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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