Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

Evaluation and communication of the relative degree of certainty in assessment findings are key cross-cutting issues for the three Working Groups of the Intergovernmental Panel on Climate Change. A goal for the Fifth Assessment Report, which is currently under development, is the application of a common framework with associated calibrated uncertainty language that can be used to characterize findings of the assessment process. A guidance note for authors of the Fifth Assessment Report has been developed that describes this common approach and language, building upon the guidance employed in past Assessment Reports. Here, we introduce the main features of this guidance note, with a Climatic Change (2011) 108:675-691

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The Reasons for Concern (RFC) framework communicates scientific understanding about risks in 1 relation to varying levels of climate change. The framework, now a cornerstone of the IPCC 2 assessments, aggregates global risks into five categories as a function of global mean temperature 3 change (GMT). We review the RFC's conceptual basis and the risk judgments made in the most recent 4 IPCC report, confirming those judgments in most cases in the light of more recent literature and 5 identifying their limitations. We point to extensions of the framework that offer complementary 6 climate change metrics to GMT and better account for possible changes in social and ecological 7 system vulnerability. Further research should systematically evaluate risks under alternative scenarios 8 of future climatic and societal conditions. 9The RFC framework was developed in the IPCC Third Assessment Report (TAR) to inform discussions 10 relevant to implementation of Article 2 of the UN Framework Convention on Climate Change (UNFCCC). 11Article 2 presents the Convention's long-term objective of avoiding "dangerous anthropogenic 12 interference with the climate system." The RFC framework and the associated "Burning Embers" 13 diagram illustrating authors' risk judgments have since been widely discussed and used to inform policy 14 decisions. For example, they informed a recent dialog between Parties to the UNFCCC and experts 1, 2 on 15 the adequacy of the long-term goal of avoiding a warming of 2°C relative to pre-industrial, contributing 16 to a strengthening of that goal in the recent Paris Agreement 3 . Elaborations of the Burning Embers have 17 been used to represent climate impacts and risks at the regional level 4 and for specific systems (e.g., 18ocean systems 5 ). 19This article reviews the conceptual basis for the RFCs (Box 1) and offers an explanation of the reasoning 20 behind associated risk judgments that is complementary to, but goes beyond, the treatment in the IPCC 21Fifth Assessment Report 6 . We focus explicitly on the evidence base for transitions from one risk level to 22 the next, incorporate post-AR5 literature in those discussions, and offer thoughts about limitations of 23 the subjective judgments behind each RFC. We also improved the synthesis of RFC-related material 24 across AR5, and in turn provide both a clearer connection to evidence from AR5 that supports the RFC 25 judgments, as well as a comparison of the RFCs to similar approaches employing metrics other than 26 GMT for characterizing risk. Perhaps most importantly, we consider improvements in the framework, 27 particularly emphasizing the dynamic nature of exposure and vulnerability, two key components of risk 28 not sufficiently covered in the current approach. 29 TEXT BOX 1: Conceptual Basis 30The Reasons for Concern (RFCs) reported in AR5 are: 31 Types of risk included in each category are discussed in the next section. The categories share an 37 emphasis on going beyond changes in biophysical systems to possible consequences for society and 38...