Monthly Archives: June 2010

Invitation On Assessing Three Climate Hypotheses

I presented the post

My Further Response To Real Climate’s Gavin Schmidt and Eric Steig

where Gavin Schmidt responded with respect to my placement of him as accepting the CO2-dominated human climate forcing perspective in the following exchange on Real Climate

Gavin – You accept Hypothesis 2b. Thank you for answering clearly. Our EOS paper concluded otherwise, and it is informative to have a discussion by your readers of the three hypotheses on Real Climate. I look forward to reading them.

[Response: Please do not put words into my mouth. First of all, I do not recognise your statements as hypotheses in any useful sense. Secondly, I see no contradiction in accepting that there are multiple sources of anthropogenic influences on climate (I think we will have over a dozen independent effects in the AR5 simulations we are doing), and acknowledging that because of the rate of the rise and the perturbation lifetime of CO2 emissions that they are the dominant issue moving forward. However that does not imply that only CO2 emission cuts are useful, and if you look at any of our recent policy-related work (Shindell et al, 2009; Unger et al, 2009), you will see a portfolio approach to calculating the impact of specific policies and sectors. See also this piece in Physics World. Thus neither 2a nor 2b properly encompass my views. Other forcings are neither negligible nor is CO2 just one issue among the rest. – gavin]

The three hypotheses that we presented in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R.
Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W.
Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,
J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate
change: The need to consider human forcings besides greenhouse gases
.
Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American
Geophysical Union

are

Hypothesis 1: Human influence on climate variability and change is of minimal importance, and natural causes dominate climate variations and changes on all time scales. In coming decades, the human influence will continue to be minimal.

Hypothesis 2a: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern  during the coming decades.

Hypothesis 2b: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and are dominated by the emissions into the atmosphere of greenhouse gases, the most important of which is CO2. The adverse impact of these gases on regional and global climate constitutes the primary climate issue for the coming decades.

Here is my invitation to readers of my weblog.  What different distinct hypotheses could be written that would be an improvement, if possible, on what we presented in our paper? The 19 co-authors of our EOS paper discussed the three hypotheses above and concluded they are well posed, but what are your conclusions on this issue?

Please send to me at pielkesr@ciresmail.colorado.edu, with your name and affliation, and I will post after obtaining a selection. If you conclude our existing hypotheses are appropriate, however, please e-mail me to report this also.

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Meeting On The Role Of Land Biospheric Processes Within The Climate System

I received another announcment of a meeting which further illustrates that there continues to be a movement to broaden the assessment of the role of humans in the climate system beyond the IPCC narrow focus on the global average radiative forcing of CO2.

Below is the announcement I received

Subject: 6th GINKGO-meeting, Frankfurt am Main, 30. Sep + 1. Oct 2010

Dear colleagues,

GINKGO is an initiative that aims at bundling activities in research on
climate and land biosphere (see www.ginkgo-network.net). We would like toinvite you to the 6th GINKGO meeting at the Biodiversity and Climate ResearchCenter (BiK-F) in Frankfurt am Main. The meeting will start on Thursday, 30thSeptember at ca 13:00h and it will end on Friday 1st October at ca 14:00. Themeeting will take place in the Juegelhaus, Building C, Mertonstrasse 17-21,Frankfurt am Main.

This meeting focuses on aspect of global versus regional modeling. We wouldlike to discuss the questions: What can we learn for global/regional models?How can we scale the processes between global and regional models? How can weintegrate global and regional models? This focus includes all aspects of earth system modelling, e.g. vegetation, climate and land use. This focus does,however, not exclude other topics; all contributions are welcome.

Please contact me (scheiter@em.uni-frankfurt.de) to register for the meeting. If you want to give a presentation please indicate the title. Please do alsoindicate if you want to join the dinner on Thursday evening. Attached is alist of hotels.

