a range of 2-4.5°C, with an average of 3.2°C.

Summary of ECS values from various reports, originally by Dr. Patrick Michaels. Updated for this essay, not part of the GWPF press release. Note the Lewis-Crok values at the bottom.

Summary of ECS values from various reports, originally by Dr. Patrick Michaels. Updated for this essay, not part of the GWPF press release. Note the Lewis-Crok values at the bottom.

This is one of the key findings of the new report Oversensitive: how the IPCC hid the good news on global warming, written by independent UK climate scientist Nic Lewis and Dutch science writer Marcel Crok. Lewis and Crok were both expert reviewers of the IPCC report, and Lewis was an author of two relevant papers cited in it.

In recent years it has become possible to make good empirical estimates of climate sensitivity from observational data such as temperature and ocean heat records. These estimates, published in leading scientific journals, point to climate sensitivity per doubling of CO2 most likely being under 2°C for long-term warming, with a best estimate of only 1.3-1.4°C for warming over a seventy year period.

“The observational evidence strongly suggest that climate models display too much sensitivity to carbon dioxide concentrations and in almost all cases exaggerate the likely path of global warming,” says Nic Lewis.

These lower, observationally-based estimates for both long-term climate sensitivity and the seventy-year response suggest that considerably less global warming and sea level rise is to be expected in the 21st century than most climate model projections currently imply.

“We estimate that on the IPCC’s second highest emissions scenario warming would still be around the international target of 2°C in 2081-2100,” Lewis says.

Full report attached.

Contacts:

Nic Lewis

e: nhlewis@btinternet.com

t: +44 (0)7462 155076.

Marcel Crok

e: marcel.crok@gmail.com

m: +31 6 16236275

Dr Benny Peiser

Director, The Global Warming Policy Foundation

t: 020 70065827

m: 07553 361717

e: benny.peiser@thegwpf.org

registered in England, no 6962749

registered with the Charity Commission, no 1131448

==============================================================

Nic Lewis comments in an email to me:

The report shows that – contrary to the impression given by the Summary for Policymakers – the observational, scientific evidence in the main IPCC AR5 report actually supports much lower estimates of how sensitive the climate system is to greenhouse gas levels, both in the long term and over the remainder of this century, than those exhibited by almost all of the CMIP5 climate models used for virtually all the projections of future climate change. The report develops sound observationally-based projections of future global warming, to the last twenty years of the century, that are 40–50% lower than the IPCC’s average projections on the same emissions scenarios.

I carried out the scientific analysis for the report and co-wrote it with Marcel Crok, a Dutch science writer. The foreword is written by Judith Curry.

==============================================================

Foreword

The sensitivity of our climate to increasing concentrations of carbon dioxide is at the heart of the scientific debate on anthropogenic climate change, and also the public debate on the appropriate policy response to increasing carbon dioxide in the atmosphere. Climate sensitivity and estimates of its uncertainty are key inputs into the economic models that drive cost-benefit analyses and estimates of the social cost of carbon.

The complexity and nuances of the issue of climate sensitivity to increasing carbon dioxide are not easily discerned from reading the Summary for Policy Makers of the Assessment Reports undertaken by the Intergovernmental Panel on Climate Change (IPCC). Further, the more detailed discussion of climate sensitivity in the text of the full Working Group I Reports lacks context or an explanation that is easily understood by anyone not actively reading the

published literature.

This report by Nic Lewis and Marcel Crok addresses this gap between the IPCC assessments and the primary scientific literature by providing an overview of the different methods for estimating climate sensitivity and a historical perspective on IPCC’s assessments of climate sensitivity. The report also provides an independent assessment of the different methods for estimating climate sensitivity and a critique of the IPCC AR4 and AR5 assessments of climate sensitivity.

This report emphasizes the point that evidence for low climate sensitivity is piling up. I find this report to be a useful contribution to scientific debate on this topic, as well as an important contribution to the public dialogue and debate on the subject of climate change policy.

I agreed to review this report and write this Foreword since I hold both authors of this report in high regard. I have followed with interest Nic Lewis’ emergence as an independent climate scientist and his success in publishing papers in major peer reviewed journals on the topic of climate sensitivity, and I have endeavored to support and publicize his research. I have interacted with Marcel Crok over the years and appreciate his insightful analyses, most recently as a participant in climatedialogue.org.

