2
Key Issues

Computer models of the coupled atmosphere-land surface-ocean-sea ice system are essential tools for understanding past climates and making projections of future climate resulting from radiative forcing changes, both natural and anthropogenic. Projections of future climate require estimates (e.g. scenarios) of future emissions of long-lived greenhouse gases, aerosols, and other short-lived gases. A number of standard scenarios have been developed for the Intergovernmental Panel on Climate Change (IPCC) assessment process, and the future impacts of these have been explored. As part of the Climate Change Science Program (CCSP) process, updated scenarios of long-lived greenhouse gases and their atmospheric concentrations were developed by the Synthesis and Assessment Product 2.1 team and served as a basis for SAP 3.2.


Understanding the impact of short-lived radiatively active species on future climate is critical and recently has become an active area of research in the reviewed literature. These types of studies encompass a realistic time frame over which available technological solutions can be employed, and focuses on those gas and aerosol species whose future atmospheric levels are also subject to mitigation to control air pollution. Thus the Climate Change Science Program’s (CCSP) Synthesis and Assessment Product (SAP) 3.2 will potentially be very beneficial to all stakeholders of climate change science. The committee commends CCSP and the National Oceanic and Atmospheric Administration (NOAA) for emphasizing the need to address this important topic.


This chapter outlines the major issues that, from the point of view of the review committee, the authors should strongly consider addressing in the revised version of SAP 3.2. In some cases, findings are simply noted without explicit recommendations. In other cases, the committee provides either a direct recommendation or alternatives for the authors to consider as they address the review findings. In subsequent chapters of this report, the committee provides further overarching thoughts on the draft document and then findings and recommendations specific to individual chapters of the draft. Comments regarding key issues follow.


This assessment provides initial information regarding the influence of short-lived radiatively active species on future climate out to 2100. The authors had to work with existing models that did not treat all parameters consistently. Despite this challenge, they have shown that these short lived species are significant in forcing climate.



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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.2, “Climate Projections Based on Emission Scenarios for Long-lived and Short-lived Radiatively Active Gases and Aerosols” 2 Key Issues Computer models of the coupled atmosphere-land surface-ocean-sea ice system are essential tools for understanding past climates and making projections of future climate resulting from radiative forcing changes, both natural and anthropogenic. Projections of future climate require estimates (e.g. scenarios) of future emissions of long-lived greenhouse gases, aerosols, and other short-lived gases. A number of standard scenarios have been developed for the Intergovernmental Panel on Climate Change (IPCC) assessment process, and the future impacts of these have been explored. As part of the Climate Change Science Program (CCSP) process, updated scenarios of long-lived greenhouse gases and their atmospheric concentrations were developed by the Synthesis and Assessment Product 2.1 team and served as a basis for SAP 3.2. Understanding the impact of short-lived radiatively active species on future climate is critical and recently has become an active area of research in the reviewed literature. These types of studies encompass a realistic time frame over which available technological solutions can be employed, and focuses on those gas and aerosol species whose future atmospheric levels are also subject to mitigation to control air pollution. Thus the Climate Change Science Program’s (CCSP) Synthesis and Assessment Product (SAP) 3.2 will potentially be very beneficial to all stakeholders of climate change science. The committee commends CCSP and the National Oceanic and Atmospheric Administration (NOAA) for emphasizing the need to address this important topic. This chapter outlines the major issues that, from the point of view of the review committee, the authors should strongly consider addressing in the revised version of SAP 3.2. In some cases, findings are simply noted without explicit recommendations. In other cases, the committee provides either a direct recommendation or alternatives for the authors to consider as they address the review findings. In subsequent chapters of this report, the committee provides further overarching thoughts on the draft document and then findings and recommendations specific to individual chapters of the draft. Comments regarding key issues follow. This assessment provides initial information regarding the influence of short-lived radiatively active species on future climate out to 2100. The authors had to work with existing models that did not treat all parameters consistently. Despite this challenge, they have shown that these short lived species are significant in forcing climate.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.2, “Climate Projections Based on Emission Scenarios for Long-lived and Short-lived Radiatively Active Gases and Aerosols” The document is not accessible to all intended audiences. The committee finds that the draft is written largely for a technical audience. The intended audiences as outlined in the prospectus also include those people engaged in scientific research, the media, policymakers, and members of the public. Policy and decision-makers in the public sector (e.g., congressional staff) need to understand the implications of these scenarios, in contrast to the research science community, who may be more interested in the actual outcomes. The draft provides relatively little information for an audience of non-technical readers, particularly information that could be used as guidelines for effective communication techniques. In general, the draft would greatly benefit from revisions to make it easier to read. Some specific suggestions follow. The committee finds that the lack of a non-technical executive summary hinders its accessibility to the audiences named in the prospectus. A concise and readable summary of the document, including key findings and recommendations, would enable all audiences -- producers of synthesis and assessment products, scientific researchers, decision-makers, media, and the general public -- to glean the main points and to locate further information that may be of interest to them. The document should include a short executive summary for a non-technical reader, such as congressional staff, local and regional governmental decision makers. The summary should not be merely descriptive, but informative on the main points of the document. The summary should use plain language to describe the goals of the report, the principal findings and why it is critical to understand the impact of short-lived species on future climate. The summary should point out that these types of studies encompass a realistic time frame over which available technological solutions can be employed, and that this study in particular, focuses on those gas and aerosol species whose future atmospheric levels are also subject to mitigation to control air pollution. The summary should define briefly but clearly the line between “long-lived” and “short-lived”, not just described as “(carbon dioxide)” and “(soot)”. An alternative approach could be to add a box consisting of a chart with temporal vs. spatial scales of various species, added by general model resolutions used in such a practice as a reference. A technical summary written for an informed general scientific audience could be included. This could be written using clearly defined technical language (without acronyms) so that the general scientific community, not just atmospheric scientists can understand the goals, findings and relevance of the study. If some chapters are to use technical language, the introduction chapter should contain a section with advice on “How to read this document” – a paragraph that describes the intent of each chapter and its target audience. For instance, the paragraph may state: Chapter 1 provides an introduction to the study and relevant findings from previous studies and is intended to provide all audiences with a general overview. Chapters 2 and 3 provide