We hope to see you in Frankfurt,

Simon Scheiter and Steven Higgins

————-
Simon Scheiter
Biodiversity and Climate Research Centre (LOEWE BiK-F)

Contact:
Department of Physical Geography
Goethe-University Frankfurt am Main
Altenhoeferallee 1
60438 Frankfurt am Main
Germany
Tel: ++49 (0)69 798-40167
Fax: ++49 (0)69 798-40169
Email: scheiter@em.uni-frankfurt.de
www.bik-f.de

_______________________________________________
For more information on ther GINKGO-network see:

 www.ginkgo-network.net

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My Further Response To Real Climate’s Gavin Schmidt and Eric Steig

I continue to be amazed at how Gavin Schmidt and Eric Steig are so convinced of their viewpoints, that they fail to see that their autocratic tenor and tone is turning off many of their colleagues. In my engagement with them at Real Climate, apparently, I never learn either (see). :-)

Here are the latest comment/replies on the Real Climate weblog reproduced below

Eric – Please elaborate, however, on why hypotheses 2a and 2b are not sufficiently distinct. The 19 authors of our 2009 EOS paper concluded that they are. Hypothesis 2b is clearly the emphasis of the 2007 IPCC reports.

As we wrote in our article

“The evidence predominantly suggests that humans are significantly altering the global environment, and thus climate, in a variety of diverse ways beyond the effects of human emissions of greenhouse gases, including CO2. Unfortunately, the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment did not sufficiently acknowledge the importance of these other human climate forcings in altering regional and global climate and their effects on predictability at the regional scale.”

Also, you state that Hypothesis 2a is not original. Please refer us to where this perspective is discussed in the IPCC (and CCSP) reports.

[Response: Eric can speak for himself, but other forcings are discussed in WG1 Chapter 2, section 2.3, 2.4, 2.5, through most of chapter 6, and all of chapter 7. The dominance of CO2 among the greenhouse gases is seen in fig 2.20 and fig 2.21 as well as the diversity of other forcings. – gavin]

[Response: To answer your question about hypotheses 2a and 2b, it simply depends on what aspect of the climate system you are talking about. In many land areas, deforestation has a huge impact on the local climate, certainly larger than CO2, so far (Hypothesis 2a). In the central Pacific, it is certainly not land use that dominates; CO2 probably does (Hypothesis 2b). To suggest these are mutually exclusive is just wrong.
–eric]

Response #1: June 25 2010

It appears that Eric, although he apparently does not want to admit it (perhaps even to himself), provides evidence to reject Hypothesis 2b. If in parts of the climate system, non-CO2 human climate forcings are more important than CO2 radiative forcing, than Hypothesis 2b must be rejected. 

We do need to better define what aspects of the climate system are important to society and the environment. In the 2005 NRC report

National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp.

it is written

Despite all these advantages, the traditional global mean TOA radiative forcing concept has some important limitations, which have come increasingly to light over the past decade. The concept is inadequate for some forcing agents, such as absorbing aerosols and land-use changes, that may have regional climate impacts much greater than would be predicted from TOA radiative forcing. Also, it diagnoses only one measure of climate change—global mean surface temperature response—while offering little information on regional climate change or precipitation. These limitations can be addressed by expanding the radiative forcing concept and through the introduction of additional forcing metrics. In particular, the concept needs to be extended to account for (1) the vertical structure of radiative forcing, (2) regional variability in radiative forcing, and (3) nonradiative forcing.

In our 2009 EOS article, we concluded

In addition to greenhouse gas emissions, other first- order human climate forcings are important to understanding the future behavior of Earth’s climate. These forcings are spatially heterogeneous and include the effect
of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008], the influence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and the role of changes in land use/land cover [e.g., Takata et al., 2009]. Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005]. As with CO2, the lengths of time that they affect the climate are estimated to be on multidecadal time scales and longer.

Apparently, Eric has chosen to ignore this broader view of the human role in the climate system, which necessarily requires that Hypothesis 2b be rejected.

 Response #2: June 25 2010

Gavin – You accept Hypothesis 2b. Thank you for answering clearly. Our EOS paper concluded otherwise, and it is informative to have a discussion by your readers of the three hypotheses on Real Climate. I look forward to reading them.

