Among the many possible climatic futures, not all are plausible. The purpose of the second Hamburg Climate Futures Outlook is to systematically assess the plausibility of a climate future in which the Paris Agreement temperature goals are attained, namely holding global warming to well below 2°C and, if possible, to 1.5°C, relative to pre-industrial levels. Assessing plausible climate futures involves addressing a complex combination of social and physical dynamics. We establish the CLICCS Plausibility Assessment Framework to guide and integrate social and physical plausibility assessments. We analyze the dynamics of ten dominant social drivers of decarbonization and of six select physical processes of public interest.

Our key findings are:

• None of the ten social drivers support deep decarbonization by 2050, as in the 2021 Outlook. Seven social drivers (i.e., United Nations climate governance, transnational initiatives, climate-related regulation, climate protests and social movements, climate litigation, fossil-fuel divestment, and knowledge production) support decarbonization, but not deep decarbonization by 2050. Two social drivers (i.e., corporate responses and consumption patterns) continue to undermine the pathways to decarbonization, let alone deep decarbonization. One driver (i.e., media) remains ambivalent insofar as its dynamics are volatile, both supporting and undermining decarbonization.
  
  
• The dynamics of virtually all social drivers of decarbonization are significantly affected by the short-, medium-, and long-term consequences of the COVID-19 pandemic and Russia’s invasion of Ukraine. Recovery programs and measures to relieve the socioeconomic impacts of the COVID-19 outbreak have locked in fossil-fuel dependence, making transformations to deep decarbonization less plausible than previously expected. There is still insufficient empirical evidence to conclude whether Russia’s invasion of Ukraine in the long term will lead to or undermine worldwide efforts for reducing dependence on fossil fuels and toward faster energy transitions.
  
  
• Three physical processes (i.e., polar ice-sheet melt, Arctic sea-ice decline, and regional climate change and variability) barely influence global mean temperature and thus do not affect the plausibility of attaining the Paris Agreement temperature goals. Three physical processes (i.e., permafrost thaw, AMOC instability, and Amazon Forest dieback) can moderately affect the global mean temperature, thus moderately inhibit the plausibility of attaining the Paris Agreement temperature goals. Global-warming-induced changes in the dynamics of all six physical processes have extensive effects on regional hydrological cycles, ecosystems’ resilience, or communities’ well-being.
  
  
• Failing to attain the Paris Agreement temperature goals has three main implications for physical processes. First, drastic or abrupt changes in the 21st century in the polar ice sheet and regional climate are plausible, but are not plausible for the Arctic sea ice or the AMOC. Second, future development of deforestation activities is a fundamental condition that can either enable or constrain the plausibility of large-scale dieback of the Amazon Forest. Third, uncertainties about the behavior of permafrost carbon preclude us from assessing the plausibility of drastic changes in permafrost thaw within the 21st century. However, we can exclude that permafrost thaw will lead to runaway warming.
  
  
• The joint social and physical plausibility assessments reveal that the prospects of attaining the Paris Agreement temperature goals through deep decarbonization are fundamentally shaped by the interaction of social and physical dynamics. The social driver assessments demonstrate that human agency has a large potential to shape the way climate futures will evolve, highlighting a series of conditions and resources for societal transformation required for the climate future scenario to become plausible. However, the assessments also show that human agency is strongly shaped by injustices and social inequalities, which inhibit social dynamics toward deep decarbonization by 2050.
  
  
• We address the interconnections between climate mitigation and climate adaptation and introduce key concepts and guiding principles toward a Sustainable Adaptation Plausibility Framework. This framework will be further developed in upcoming editions of the Hamburg Climate Futures Outlook.
  
  
• In light of these findings, we conclude that reaching worldwide deep decarbonization by 2050 is currently not plausible, given the observable trajectories of social drivers. The select physical processes of public interest only moderately, if at all, inhibit the plausibility of attaining the Paris Agreement temperature goals, although they can substantially modify the physical boundary conditions for society. Meeting the 1.5°C Paris Agreement temperature goal is not plausible, but limiting the global temperature rise to well below 2°C can become plausible if ambition, implementation, and knowledge gaps are closed.

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Key Findings: the most importand findings at a glance (PDF 1,5 MB)

Publication

Engels, Anita; Jochem Marotzke; Eduardo Gonçalves Gresse; Andrés López-Rivera; Anna Pagnone; Jan Wilkens (eds.); 2023. Hamburg Climate Futures Outlook 2023. The plausibility of a 1.5°C limit to global warming—Social drivers and physical processes. Cluster of Excellence Climate, Climatic Change, and Society (CLICCS). Hamburg, Germany.

• ePUB version of the publication (4 MB): Hamburg Climate Futures Outlook 2023:  The plausibility of a 1.5°C limit to global warming - Social drivers and physical processes.
• PDF version of the publication (236 pages, 6 MB): Hamburg Climate Futures Outlook 2023: The plausibility of a 1.5°C limit to global warming - Social drivers and physical processes.

