Government Comments on the Final Draft of the SPM IPCC Working ...

<strong>Government</strong> <strong>Comments</strong> on the Final Draft of the SPM
IPCC Working Group I Fourth Assessment Report
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SPM-199 A 3:25 3: (figure SPM-2, continued:) It is important to understand that in the paper from which this number 0.01 W/m2 is taken (Sausen
et al 2005), the forcing for "linear contrails" (i.e. those easily identifiable by satellite), which is much reduced since the IPCC
Special Report on Aviation and the Global Atmosphere (1999), is separated from other aviation-induced cirrus whose forcing
estimate has substantially increased and is now much larger, whilst much more uncertain.
Numbers in Table 2.9 show that "Aviation-Induced-Cloudiness" (AIC, with persistent contrails) is 0.030 (0.010–0.080) W/m2
in 2000, or 0.034 in 2004. That number 0.03 is also derived from the review of Sausen et al (2005) although the original
source to which it refers (Stordal et al) gives 0.05 W/m2 (the reason for the reduction by Sausen et al is not clear in their
paper).
The detailed text on this topic in section 2.6 (especially 2.6.3 and 2.6.4) is reasonable and comprehensively balanced. The
problem lies in the summary and the summary figures.
[Govt. of Belgium (Reviewer’s comment ID #: 2003-18)]
SPM-200 A 3:25 3: (figure SPM-2, continued:) Note that in the May draft both the summary of Chapter 2 and TS 2.3 included the helpful
sentence: “Observational studies provide evidence that the net radiative forcing from spreading contrails and their effects on
nearby cirrus cloudiness may be 2-10 times greater than the radiative forcing from persistent linear contrail cover." However
this information has been removed in the new October draft. This is unfortunate, and those sentences or similar information
should be re-inserted to provide the necessary context. This is VERY policy-relevant.
We note that this issue is especially important to Belgium considering the high concentration of aviation cirrus above our
region – for example figure 3-24 of the IPCC Special Report on Aviation (1999) suggested that radiative forcing from this
source in 2050 could be up to 2.5 W/m2 over our country and its neighbours, a similar magnitude to that from global CO2.
Although the specific estimate of forcing for linear contrails has decreased since then, the total effect including spreading
cirrus /AIC could still be of this magnitude.
[Govt. of Belgium (Reviewer’s comment ID #: 2003-19)]
SPM-201 A 3:25 3:33 Figure SPM-2 would be better presented to policymakers if rotated 90degrees to be consistent with the TAR. It would also be
useful for a short discussion of the increased confidence in most of these estimates of RF since the TAR. Also, Panel B as in
technical summary Figure TS-5 should be included in the SPM. The conclusion of this Figure is easily grasped and effectively
conveyed.
[Govt. of Canada (Reviewer’s comment ID #: 2004-16)]
SPM-202 A 3:25 3: Fig. SPM-2: Readability could be improved: The order of the radiative forcing components should be the same as in the text
(by changing either the figure or the text).
[Govt. of European Community (Reviewer’s comment ID #: 2008-3)]
SPM-203 A 3:25 3: Fig. SPM-2: Readability could be improved: Three different colours should be used to distinguish CH4, N2O, and
Halocarbons. Currently CH4 as well as halocarbons are marked in red, which makes the correspondence between these gases
and the numerical values unclear.
[Govt. of European Community (Reviewer’s comment ID #: 2008-4)]
SPM-204 A 3:25 3: Figure SPM-2: Unclear as to why the timescale values in the previous draft were replaced by RF values for the Final Draft.
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