The collaboration of these two authors in writing this report has resulted in a technically sound, well-organized and readily comprehensible report on the scientific issues surrounding climate sensitivity and the deliberations of the IPCC on this topic.

While writing this Foreword, I considered the very few options available for publishing a report such as this paper by Lewis and Crok. I am appreciative of the GWPF for publishing and publicizing this report. Public accountability of governmental and intergovernmental climate science and policy analysis is enhanced by independent assessments of their conclusions and arguments.

Judith Curry

Atlanta, GA, USA

February 2014

===============================================================

PDF’s short and long (technical) versions

Oversensitive – Final

A Sensitive Matter – Final

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Bob Diaz

This does NOT come as a surprise considering that all past IPCC models were way off because they assumed a high positive feedback. The logical conclusion is that the feedback is a lot lower than IPCC estimates. I’ll bet that the major news media is going to be very silent about this.

Chuck L

At Dr. Curry’s blog, serial troll David Appell already remarked that he can’t take this report seriously because it wasn’t peer-reviewed.

REPLY:
Well, it was never intended to be. The fun part is that when people Google David Appell, they won’t take him seriously either. See https://www.google.com/search?q=david+appell
Pretty bad when your flaws show up as #1 on Google ahead of your web page.
– Anthony

Lance Wallace

If the media mention this, they will dismiss it because it is under the auspices of Nigel Lawson’s political organization, the GWPF. But as Judy Curry says, where else can you publish these days? A sad commentary on the takeover of part of science by a Perfect Storm of econuts, landed gentry in England making money by putting up birdmincer eyesores, reinsurance companies hiking premiums for baseless fears, politicians riding the swells of current opinion, and, not least, scientists willing to trim their sails for continuation of the grants.

Nic Lewis and Marcel Crok,
I look forward to reading your report. I like Judy Curry’s ‘Forward’.
The merits of science evaluation activities, such as yours, which are independent of IPCC processes, make for a much needed increase in overall science balance and robustness.
John

Jim G

It will become very interesting should the long term increase in CO2 level off or begin to actually drop if the oceans actually begin to cool long term, as some believe.

Zek202

AGW put in new light by following evidence, dogma disturbed, character assasinations to follow.

Gail Combs

How many of the papers included in the IPCC report are based on raw temperature data vs the manipulated data?
As Steve Goddard has continually shown all of the ‘Adjustments’ are always positive so this would have an impact on the climate sensitivity calculations.

Konrad

[snip – sorry, I’m not going to let you start that flame war – Anthony]

Two questions:
“NEW REPORT: CLIMATE LESS SENSITIVE TO CO2 THAN MODELS SUGGEST
The GCMs overestimate future warming by 1.7–2 times relative to an estimate
based on the best observational evidence.
Press Release
A new report published by the Global Warming Policy Foundation shows that the best observational evidence indicates our climate is considerably less sensitive to greenhouse gases than climate models are estimating.

(Bold mine)
CO2 only or all greenhouse gasses?
and
…point to climate sensitivity per doubling of CO2 most likely being under 2°C for long-term warming, with a best estimate of only 1.3-1.4°C for warming over a seventy year period.
Is this climate sensitivity per doubling consistent, from 20 to 40, 40 to 80, 80 to 160, etc. or does it at some point reach an effective “saturation point” where the doubling of CO2 provides a virtually undetectable warming?

Anthony,
Your cute post title ‘The Lewis and Crok exposition’ purposely echoed the famed Lewis and Clarke Expedition . . . . it gave me a laugh.
John
REPLY: well is is a pioneering expedition of sorts. Glad your noticed. – A

[snip – nope, sorry, not going to allow that off topic stuff here to start a flame war – Anthony]

I wonder if these AGW folks understand that the worst possible case is that they might succeed in reducing CO2 levels to pre-industrial levels. Mass starvation.