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Review copies can be ordered from Franziska Neigenfind.

DOI: 10.25592/uhhfdm.11230

The material below is the property of CLICCS/University of Hamburg and is protected by intellectual property laws. Please respect the copyright.

Climate future scenario. The circle represents the climate future scenario of the 2023 Outlook, which combines deep decarbonization by 2050 with the Paris Agreement ... Download figure as jpeg file 

Figure 1: The circle represents the climate future scenario of the 2023 Outlook, which combines deep decarbonization by 2050 with the Paris Agreement temperature goals. The social plausibility of deep decarbonization is central to limiting global surface temperature increase to 1.5°C above pre-industrial times, whereas the physical plausibility is assessed also with respect to a global surface temperature increase of below 2°C.

CLICCS Plausibility Assessment Framework. The figure illustrates how social drivers of decarbonization and physical processes are  embedded ... Download figure as jpeg file 

Figure 2: The figure illustrates how social drivers of decarbonization (in the gray arrow) and physical processes (all around the arrow) are embedded, act, and exist within the physical boundary conditions (encircled in the lines). Social drivers and physical processes influence each other and affect both the global opportunity structure (blue area in the center) and the physical boundary conditions. Thus, both are not static but dynamic. On the right side, the figure shows the chosen climate future scenario, which combines deep decarbonization by 2050 with the Paris Agreement temperature goals. The assessment of the social drivers’ and physical processes’ dynamics, and their enabling and constraining conditions, leads to a conjecture about the plausibility of the selected climate future scenario (for details, see Section 6.1 and 6.2).

Path toward climate goals. The plausibility assessments indicate where social drivers and physical processes position themselves on the path toward the climate future ... 
Download figure as jpeg file 

Figure 3: The plausibility assessments indicate where social drivers and physical processes position themselves on the path toward the climate future scenario in which global climate mitigation goals are attained. Drivers and processes situated in the gray area are ambivalent with regard to reaching deep decarbonization by 2050 (social drivers), or do not affect the plausibility of attaining the Paris Agreement temperature goals (PAtg) (physical processes). Several social drivers are positioned closer to the goals as they support decarbonization (light blue area). However, the path toward climate goals is obstructed by physical processes which moderately inhibit the plausibility of attaining the Paris Agreement temperature goals (light red hexagon), and even more by social drivers which inhibit decarbonization (red hexagon). Currently, no social driver positions itself on the path of supporting deep decarbonization. More information can be found in Tables 1 and 2 and in the assessments in Chapter 6.

Percentage of climate change related articles ... of the total number of articles published in leading German online news outlets. Download figure as jpeg file 

Figure 7: Monthly mean percentage of climate change related articles of the total number of articles published in leading German online news outlets. Source: Brüggemann and Sadikni (2022), Online Media Monitor on climate change (OMM).

Daily number of tweets related to climate change ... in English and German showing at least five retweets. Download figure as jpeg file 

Figure 8: Daily number of tweets related to climate change in English and German showing at least five retweets.
Source: Brüggemann and Sadikni (2022), Online Media Monitor on climate change (OMM).

Thawing permafrost effects on carbon budget. With increasing temperature in response to fossil-fuel emissions in the climate stabilization scenario ... Download figure as jpeg file 

Figure 9: Left y-axis: global carbon emissions, right y-axis: global temperature change. With increasing temperature in response to fossil-fuel emissions in the climate stabilization scenario (red line), carbon emissions due to thawing permafrost (blue line) lead to an additional increase in temperature reducing the remaining carbon budget for a given temperature target. Permafrost emissions continue on long timescales after fossil-fuel emissions are reduced to zero (on a short timescale), implying either warming exceeding the target temperature or the necessity of negative carbon emissions for climate stabilization.

Key Findings (PDF, 1,5 MB)

Tables

• Table 1: Summary of social plausibility assessments (PDF, 9 MB)
• Table 2: Summary of physical plausibility assessments (PDF, 4 MB)

Complementary Aspects

• How the Hamburg Climate Futures Outlook corresponds to other global assessments of climate futures (BOX I)
  (Download Box I as PDF, 1,6 MB)
  
  
• The Planck response and the stabilization of the global surface temperature (BOX II)
  (Download Box II as PDF, 1 MB)
  
  
• Implications of Russia’s invasion of Ukraine for decarbonization (BOX III)
  (Download Box III as PDF,  1,6 MB)
  
  
• Technology and the Plausibility of Climate Futures (BOX IV)
  (Download Box IV as PDF, 1,6 MB)

Aykut, Stefan C. (Chapter 2, 2.1, 2.2, 6.1.1, 6.1.5, Box 4, Table 1)

Bähring, Jill (Chapter 6.1.5, Table 1)

Bassen, Alexander (Chapter 6.1.7, Table 1)

Beer, Christian (Chapter 6.2.1, Table 2)

Brovkin, Victor (Chapter 6.2.1, 6.2.5, Table 2) 

Brüggemann, Michael (Chapter 6.1.9, Table 1)