[Response: Please do not put words into my mouth. First of all, I do not recognise your statements as hypotheses in any useful sense. Secondly, I see no contradiction in accepting that there are multiple sources of anthropogenic influences on climate (I think we will have over a dozen independent effects in the AR5 simulations we are doing), and acknowledging that because of the rate of the rise and the perturbation lifetime of CO2 emissions that they are the dominant issue moving forward. However that does not imply that only CO2 emission cuts are useful, and if you look at any of our recent policy-related work (Shindell et al, 2009; Unger et al, 2009), you will see a portfolio approach to calculating the impact of specific policies and sectors. See also this piece in Physics World. Thus neither 2a nor 2b properly encompass my views. Other forcings are neither negligible nor is CO2 just one issue among the rest. – gavin]

 Gavin fails (deliberately?) to accurately present what our three hypotheses actually state. In Hypothesis 2b, we do not state that the other human climate forcings are negligible, just that the dominant human climate forcing is the radiative forcing from CO2, as is assumed by the IPCC. When Gavin writes

 “acknowledging that because of the rate of the rise and the perturbation lifetime of CO2 emissions that they are the dominant issue moving forward”

this is a rejection by Gavin of Hypothesis 2a [whose rejection conflicts with what Eric has written in his response].

Finally, statements by Eric such as

Response: To use an overused metaphor, you are basically asking “Do you still beat your wife, sir?”. The point is that the “AGW” mainstream (at least, as represented by the IPCC) is not in conflict with Pielke’s position, at least not the way it is expressed by you. As I note above the real problem with his position is that it is a false dicotomy. Both 2a and 2b are correct depending on what part of the system you are talking about. Pielke Sr. makes it sound like the ‘mainstream’ is missing something when they aren’t. Hence the definition of a loaded question: one that presupposes something that has not been proven or accepted by all the people involved. If Pielke Sr. gets classified as a skeptic, or contrarian, or whatever, it is due to this sort of misleading rhetoric. It may not purposefully intend to mislead, but it is misleading nevertheless, and in quite substantive ways (because it implies that the mainstream view that we probably ought to cut CO2 emissions is based on faulty science). Note, however, this none of this has anything to do with Anderegg et al., except that, if in fact he gets classified as a ‘denier’ in their analysis, this is probably why. –eric]

and

[Response: Being listed on our blogroll does not constitute endorsement. In general, the sites we do list — whether they are run by scientists or not — tend to get the science right much of the time, and hence are consistent with our mission. Being not-listed could mean that a) we haven’t heard of the site, b) that it is uninteresting or unimportant, or c) that we consider it dishonest or disingenuous with respect to the science. Pielke Jr, Blackboard, and ClimateAudit all fall squarely into the latter category.–eric

illustrate their continued use of ad hominem labels to attempt to promote their viewpoint rather than constructively engage in the discussion of the science. Unfortunately, this also means that when they (and those colleagues who agree with them) review one of my proposals or papers (and those others with whom Gavin and Eric disagree), they likely are going to provide biased reviews. This is why the term “black list” is so appropriate.

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The Forecasting Potential of Complex Models by Antonis Christofides

We are glad to have another guest post by Antonis Christofides (see his earlier post here)

Guest Weblog Post By Antonis Christofides

If you gave me the following paper after replacing the author’s examples of econometric and energy models with climate models, I could not have told it had been written in 1981.

 Ascher, W. (1981). The forecasting potential of complex models. Policy Sciences, 13(3), 247-267. doi:10.1007/BF00138485

Here are some extracts.

On the contrast between bad performance record and large volume of research:

Unless forecasters are completely ignorant of the performance  record, or are attracted solely by the promotional advantages of  the scientific aura of modeling, they can only be attracted to its potential benefits not yet realized.

On the difficulty of retrospective evaluation of model performance when there are competing scenarios:

When no scenario is designated as most likely, the scenarios must  be regarded as exogenous factors, whose likelihoods are not at issue in the modeling exercise. The model produces a set of projections, each posited as correct if the corresponding condition or scenario were to hold, but without implying that any particular one will hold or that some are more likely than others. In this case, the retrospective evaluation of forecast accuracy must proceed by first establishing which condition actually  prevailed, and then measure the discrepancy between the projection tied to that condition and the actual level of the predicted trend. If it is still too early to evaluate a set of conditional forecasts retrospectively, the spread of conditional forecasts of the same trend for the same year can be used as one indication of  uncertainty or minimum error, but only if the conditional is the same for every forecast of the set.

