2021 Hamburg Climate Futures Outlook Assessing the plausibility of deep decarbonization by 2050 Cluster of Excellence Climate, Climatic Change, and Society (CliCCS) About CLICCS Researchers from a wide range of disciplines have joined forces at the Cluster of Excellence CLICCS (Climate, Climatic Change, and Society) to investigate how climate and society will co-evolve. The CLICCS program is coordinated through Universität Hamburg’s Center for Earth System Research and Sustainability (CEN) in close collaboration with multiple partner institutions and is funded by the Deutsche Forschungsgemeinschaft (DFG). About the Outlook In the annual Hamburg Climate Futures Outlook, CLICCS researchers make the first systematic attempt to assess which climate futures are plausible, by combining multidisciplinary assessments of plausibility. The inaugural 2021 Hamburg Climate Futures Outlook addresses the question: Is it plausible that the world will reach deep decarbonization by 2050? DOI: 10.25592/uhhfdm.9104 URL: www.cliccs.uni-hamburg.de/results/hamburg-climate-futures-outlook.html Citation Stammer, Detlef; Anita Engels; Jochem Marotzke; Eduardo Gresse; Christopher Hedemann; Jan Petzold (eds.); 2021. Hamburg Climate Futures Outlook 2021. Assessing the plausibility of deep decarbonization by 2050. Cluster of Excellence Climate, Climatic Change, and Society (CLICCS). Hamburg, Germany. Key Findings There are many possible futures of climate, but Therefore, we find that unless the enabling condi- not all possible futures are also plausible. Because tions of social drivers deliver a radical boost to these climate futures arise from a complex combination drivers in the coming years, reaching worldwide of social and physical dynamics, estimating their deep decarbonization by 2050 is not plausible (see plausibility requires insights from multiple disci- Figure 1). This result implies that, even if techno- plines. The inaugural Hamburg Climate Futures economic options for decarbonization are theoret- Outlook makes the first systematic attempt to ically available, reaching deep decarbonization by assess the plausibility of various climate futures. 2050 constitutes a societal challenge that may well We deem climate futures to be plausible if we ex- be much larger than assumed by many. pect them to unfold with appreciable probability, However, six of the evaluated social drivers given the existing evidence from the physical and show movement toward decarbonization, and social worlds. many drivers offer resources that could be utilized In this Outlook, we combine complementa- by societal actors to strengthen the enabling con- ry assessments of physical and social dynamics, ditions and therefore increase the plausibility of starting with a review of the techno-economic decarbonization in the future. Therefore, partial plausi bility of very high and very low CO2 emissions decarbonization by 2050 remains plausible under scenarios. We find evidence from the scenario lit- our current social assessment. erature which suggests that very high emissions The finding that deep decarbonization by 2050 scenarios are internally inconsistent, due to the is currently not plausible adds to the evidence extent of economic damages from climate change, speaking against the overall plausibility of very the falling cost of clean energy, and limits to recov- low emissions scenarios for the entire twenty-first erable coal reserves. The literature also provides century. Combined with the recently identified, some degree of evidence against the plausibility of narrower range of climate sensitivity, this indicates large-scale deployment of carbon dioxide removal that limiting global surface warming below about technologies, which is a common requirement of 1.7°C by 2100 is currently not plausible. very low emissions scenarios. The new climate sensitivity range, combined Very low emissions scenarios, if they are de- with our techno-economic plausibility assessment, signed to achieve the Paris Agreement’s 1.5°C target, also constrains the upper bound of plausible warm- additionally require decarbonization of the global ing, so that global surface warming above about economy by around the year 2050. Many known 4.9°C by 2100 is likewise currently not plausible. technical or economic options would in principle This assessment of plausible climate futures achieve this decarbonization goal in time. Yet ex- represents a judgement that synthesizes currently isting assessments have only begun to evaluate the available evidence. However, social agency can al- plausibility of the societal transformations neces- ways produce departures from expected trajecto- sary for deep decarbonization. Such a plausibility ries. For deep decarbonization by 2050 to become assessment requires