Gail Combs

Col Mosby says: @ March 5, 2014 at 4:56 pm
I wonder if these AGW folks understand that the worst possible case is that they might succeed in reducing CO2 levels to pre-industrial levels. Mass starvation.
>>>>>>>>>>>>>>>
They would not care and probably would rejoice. See Dr Ball’s Overpopulation: The Fallacy Behind The Fallacy Of Global Warming

Latitude

The new report suggests that the inclusion of recent evidence…
…we discovered the real temperature record
the one that wasn’t jiggled
Here’s my problem with all of these papers…it doesn’t matter!
Every one of them are using temperature records that have no relation to reality at all……
When you “adjust” the past to show faster warming…even if someone could make the perfect computer weather/climate program..that was 100% accurate
..it will never be right..and will always show faster/higher warming than is real
Lewis and Crok’s best estimate is 1.75C…well great….when almost all of that is from jiggling the temp record..the record they had to use in the first place

Niff

The accusation in this paper need to be vigorously repudiated by the IPCC. Yeah Right!
The techniques used to misrepresent the science that the IPCC is supposed to be assessing would appear to be ‘fraudulent’. It’s hard to see how any competent assessor could be seen to be otherwise?
Wouldn’t it be good for this paper to be debated and discussed on a wide basis. What are the chances? Fat and none. /sarc

Bill Illis

I would disregard any climate sensitivity number estimated from the paleoclimate because the actual results are literally +/- 1000C without an accurate Albedo figure. I have the numbers.
Climate science cannot estimate CO2 sensitivity from the paleoclimate without making an assumption about what Albedo was. What was it at the last glacial maximum with all that extra land glacier, sea ice and cloud cover (the feedback assumption calculates that cloud cover was higher). What was it at the Holocene Optimum (when ice was at its minimum). What was it in the Miocene or the Cretaceous. What about the Carboniferous Ice Age or the Ordovician Ice Age. The early pre-Cambrian Earth. All the CO2 sensitivity estimates from the paleoclimate are based on a very key apriori assumption about Albedo that is never outlined in the study.

David L. Hagen

Compare:

Global climate trend since Nov. 16, 1978: +0.14 C per decade

i.e. 0.98 deg/70 years – by UAH Global Temperature Feb. 2014
With

These estimates, published in leading scientific journals, point to climate sensitivity per doubling of CO2 most likely being under 2°C for long-term warming, with a best estimate of only 1.3-1.4°C for warming over a seventy year period.

The Lewis and Crok exposition – Climate less sensitive to Carbon Dioxide than most models suggest
Remarkable!

Janice Moore

@ Gail! LOL. Here I am again. I missed you on the Antarctic sea ice thread…. then, today on the Great Lakes ice thread…. so, here I am AGAIN — dashing up with my bouquet of tulips for you, with thanks:
Link to original tulip bouquet delivery:
Gail’s Bouquet
http://wattsupwiththat.com/2014/03/01/southern-sea-ice-area-minimum-2nd-highest-on-record/#comment-1581857
Janice

Richard M

The number is still too high as it still contains UHI, micro-site contamination, homogenization, etc. while still ignoring ocean cycles. However, that is alright. What it shows, and this is important, is even using their own data the estimates are way too high. This is the right place to start because otherwise the other issues over shadow this important conclusion. If we first eliminate the concept of dangerous warming then the propagandists have no place to hang their hats.

Given the raw data (even adjusted) even their estimate appears to be a bit high.

Shall we make a long term wager? What will the locus of climate sensitivity estimates in the climate science literature be in the year 2020? Will it be 0,5 C +/- 0.5? Will it be 1.0 C +/-1? Will be 1.5 C +/-1.5? Or . . . what number and range do you think it will be?
I am going with 0.5 C +/-0.5 as the locus of climate sensitivity estimates in the climate science literature in 2020.
John

Chad Wozniak

Climate not so sensitive to CO2? Duh.
@Gail Combs –
Yes, Eichmann (uh, Holdren) would surely rejoice.

