Letter to Congressman Brown from Dr. Robert Corell, March 4, 1996 . United States Department of State Bureau of Oceans and International Washington, D.C. 20520 July 7, 1995 Sir John Houghton Dr. L.G. Meira Filho IPCC Working Group I c/o Technical Support Unit The Hadley Centre Meteorological Office Bracknell RG12 2SY United Kingdom Dear Sirs: Attached please find the United States' comments on the text of the individual chapters of the IPCC Working Group I Draft Second Assessment Report. At this time, we have not completed our internal review of the draft Summary for Policymakers. We will fax and e-mail the text of our comments on that section to you early the week of July 10th. While the U.S. comments are extensive, reflecting the importance we attach to this assessment, we believe the present report, although uneven, is a good start on a useful final product. Our comments are reflective of the broad range of input we sought in their development: we circulated the draft text to an extensive group of experts both inside and outside of the U.S. government. Comments received through this process were, where appropriate, incorporated into the U.S. recommendations included in this package although we have also forwarded a full set of all comments received as part of our review. - In addition to the attached volume of material, we note the following general points with respect to the document: Each chapter should stand on its own. This assessment can be expected to serve as an important reference for many years to come, and will need to present data in a manner that does not rely on the reader's familiarity with or access to additional literature to provide an understanding of the material. This is not now the case for Chapter 2 which will consequently require revision. - The major new result in this assessment is the -2 change into analyses of greenhouse gas forcing. The The chapters on ecosystems need to be coordinated with the It is critical for this assessment to base its results on published and generally available information; the conclusions must be supported by reviewed literature and findings. This "Second Assessment Report" (with its 1995 or 1996 publication date) must emphasize the state of the science in the 1990's. One case where information is not entirely updated is the global carbon budget for the 1990's; there are other instances where the Assessment could be substantially more up-to-date as well. · The United States appreciates the enormous time and effort that has gone into the preparation of this Report. Through these comments we hope to assure that the final product is not only accurate, but of value to the policy community. We look forward to receipt of the revised draft text prior to the WG I Plenary session, and wish the IPCC Bureau and the lead authors every success in their continued efforts to prepare this important document for publication. Sincerely, Rafe Pomerance Deputy Assistant Secretary Attachments: 1. U.S. comments on draft chapters 2. 3. Computer disk containing all U.S. comments (in MSWord 5.1 Compiled comments from all reviewers U.S. COMMENTS: CHAPTER 6: CLIMATE MODELS - PROJECTIONS OF FUTURE CLIMATE UNITED STATES IPCC Working Group I Second Draft Assessment Washington DC 20520 USA 202-647-4069 202-647-0191 office@usgcrp.gov Technical Contact: Michael MacCracken Office of the USGCRP 300 D St. SW, Suite 840 Washington DC 20024 USA 202-651-8250 202-554-6715 office@usgcrp.gov GENERAL COMMENTS ON CHAPTER 1. The writing of the first part of this chapter (the Summary and Section 6.2) needs to be improved for clarity, accuracy, and completeness of references. In Section 6.2 especially, the discussion is often in the form of a series of disconnected sentences, each of which may state some fact or thought, but which are not always logically related to one another. Thus the section appears to be a collection of miscellaneous information on model predictions rather than a well-focused discussion of model results and what they mean. Some of the information given is inconsistent with other information. References are not given for some of the factual statements. See also the Specific Comments below. In contrast, Sections 6.4 through 6.6 are well written and well referenced. UNITED STATES 2. In several places, the chapter cites results which are unpublished. We take the position that unpublished results should not be used in this report unless the actual publication is pending (i.e., accepted for publication in an appropriate journal, with an announced date) or unless there is absolutely no alternative published information available. In the latter case, wording should clearly indicate that such results are preliminary, have not been reviewed, and are subject to change. A related issue concerns the availability of the details of model simulations/predictions to scientists who ask to see them. We believe that detailed information should be generally available for all model results that are cited in the report. UNITED STATES 3. There is inadequate referencing of material in other chapters, especially Chapter 5. Some examples are contained in the Specific Comments below. UNITED STATES 4. Section 6.7 seems inappropriate for this chapter and should be placed elsewhere, probably in Chapter 5. It is concerned with making improvements to models, not with projections of future climate. UNITED STATES 5. Section 6.A.2 of the "Annex" (or its revised version) should be moved to Chapter 2, as suggested in the note at the beginning of the section. There is a difference of opinion about Section 6.A.3. Since this section is concerned with projections made with energy-balance models, whereas the rest of Chapter 6 uses GCM projections or regional model projections keyed to GCMs, and since the discussions in 6.A.2 and 6.A.3 are intimately related, one line of thought is to move the entire Annex material to Chapter 2. The other line of thought is to keep the material of 6.A.3 in Chapter 6 because it does consist of climate projections. If 6.A.3 or its revision is retained in Chapter 6, detailed reference to the material moved to Chapter 2 will be necessary. UNITED STATES SPECIFIC COMMENTS ON CHAPTER Page 6.1: The Summary contains no references. Therefore, the statements it contains should be cross-referenced to the appropriate sections where the references are (or should be) given (cf. the Executive Summary of the Summary for Policymakers). UNITED STATES Page 6.1, line 5 (and several other places in the chapter): "a rate of about 0.1 to 0.2 C per decade" should either be "an average rate of 0.1 to 0.2 C per decade, sustained over several decades" or "an average rate