 On using model consensus to judge model validity:

[E]ven the agreement across models need not be an indication of  validity; they could all be wrong. For example, all energy models predicting the 1975 levels of U.S. electricity, petroleum, and  total energy consumption projected these levels higher than they actually turned out to be. This confident consensus was no guarantee that the models were correct then; any consensus among  models’ predictions in the future may be equally misleading.

… [S]imilar models undergoing similar judgmental censorship by  modelers holding similar outlooks on the future can so easily reassure all parties that the future is seen with certainty.

On using the fact that models are physically based as an argument for model correctness:

Complex models are formulated by specifying assumptions and hypothesized relationships as explicit, usually mathematical propositions. While this procedure is often very helpful in uncovering inconsistency and vagueness in the initial ideas or verbal formulations, it cannot establish the correctness of the model’s propositions. Models express assumptions, but do not validate them. If the modeler tries to ensure the validity of the  model’s propositions by focusing on disaggregated behavior of presumably greater regularity, the problem of reaggregating these behaviors to model overall patterns becomes another potential source of error. If the modeler only includes relationships proven  by past experience, there is no guarantee they will hold in the future. There is no procedure or format of model specification that guarantees the validity of this specification.

On the effort required:

Since rigorous, elaborate analysis [of models and their outputs]is time consuming and expensive, there has been a natural tendency  for forecasters to pour their efforts into grand, once-and-for-all projects, carried out only infrequently and yet used long after  they are produced because the immense effort makes them seem definitive.

On the likelihood of modelers to reconsider:     

[A]fter the modeler has spent years developing optimization  routines, apparent violations of … assumptions are more likely  to be accommodated by patchwork modifications, or disregarded altogether as short-term aberrations, than they are to trigger the abandonment of the model altogether.

… [M]odel revision, which seems to the cynic to be an ad hoc effort to keep a fundamentally misspecified model more-or-less in line with reality, is often regarded by the model builder as the normal routine of science.

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Comment On Real Climate

Second Update Thursday Evening June 24 2010

Here is the response by Gavin Schmidt

[Response: Eric can speak for himself, but other forcings are discussed in WG1 Chapter 2, section 2.3, 2.4, 2.5, through most of chapter 6, and all of chapter 7. The dominance of CO2 among the greenhouse gases is seen in fig 2.20 and fig 2.21 as well as the diversity of other forcings. – gavin]

My reply

REPLY

Gavin – You accept Hypothesis 2b. Thank you for answering clearly. Our EOS paper concluded otherwise, and it is informative to have a discussion by your readers of the three hypotheses on Real Climate.  I look forward to reading them.

Update Thursday Evening June 24 2010

The response from Real Climate reads

[Real Climate Response: The problem with this paper is that it strongly implies that Hypothesis 2a is original, and that the rest of the climate science research community thinks that only 2b applies. That’s a strawman argument. Furthermore, these are not well-separated, mutually exclusive hypotheses, which means that choosing one over the other is misleading. Yes, I have read the paper.–eric]

I responded with the comment

My Reply:

“Eric – Please elaborate, however, on why hypotheses 2a and 2b are not sufficiently distinct. The 19 authors of our 2009 EOS paper concluded that they are. Hypothesis 2b is clearly the emphasis of the 2007 IPCC reports.

As we wrote in our article

“The evidence predominantly suggests that humans are significantly altering the global environment, and thus climate, in a variety of diverse ways beyond the effects of human emissions of greenhouse gases, including CO2. Unfortunately, the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment did not sufficiently acknowledge the importance of these other human climate forcings in altering regional and global climate and their effects on predictability at the regional scale.”

Also, you state that Hypothesis 2a is not original. Please refer us to where this perspective is discussed in the IPCC (and CCSP) reports.”

Original Post

Gavin Schmidt has a post on Real Climate titled

What do climate scientists think?