the definition of the political, plausible, much will depend on public pressure via economic, and cultural conditions under which the protests, organized action, and climate litigation, so necessary transformations become plausible. The that governments around the globe are increasing- existing empirical evidence can then be weighed ly driven towards policies that support change, not against this theoretical model of transformation. only via goals and pledges, but by consistent action. We therefore propose the Social Plausibility Furthermore, the complex interrelations within Assessment Framework, a framework that enables social dynamics can produce unforeseen disrup- the analysis of the social drivers of decarboniza- tions, and events like the COVID-19 pandemic can tion, their enabling and constraining conditions, happen at any time. Should additional evidence, and emerging resources and structures that could including that from unexpected events, necessitate influence plausible future developments of these modifications of our assessment, this will be re- drivers. None of the ten social drivers studied show flected in future editions of the Hamburg Climate sufficient movement toward deep decarbonization. Futures ­Outlook. Some of these drivers—namely United Nations climate governance, transnational initiatives, cli- mate-related regulation, climate litigation, fossil fuel divestment, and knowledge production— support decarbonization, but without sufficient momentum to drive deep decarbonization by 2050. For two drivers—climate protests and social move- ments, and journalism—the momentum toward or away from deep decarbonization by 2050 could not be assessed. Two further drivers—consump- tion patterns and corporate responses—currently oppose decarbonization. 4 Hamburg Climate Futures Outlook 2021 ILITY SIB AU PL L N A O I IC T C OM PLA O ON U A S C S S E IB S - I E O L S I N T S Y E H D C E T VERY HIGH EMISSIONS ZATION Reference: approximate net CO emissions in 2020 Figure 1: Plausibility of net global CO2 emissions by 2050. The speedometer shows the wide range of possible emissions in the year 2050 as described in existing emissions scenarios. Emissions could reach net-zero by 2050 (deep decarbonization) or could increase up to a doubling of current emissions (very high emissions). Approximate emissions in 2020 are indicated by the speedometer needle. Here we find a reduced range of plausible emissions scenarios, supported by a techno-economic plausibility assessment (Chapter 3) and a social plausibility assessment ( Chapter 5), indicated by the shaded bands. Increasing emissions are not yet considered in the social plausibility assessment (gray band). 5 Author List PART I: Plausibility assessment of PART II: Social driver assessments climate futures Chapter 8: Chapter 1: Detlef Stammer, Anita Engels, Jochem 8.1: Stefan Aykut, Felix Schenuit, Emilie d’Amico, Marotzke, Eduardo Gresse, Christopher Hedemann, Cathrin Zengerling, Jürgen Scheffran Jan Petzold 8.2: Jürgen Scheffran, Cathrin Zengerling, Chapter 2: Detlef Stammer, Anita Engels, Jochem Andreas Lange, Emilie d’Amico Marotzke, Eduardo Gresse, Christopher Hedemann, Jan Petzold 8.3: Grischa Perino, Johannes Jarke-Neuert, Cathrin Zengerling, Martin Wickel, Felix Schenuit Box 1: Eduardo Gresse, Christopher Hedemann, Jan Petzold 8.4: Grischa Perino, Johannes Jarke-Neuert, Jan Wilkens, Christopher Pavenstädt Chapter 3: Hermann Held, Stefan Aykut, Christopher Hedemann, Chao Li, Jochem Marotzke, 8.5: Cathrin Zengerling, Stefan Aykut, Antje Wiener, Jan Petzold, Uwe Schneider Martin Wickel Box 2: Beate Ratter, Jürgen Oßenbrügge, Peter 8.6: Matthew Johnson, Timo Busch Fröhle, Hermann Held, Michael Köhl, Jan Petzold 8.7: Anita Engels, Alexander Bassen, Timo Busch, Chapter 4: Stefan Aykut, Antje Wiener, Anita Jürgen Beyer, Thomas Frisch Engels, Eduardo Gresse, Christopher Hedemann, Jan Petzold 8.8: Eduardo Gresse, Anita Engels, Junis Sander Chapter 5: Anita Engels, Eduardo Gresse, 8.9: Lars Guenther, Michael Brüggemann Christopher Hedemann, Jan Petzold 8.10: Antje Wiener, Felix Schenuit, Jan Wilkens 5.1: Eduardo Gresse, Christopher Hedemann, Jan Petzold, Anita Engels FAQs: Maike Nicolai 5.2: Stefan Aykut, Alexander Bassen, Jürgen Beyer, Michael Brüggemann, Timo Busch, Emilie d’Amico, Anita Engels, Thomas Frisch, Eduardo Reviewers Gresse, Lars Guenther, Christopher Hedemann, Enric Bas, Jörn Behrens, Leonie Färber, Gregory Johannes Jarke-Neuert, Matthew Johnson, Andreas Flato, Pierre Friedlingstein, Oliver Geden, Sue Lange, Christopher Pavenstädt, Grischa Perino, Grimmond, Jim Hall, Franziska Hanf, Peter Haugan, Jan Petzold, Junis Sander, Jürgen Scheffran, Felix Gabriele Hegerl, Charlotte Huch, Kerstin Jantke, Schenuit, Martin Wickel, Antje Wiener, Jan Wilkens, Andreas