Berényi Péter

With no general physical understanding of quasi stationary non equilibrium thermodynamic systems at all, especially the irreproducible case (when microstates belonging to the same macrostate can evolve to different macrostates in a short time), current climate modelling paradigm is doomed to failure anyway.
I would not trust “observational” estimates of climate sensitivity either, because all datasets were tampered with based on the same flawed computational models.
Let me introduce a fat example. Figure 2.5 (at bottom of page 55) of Energy and Climate Studies in Geophysics (1977) shows “Recorded changes of annual mean temperature of the northern hemisphere”, according to the caption (between 1880 and 1975). If we compare it to HadCRUT4 Northern Hemisphere Temperatures, a current dataset, which contains the interval for which data were known in 1977, shows only one third of the cooling in 4 decades between the mid thirties and mid seventies compared to the dataset assembled by Budyko (1969) and updated after 1959 by H. Asakura of the Japan Meteorological Agency.
Now, history is not supposed to change, but in climate science it does. And all this because computational climate models had insurmountable difficulties in hindcasting the mid 20th century cooling. Solution? Rewrite datasets retrospectively, make that inconvenient cooling all but disappear. Huh.

Konrad

Moderators – For Anthony, no need to publish.
[snip – sorry, I’m not going to let you start that flame war – Anthony]
Understood. I was not my intention to start a flame war, but rather raise the “how do we get there from here” issue. I am not one who believes the current route leads where sceptics need to go.
For sceptics there are two apparent paths, slightly positive ECS and slightly negative ECS. Current observational evidence supports both sceptic paths at this time. The CAGW believer path is clearly not supported by current observational evidence.
The instantaneous radiative flux calculations applied to transparent substances and moving fluids have proved the dead end of the CAGW path. Observational evidence supports both sceptic paths. So the question now is which of the sceptic paths is best supported by empirical experiment? They can’t both be right.

Pamela Gray

I have a hunch that many, many scientists are at this very moment seriously considering the extensive publications made by Bob Tisdale. It is my considered opinion that those involved in peer reviewed climate science in the rarefied and Ivoried towers of university supported research will eventually come to the same conclusions as this humble man first proposed.

GBmoore

This how to answer people who say that co2 is bad. What worries me is that co2 levels are too low.
http://www.ehleringer.net/Jim/Publications/329/Chapter%2011.pdf

Gail Combs

Janice Moore says: @ March 5, 2014 at 5:24 pm
Thanks for the tulip pictures, they look like the ones I had up north. I haven’t done any planting except grass since I moved south. Escape artist goats make it not worth the effort unless I goat proof the garden. (I am looking for a Round Tuit for my garden that I have been planning)
I do have some wonderful wild flowers though IMAGE
Sorry mods, the spring fever is breaking out, no doubt aided by all that man-made CO2.

Gail Combs

GBmoore says:@March 5, 2014 at 6:16 pm
This how to answer people who say that co2 is bad. What worries me is that co2 levels are too low.
>>>>>>>>>>>>>
That is a definite problem. Part of our gain in crop production is because of increased CO2. We have closed to doubled our yield per ac here in the USA over the last century.
I use this as an illustration for others: Carbon starvation in glacial trees recovered from the La Brea tar pits, southern California.

Dr Burns

Based on the past 17 years, climate sensitivity is zero. Based on the past hundreds of thousands of years, there is still no hard evidence that CO2 has any influence on climate, or that CO2 changes are an effect rather than a cause of climate changes.

john robertson

What no catastrophe?
Another Howard Camping moment for the Cult of Calamitous Climate?
So far it would be just as reasonable, to assert that climate sensitivity is indistinguishable from zero.
If this nonsense survives to host a 6th Assessment report and the weather cycle has taken that imaginary AGT negative, will the IPCC then claim a negative sensitivity?
or will science break free of climatology?

CC Squid

What determines how much a peer reviewer earns for an article? How are they chosen?

Pat Frank

Any empirical estimate of transient climate sensitivity (TCS) has to proceed by assuming some part of the recent warming is caused by CO2 or GHGs. It matters not what reasoning is used in the estimate. The analytical assumptions determine the outcome. That makes any empirical TCS circular and tendentious. Fun to play with but of no particular physical meaning.

Pat Frank

By the way, as David Appell’s comment wasn’t peer-reviewed David Appell cannot take it seriously.

Lindzen’s seems the most likely.