I have submitted a comment to his post on Real Climate, and I recommend readers of my weblog look at the comments that result. My comment is reproduced below.

 I have posted on the PNAS paper on my weblog – Comments On The PNAS Article “Expert Credibility In Climate Change” By Anderegg Et Al 2010 [http://pielkeclimatesci.wordpress.com/2010/06/21/comments-on-the-pnas-article-expert-credibility-in-climate-change-by-anderegg-et-al-2010/]

If Real Climate and others want to focus on actual science questions [this text was (quite surprisingly) edited out by Real Climate as being an “offensive statement“], I propose your readers comment our paper (of which all of the authors are AGU Fellows]:

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R.
Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W.
Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,
J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate
change: The need to consider human forcings besides greenhouse gases
.
Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American
Geophysical Union.
http://pielkeclimatesci.files.wordpress.com/2009/12/r-354.pdf

including comments on the three hypotheses, discussed in our paper,  that I reproduce below

Hypothesis 1: Human influence on climate variability and change is of minimal importance, and natural causes dominate climate variations and changes on all time scales. In coming decades, the human influence will continue to be minimal.

Hypothesis 2a: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and involve a diverse range of first order climate forcings, including, but not limited to, the human input of carbon dioxide (CO2). Most, if not all, of these human influences on regional and global climate will continue to be of concern  during the coming decades.

Hypothesis 2b: Although the natural causes of climate variations and changes are undoubtedly important, the human influences are significant and are dominated by the emissions into the atmosphere of greenhouse gases, the most important of which is CO2. The adverse impact of these gases on regional and global climate constitutes the primary climate issue for the coming decades.

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Informative News Article In EOS By Randy Showstack On A Climate Change Debate At The May 2010 EGU General Assembly

There is an informative news article by Randy Showstack in the June 8 2010 issue of EOS. It is titled

Great Debate on Climate Change Featured at EGU Meeting [subscription required]

The article includes the interesting two contradictory sentences

Panelists largely agreed with climate change findings incorporated in the recent Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). However, some also emphasized large areas of uncertainty related to understanding the Earth system and sociological factors, the quality of models, and regional and local climate impacts.

Text in the article that clearly conflicts with the IPCC view include

Günter Blöschl, head of the Department of Hydrology and Water Resources Management at the Institute for Hydraulic and Water Resources Engineering of the Vienna University of Technology, Austria, noted “huge differences” in the ability to predict climate changes depending on what variables are used. As an example, he showed that flood patterns on the Danube River could be interpreted differently depending on the time frame. “The IPCC report is overly optimistic in terms of the ability to predict changes in extremes and overly pessimistic as to adverse outcomes of such changes,” Blöschl said…..Blöschl added that too much focus is on carbon dioxide (CO2) alone, and that many other changes are occurring along with or independent of CO2.

He said significant impacts to hydrology include increased water withdrawals, pollution, and human migration. “Whether the climate signal is the most important one” related to hydrology, he said, “I would challenge.”

There is also an insightful communication regarding the limits to skillfull prediction.

An additional factor related to poor communication of climate science is the inability to provide detailed, reliable, and specific information that can be used to determine risks, Berkhout [Frans Berkhout, director of the Institute for Environmental Studies at Vrije Universiteit, Amsterdam, Netherlands,] asserted. He wondered whether science ever would be able to offer specific predictions regarding precipitation and other changes on which decision makers could rely. “It may be better to say that we will never, or not in the coming decades, be able to do that, and the way to manage is to accept uncertainty and try to respond adaptively,” he said.

These two colleagues are reinforcing the views that we present in our EOS paper

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union.

I am pleased that the wider view that we present is finally becoming better recognized as a more scientifically robust approach as well as being more societally useful than the unnecessarily narrow IPCC and CCSP perspectives.

There is also an interesting comment on the connection between science and policy.