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.2, “Climate Projections Based on Emission Scenarios for Long-lived and Short-lived Radiatively Active Gases and Aerosols” detailed technical information about specific models, model runs and trends and are intended primarily for the scientific community. Chapter 4, which is intended for all audiences, provides a summary of the major findings and identifies new opportunities for future research. A clear concise description of the models employed in the study needs to be included. This description should clearly outline the strengths, weaknesses, and critical assumptions for each model. The models should be referred to by what they do, not necessarily by the name of the team that developed it. For example, explain what the GFDL, GISS, and the two NCAR models do when they are referred to in the document. This could either go in the introduction or in Chapter 3. Introductory material is lacking. The draft would be improved if the introduction section provided a clear framework and context for the rest of the document. At present the scope of and motivation for the study are not well explained. The authors could clearly state what this study does and does not address in terms of responses, relative feedbacks and species. In its current form, the transition to technical material is far too abrupt. Specific ways to improve the introduction follow. The authors could define what a scenario is, describe the models used in the study and differentiate between the different types of models. The introduction section could outline the charge to the authors as they perceived it, and clearly define the goals and objectives of the document. As an alternative, this material could be included in a foreword. The foreword or introduction could also state explicitly what the document does not address. There could also be a description of how adequate the adopted methods in the 3 models are in comparison to the current findings regarding the related processes. A discussion explaining the coupling between climate effects of long-and short-lived species is needed in the introduction. A reader with less technical background might wonder specifically about the relationship between the predictions for the well-mixed greenhouse gas scenarios and the predictions where these are combined with the short-lived species. Are effects adding? What changes in climate response, and on what time and space scales, when they are considered together? Details about the models used are lacking. In addition to a general description of model functions, many details about the models used in this assessment are not

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product 3.2, “Climate Projections Based on Emission Scenarios for Long-lived and Short-lived Radiatively Active Gases and Aerosols” stated. Model resolution, inputs, reactive chemical mechanisms, emissions assumptions, and removal mechanisms, and residence times should be more clearly presented. In addition, there is insufficient detail about how the experiments were run. It is not possible to decipher what radiatively active species are predicted (emissions) vs. those prescribed (concentrations) and how they vary temporally and spatially. The technical detail could either be included in a table in Chapter 3 or described in the text of Chapter 3. The more general information about the models used could be included in the introduction (see reviews of specific chapters for suggestions). Details about statistical methods employed are lacking. At present there is no discussion about how statistical significance was determined. The statistical significance of certain trends is discussed and judgments are made about the relative significance, yet there is no description of how this was calculated. This information could be provided in an appendix and should clearly describe the statistical approaches used to determine the relative significance of trends and explain the rationale behind why judgments were made. Many of the figures and captions presented could be improved for ease of interpretation. The figures presented in the report do not have similar scales or projections, which makes comparison of the data difficult. In addition, key points that are made in the discussion are not necessarily obvious from the present figures. For example, it is not entirely clear that the pattern of temperature response to short-lived species is of similar magnitude and distribution as the pattern of long-lived species. A graphic comparison of the temperature response to short-lived species vs the response to long-lived species should be presented.