NZ Willy

This CO2 sensitivity value reminds me of the Millikan oil drop experiment to quantify the electon’s charge — the original result was a bit off, and subsequent experiments incrementally changed the result until the true value was reached — but the migrating results results showed that the science was a hybrid of observation and expectation — only when the expectations reached the true value were the observational results allowed to go there. And those scientists were not funded to cling to their previous expectations, like today’s climate scientists are.

Janice Moore

Oh, hurrah! Thanks, Gail, for letting me know you received my bouquet. And, thanks for that gorgeous photo. WOW. Janice

HankH

CC Squid says:
March 5, 2014 at 6:43 pm
What determines how much a peer reviewer earns for an article? How are they chosen?

It varies from discipline to discipline. Typically the editor of the journal chooses the reviewers. Some journals ask that the author name individuals considered authoritative to review the paper and also name those who, for possible reasons of conflict of interest or whatnot, should not be selected to review the paper. In those cases, the authors must explain why that person should not be selected.
In my field it is considered inappropriate to hame friends or colleagues to review a paper. Doing so will likely result in chastisement by the editor. In most cases peer reviewers are not paid (at least they are not paid in my field).

I dunno – when you take the longer view of temps since the last ice age, there does not seem to be much correlation to CO2 at all. Even this pro-skeptic report seems to concur that CO2 is the Earth’s temperature thermostat, just not as powerful a thermostat at the alarmists feel. Clearly the ice core graphs show CO2 is an effect to temps rather than a cause.
In the shorter time frame, you have the IPCC saying mid last century is when man’s CO2 contribution to climate started to be significant, yet we have the same general rate of increase coming out of the little ice age way before man’s 1950+ contribution to the rise in C02, yet the temps are not doing anything they have not done in the past.
Ultimately it will take some ‘unexplained’ cooling to break this meme that C02 is in any way a thermostat that can overcome nature’s own, as yet, not fully understood effects on climate that clearly dwarf any minor input man has made.
http://i.snag.gy/BztF1.jpg

SIGINT EX

I would say that Earth’s atmosphere is insensitive to CO2 in any presumed and measured concentration to date.

Ron Richey

What’s a flame war?

Herbert

In 1957,Hans Suess and Roger Revelle stated that human beings were carrying out a unique large scale geophysical experiment by increasing the amount of CO2 in the earth’s atmosphere.Scientifically, it is wrong to prejudge the outcome of a unique experiment.Beforehand ,the laws of physics don’t predetermine or mandate the climate sensitivity of greenhouse gases.Therefore, no one knows whether the climate sensitivity assumed by the IPCC will turn out to be correct.We must wait until the amount of CO2 in the atmosphere has doubled from pre-industrial levels some time later this century.
What is happening is a guessing game based on past indicia.
Another way of putting this is Nobel Prize Winner and Harvard Physicist ,Percy W. Bridgman’s view of the “inscrutable character of the future “-
” I personally do not think that one should speak of making statements about the future. For me, a statement implies the possibility of verifying its truth, and the truth of a statement about the future cannot be verified.”

Konrad

Ron Richey says:
March 5, 2014 at 8:42 pm
“What’s a flame war?”
————————————–
RL or Internet? Context, as always, is everything 😉
In RL “flame war” may refer to violent conflict involving the use of Fuel Air Explosives (aluminium powder/kerosene), Thermo Kinetic weapons (requiring rail gun/gas cannon launch) or more advanced non air-burst variants thereof (hydrogen strippers).
On the Internet “flame war” refers to text based arguments starting with entrenched positions, escalated by misinterpretation and resulting in foul language and regrets. (Generally safer than the RL version, but still unpleasant).
[That RL = RealLife Mod]

A few hundred parts per million CO2 can not have an appreciable effect. The last 20 years have proved that is the case. Time to dump this theory and move on to the next one. Better luck on their next guess. Next, Man caused cooling due to the huge amounts of dandruff from the 6.5billon people that cover the earth! The Ice will soon be on the march destroying everything before it. pg

davidmhoffer

There’s a model that comes in at 1.1 degrees? Curious to know which one that is. In fact it would be very interesting to contact the groups with models closest to observations and ask for their comment.