of 1 to 2 C per century." The latter wording is preferred. The present wording is misleading to some readers in that they interpret it to mean. e.g., 0.1 to 0.2°C each decade, thereby neglecting the role of natural climate variability on these time scales. UNITED STATES Page 6.1, lines 11-13: The numbers given are inconsistent here. A warming rate of between 0.2 and 0.35 C per decade, sustained for approximately 50 years (to the middle of the next century) would produce a total warming of between 1.0 and 1.8 C, not between 2 and 4 C as stated. UNITED STATES Page 6.1, lines 37-38: An inconsistency similar to the preceding appears here. UNITED STATES Page 6.5, lines 2 and 3: This sulphate aerosol forcing change implies that annual unscrubbed coal combustion will double by 2050. The realism of such a scenario is doubtful. UNITED STATES Page 6.5, Section 6.2.3: The discussions of climate changes due to greenhouse and/or aerosol forcing are mostly qualitative here. Inclusion of quantiative estimates of these changes, with some indication of their uncertainty or "range", should be included in this section. One way to do this is to include a sidebar ("box"), such as the one on page 35 of the Summary for Policymakers, that summarizes the quantitative information. UNITED STATES Page 6.6, line 5: The discussion here is a bit confused. A precipitation increase in the equatorial Pacific and a decrease in the ITCZ are contradictory statements. UNITED STATES Page 6.6, line 25ff: There is insufficient information on the source function assumed for aerosol forcing. A more detailed explanation is needed on how the forcing was prescribed in the model simulations. Was it kept constant in time? If not, what was the temporal change of the forcing? UNITED STATES Page 6.6, line 30: At this point, mention at least should be made of the other known aerosol forcings (biomass burning, Saharan dust, and indirect forcing due to cloud enhancement) and ranges of estimates of these forcings, even though they may not be included in the following model simulations (which should also be clearly mentioned here, if that is the case). UNITED STATES Page 6.6, line 32: The Chapter 2 figure for sulphate aerosol forcing is 0.5 W/m2. The expression "with a factor of two uncertainty" is ambiguous (does it mean uncertainty ranging from 0.2 to 0.8, or 0.2 to 0.6, or something else?) and should not be used. Instead, a definite range of uncertainty should be given, e.g. "0.4+/-0.2", or "0.4 with uncertainty ranging from 0.2 to 0.8." UNITED STATES Page 6.7, line 1: A reference should be given for the information stated in this paragraph. Page 6.7, line 10: The two references given here are unpublished manuscripts and should not be included (See General Comments). This paragraph is poorly written and contains very tentative results. It would be best to delete it from the report. UNITED STATES Page 6.8, line 5: "1950-1980 to 1980-1990", what does this mean? Shouldn't it be just "1950 to 1990"? UNITED STATES Page 6.8 lines 7-8: "The direct effect of sulphate aerosols is to cool climate. This leads to a reduction in evaporative cooling of the earth's surface, and an increase in global mean diurnal range.." This seems to misrepresent, or at least distort, the underlying science. It is true that a cooling climate gives larger diurnal range. However, this has less to do with evaporative cooling than with the longwave radiative effect of the lower concentrations of water vapor that result. The text leaves out the point that the direct radiative effect of the sulfate aerosol is to reduce the diurnal range by its reduction of daytime solar heating; indeed, Hansen's paper emphasizes the latter. Presumably the relative importance of the increase versus decrease of diurnal range depends on how much the temperature has changed, which in turn depends on climate sensitivity, transient effects., etc. UNITED STATES Page 6.8, Section 6.2.5: This section continues the pattern of rambling qualitative discussion that is characteristic of Section 6.2. Quantitative values should be part of the discussion where appropriate. Some of the results are apparently from Tables 6.2.3 and 6.2.4, but they are not referenced. There is better, but incomplete, referencing of the relevant Figures. This section confuses continental and regional scale results by using the term"regional" in both cases. UNITED STATES Page 6.9 line 34ff: A general conclusion that warmer climates give more soil moisture in high latitudes is suspect, based on the fact that the GCMs have made little or no effort to include anything reasonable for soil hydrology in high latitudes. None of the models appear to have any treatment of soil freezing, which is a dominant control on winter soil moisture dynamics. Furthermore, in many if not most high latitude regions, during the winter the soil water storage is already near maximum, so it would be difficult for it to increase. Thus, without much more attention given to development and validation of the GCM treatments of high latitude soil moisture, the general conclusion stated should be viewed with suspicion. UNITED STATES Page 6.9, line 34: This sentence contains an apparent contradiction: "...all models"..."though in some"... UNITED STATES Page 6.10, line 18ff: The "surprising" result noted here is probably related to the summer-dry character of the climate in this region and to the likelihood that soil moisture is strongly depleted even in the control runs, thereby minimizing its further impact on climate change simulations. The relevant control runs should be examined, and a statement such as that above added to the text if warranted. UNITED STATES Page 6.10, Section 6.26: The authors make a bold claim that "there is no evidence from these simulations that the use of flux adjustments are substantially distorting the model's response to increases in greenhouse gases.". This is a highly relevant conclusion for policy purposes. As it currently reads, however, this statement appears simply to be an opinion being expressed by the authors. Others may disagree with this statement. It should be remembered that the purpose of an IPCC report is to assess the literature, not to create it. For this reason, the conclusions expressed in this report should be fully documented by reference to the literature. For example, the magnitude of flux adjustments, as noted by Murphy and Mitchell (1995) for the western oceanic gyres, should be explicitly stated here. Chapter 5 contains some very useful information regarding this subject. For example, on page 5.5, lines 3-4, it states "Flux correction is strictly justified only when the corrections are |