The United States Global Change Research Program (USGCRP) is moving toward a sustained assessment process that allows for more fluid and consistent integration of scientific knowledge into the mandated quadrennial National Climate Assessment. As part of this process, the USGCRP is developing the Climate Science Special Report (CSSR), a technical report that details the current state of science relating to climate change and its physical impacts. The CSSR is intended to focus on climate change in the United States and to inform future USGCRP products, including the Fourth National Climate Assessment, Box 1. The Committee to Review the Draft Climate Science Special Report (“The Committee”) evaluated the draft CSSR and this document presents consensus responses to the Statement of Task questions (see the Introduction and Appendix B for the full Statement of Task). Broadly, these questions focus on determination of whether the draft CSSR accurately presents the scientific literature in an understandable, transparent, and traceable way; whether the CSSR authors handled the data, analyses, and statistical approaches in an appropriate manner; and the effectiveness of the report in conveying the information clearly for the intended audience. Responses to the Statement of Task questions in this report include overarching comments that apply to the entire draft CSSR, as well as comments specific to the Executive Summary (ES) and individual chapters. A collection of line comments provided by committee members is also included in Appendix A.
The Committee commends the CSSR authors for producing an impressive, timely, and generally well-written draft report and was impressed with the breadth, accuracy, and rigor of the draft CSSR. The draft CSSR is new and significant in several ways. First, it focuses on changes in the climate system as they affect the United States. Previous reports on this topic, such as those produced by the Intergovernmental Panel on Climate Change (IPCC), have focused on global-scale changes, which may not always translate directly to climate changes occurring in the United States. Second, the report provides a synthesis of recent manifestations of continued climate change observed since the publication of the last IPCC report in 2013, including: a new global temperature record set in 2014, which was broken in 2015 and again in 2016 thanks in part to a strong El Niño event; continued decline in Arctic sea ice; and record high globally averaged atmospheric carbon dioxide which has now passed 400 ppm. Third, the draft CSSR includes several significant advancements that have been made in the science of climate change, including the rapid development of the field of extreme event attribution, and new evidence concerning the Antarctic ice sheet that raises and better quantifies the upper bounds of projected sea level rise. These recently observed changes in Earth’s climate system and substantial advancements in the science of climate change underscore the importance of up-to-date assessments like the draft CSSR. The draft CSSR, by building on previous solid work and incorporating recent advances, provides a valuable update.
In this document, the Committee also provides recommendations for how the draft CSSR could be strengthened. Some notable overarching comments include:
- The key findings throughout the draft CSSR would benefit from greater inclusion of quantification statements, where possible. Values are provided for some key findings (usually related to temperature) and are effective in making the messages more impactful, but more values could be reported.
- The traceable accounts that support the key findings often contain an insufficient level of detail and should be better utilized. The “Description of Evidence Base” provided for many key findings across many chapters list citations to support the finding, but do not summarize the evidence contained within those citations. This low level of detail makes it difficult for readers to understand the evidence base and lessens the impact of the finding.
- The draft CSSR includes many time series datasets and analyzes trends that have been observed or simulated, however the selected time periods for trend analysis are not presented in a consistent manner. The Committee recommends that the CSSR authors standardize the time periods used for the present and historical baseline, wherever possible, and include significance statements and/or ranges in values where appropriate.
- For select chapters, the Committee recommends expanding the discussion of specific topic areas, to better reflect the full breadth of literature and understanding of the subject.
The Committee appreciates the opportunity to provide recommendations for this important draft report and notes that attention to the suggestions provided here will further enhance this document and contribute positively to the foundational role the draft CSSR will play in the forthcoming National Climate Assessment.