Legras [Bernard Legras, director of research at the Laboratoire de Météorologie
Dynamique, École Normale Supérieure, Paris, France,] said that what he reads on the blogosphere makes him concerned about a growth of science illiteracy. “Science is used for a pretext for issues that are mostly ideological and political. It is very easy to use climate science as a punching ball, because it is a complex problem,” he said. However, he added it is a good thing for science to get mixed in with politics. “The social contract with science has changed” and scientists are not sitting in ivory towers, he noted. “It’s wonderful to take part in a social debate which matters to people. Science should be mixed with politics and society. We need to be careful about our procedures, but that mixing in is inevitable”

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Comment On An Error In A Figure Caption In The Statement For Policymakers In The WG1 2007 IPCC Report

While there has been considerable discussion of errors in other working group reports of the 2007 IPCC assessment, there has been little discussion of errors in the WG1 report. I have documented obvious errors of omission (i.e. see) but in this post, I want to highlight a specific error in Figure caption [an e-mail exchange with Marcel Crok encouraged me to report on this]. I have identified this error in the past (e.g. see), but it is worth repeating here.

The erroneous IPCC text is in the caption to Figure SPM.2 (see);

IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

The figure caption reads [where I have highlighted the error in bold font]

Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness.

While the footnote corrects this error; i.e.

Radiative forcing is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it. In this report, radiative forcing values are for 2005 relative to pre-industrial conditions defined at 1750 and are expressed in watts per square metre (W m–2)

the WG1 report leaves an erroneous impression in terms of how they present the radiative forcing estimates in figure SPM.2. Indeed, since some of the radiative forcing since 1750 presumably has equilibrated with an increase in global heat content, the actual 2005 radiative forcing must be less than the 1.6 Watts per meter squared that is presented in their figure.

I have reproduced below comments by James Annan and Gavin Schmidt on this subject that appeared in my January 4 2008 post.

This is an important issue. As James Annan stated in a reply on the weblog Stoat

“I think RP is really asking about the current radiative imbalance: while I do not think it is wrong or misleading to talk about total forcing (with a 1750 baseline) as the IPCC do, the other question is also interesting as it relates directly to warming “in the pipeline”. Of course the answer is we do not know for sure, since it directly depends on the climate sensitivity (and even the effective climate sensitivity of the current climate state, which may be slightly different again). But a rough ballpark estimate would be that a little more than half of the total forcing (IPCC terminology) remains as a current imbalance (the commitment runs in the AR4 show the future warming due to this imbalance). Of course splitting this up further into the contribution of each component would then become rather arbitrary.”

Thus, while he writes that this is a “rough ballpark estimate”, his insight that

“…. a little more than half of the total forcing (IPCC terminology) remains as a current balance”,

is the type of answer that is being requested.

Gavin Schmidt on Real Climate also added constructively to this when he responded that

“I don’t think it can be done robustly. A straight-forward apportioning based on the fractional contribution to the original forcing neglects the differing transient behaviour. For instance if one forcing agent rose quickly and stabilised, while another increased later, then the impact of each on the current imbalance should be weighted towards the latter. So that’s no good. Maybe you could do it by examining the single forcing transient runs we did for our recent paper (table 1) and looking at the year 2000-2003 (say) imbalances in Ann/Net TOA radiation. You’d need to check that the individual components do in fact add up to something close to the combined effect (not obviously true). However, different models might give quite different results, and you can only do this for forcings we’ve run. Other groups didn’t do as many single forcing experiments and so you might not be able to find another set of numbers to compare with. Attribution requires models however, and so I don’t see how you could do it any other way.”

The reason that this issue is so important is that

“if one forcing agent rose quickly and stabilised, while another increased later…”

as Gavin wrote, than the fractional contribution to the current radiative imbalance is weighted towards the more recent forcings. Since CO2 has been rising since 1750, at least part of the radiative forcing of CO2 has equilibrated. Thus the claim that CO2 is 50% (or about 30% as estimated on Climate Science based on the 2007 IPCC figure SPM.2; see) is an overstatement of its actual current radiative forcing.

A challenge for the next IPCC assessment, and for climate research in general, is what is the current (now 2010) global average radiative forcing? While there has been considerable discussion of the radiative imbalance (e.g. see) this imbalance includes both the radiative forcing and the radiative feedbacks.

This subject needs discussion in the next IPCC WG1 assessment, as well as a figure like SPM.2 but with the best estimate of the current radiative forcing.

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