Ed, 'Mr' Jones

Herbert says:
March 5, 2014 at 9:10 pm
“Another way of putting this is Nobel Prize Winner and Harvard Physicist ,Percy W. Bridgman’s view of the “inscrutable character of the future “-
” I personally do not think that one should speak of making statements about the future. For me, a statement implies the possibility of verifying its truth, and the truth of a statement about the future cannot be verified.”
I like Yogi Berra’s better – “Predictions are hard, especially about the future.”

This Lewis and Crok estimate of climate sensitivity is still too high because as I understand it still uses an estimate of the trend in the OHC which bears little relation to the current best estimate.
The best analysis of the changing OHC is at
http://www.pmel.noaa.gov/people/gjohnson/OHCA_1950_2011_final.pdf
Look at table1. This shows that the heat flux from 0-100 m dropped 90% when the period 1983 – 2011 is compared with 2004-2011. The flux at deeper levels also declined substantially. Trenberth’s hidey hole for the “missing ” heat is in reality non existent. This shows that, as one might expect on a cooling earth, the oceans are cooling from the top down.
Another problem with the Lewis and Crok analysis is that it follows the IPCC modelers simple minded and almost certainly wrong assumption that the natural trends can be projected forward on a straight line basis. This total disregard for the quasi periodic quasi repetitive periodicities in the temperature record perpetuates the gross scientific malfeasance which vitiates the entire IPCC modeling program and all the impact studies which derive from them.
Furthermore computer models are inherently useless for forecasting a system with as many variables as climate has , let alone the fact that the IPCC models are structured so badly that their range of outputs is likely mostly outside the range of the real climate system.For the inherent model limitations see

A new forecasting paradigm is required. The IPCC model outputs are not really worth discussing except to try to convince the politicians that their climate policies have no sound foundation. For estimates of the probable coming cooling based on the 60 year and 1000 year quasi periodicities in the temperature data and the neutron count as a proxy for solar “activity” see several posts at
http://climatesense-norpag.blogspot.com.

Alan Robertson

I just encountered the latest “Climate __________” catastrophic description today. Gone are Global Warming, Climate Change, Climate Disruption and Extreme Weather. It’s now Climate Shift.

This is a useful contribution to climate science, but many of the estimate are too high because they implicitly assume a total forcing that excludes indirect solar forcings, or century+ cyclical natural changes in ocean oscillations.
A recent addition to the list of ECS estimates was the study by Craig Loehle discussed at
http://wattsupwiththat.com/2014/02/28/more-evidence-for-a-low-climate-sensitivity/
which finds the temperature response (TCR =1.09 C) is 61% of the climate models, and ECS = 2.0 C. Unfortunately, the paper assumes without justification that the warming trend from 1950, after removing the affects of the PDO and AMO, is entirely due to greenhouse gas emissions. If this were true, there could not have been the Roman Warm period, the Dark Age Cold period, the Medieval Warm period. However, Craig writes in comments “I argue that the true equilibrium (or at least what we will see by 2100) is probably lower than my calculated value.”, but this disclaimer is not found in the abstract. These large changes in climate over the Holocene were not caused by PDO, AMO or SUVs, but correlate to solar changes. The solar magnetic flux increase throughout the 20th century, and likely had a significant affect on climate.
http://friendsofscience.org/assets/documents/HMF.jpg
Therefore, the warming trend net of PDO, AMO, should not be assumed to be all due to GHG emissions.

“The climate sensitivity” is a logically and scientifically illegitimate concept for there is no empirical support for the contension that either the equilibrium climate sensitivity or the transient climate sensitivity is a constant.

strike

@Konrad
“So the question now is which of the sceptic paths is best supported by empirical experiment? They can’t both be right.”
No, the sceptics are just trying to falsify the CAGW-theory, doesn’t matter which of your two or any other way.
In my personal opinion, CAGW is FALSIFIED already, maybe the climatologists don’t realize the fail right now. So there has to be a follow-up-theory, that’s the scientific way. This new theory will be closely inspected by us, the sceptics. If there is no new theory there would be lots of unemployed climatologists, activists and politics looking for new jobs.
I personally think, climate sensitivity is a non-linear function and has no informative value, just history management. If global temperatures go down from now on, there will be a further lowered climate sensitivity, else it goes up again.