Busch, Timo (Chapter 6.1.6, 6.1.7, Table 1)

Commelin, Solange (Chapter 6.1.4, 6.1.6, 6.1.7, Table 1)

D`Amico, Emilie (Chapter 6.1.1, 6.1.2, 6.1.5, Table 1)

De Verse, Philipp (Chapter 6.2.1, Table 2)

Engels, Anita (Chapter 1, 2, 2.1, 3, Box 1, Table 1)

Frisch, Thomas (Chapter 6.1.2, 6.1.6, 6.1.7, Table 1) Reviewer

Fröhlich, Christiane (Chapter 6.2.3, Table 2)

Fünfgeld, Anna (Chapter 6.1.1, Table 1)

Gonçalves Gresse, Eduardo (Chapter 1, 2, 2.1, 2.2, 3, 4, 4.1, 4.2, 4.4, 5, 6.1.8, Box 1, Table 1)

Guenther, Lars (Chapter 6.1.9, Table 1)

Guillén Bolaños,Tania (Chapter 4, 4.1, 4.4)

Hanf, Franziska S. (Chapter 4, 4.1, 4.3, 4.4)

Hawxwell, Tom (chapter 4, 4.2)

Held, Hermann (Chapter 4)

Hoffmann, Peter (Chapter 4, 4.2, 4.3, 4.4)

Huang-Lachmann, Jo-Ting (Chapter 4, 4.1, 4.2)

Huch, Charlotte (Chapter 6.1.4, Table 1)

Jantke, Kerstin (Chapter 4, 4.2, 4.3)

Jarke-Neuert, Johannes (Chapter 6.1.3, 6.1.4, Table 1)

Johnson, Matthew (Chapter 6.1.6, Table 1)

Kleinen, Thomas (Chapter 6.2.1, Table 2)

Kleinen-von Königslöw, Katharina (Chapter 6.1.9, Table 1)

Knoblauch, Christian (Chapter 6.2.1, Table 2)

Köhl, Michael (Chapter 6.2.5, Table 2)

Kutzbach, Lars (Chapter 6.2.1, Table 2)

Langendijk, Gaby S. (Chapter 4, 4.2, 4.3)

Li, Chao (Chapter 6.2.2, 6.2.3. 6.2.4, Table 2)

López-Rivera, Andrés (Box 1, Chapter 1, 2, 2.1, 2.2, 3, 5, 6.1.10, 6.2.2, Table 1, 2)

Marotzke, Jochem (Chapter 1, 2, 2.1, 2.3, 5, 6.2.2, 6.2.4, Box 1, 2, Table 2)

Monsuela, Cleovi (Chapter 6.1.4, Table 1)

Müller, Franziska (Chapter 6.1.3, 6.1.7, Table 1)

Neuburger, Martina (Chapter 4, 4.3, 6.2.5, Table 2)

Neumann, Manuel (Chapter 6.1.7, Table 1)

Notz, Dirk (Chapter 6.2.2, 6.2.3, 6.2.4, Table 2)

Pagnone, Anna (Chapter 2, 2.1,2.3, 3, 5)

Pavenstädt, Christopher N. (Chapter 6.1.4, Table 1)

Pein, Johannes (Chapter 4, 4.2, 4.3, 4.4)

Perino, Grischa (Chapter 6.1.3, Table 1)

Reveco Umaña, Cristóbal (Chapter 4, 4.1, 4.2, 4.3)

Rödder, Simone (Chapter 2, 2.1, 2.3, 6.2.1, Table 2)

Rothe, Delf (Chapter 6.1.10, Table 1)

Rötzel, Theresa (Chapter 6.1.6, Table 1)

Scheffran, Jürgen (Chapter 4, 4.2, 6.1.2, Box 3, Table 1)

Schenuit, Felix (Chapter 6.1.10, 6.1.3, Table 1)

Schneider, Uwe A. (Chapter 4, 4.2, 4.3)

Schröder, Ursula (Box 3)

Schrum, Corinna (Chapter 4, 4.1, 4.2, 4.3, 4.4)

Seifert, Rita (Chapter 4, 4.3)

Sillmann, Jana (Chapter 2, 2.1, 2.3, 4, 4.3, 6.2.6, Table 2)

Soans, Erika (Chapter 6.1.8, Table 1)

Struve, Svenja (Chapter 6.1.8, Table 1)

Wickel, Martin (Chapter 4, 4.1, 4.3, 6.1.3, Table 1)

Wiener, Antje (Chapter 2, 2.1, 2.2, 6.1.5, 6.1.10, Table 1)

Wilkens, Jan (Chapter 1, 2, 2.1, 2.2, 5, 6.1.4, 6.1.10, Box 1, 2, 4, Table 1)

Volger, Anselm (Box 3)

Zengerling, Cathrin (Chapter 6.1.2, 6.1.3, 6.1.5, Table 1)

Reviewers

Philipp Degens, Gregory Flato, Thomas Frisch, Franziska S. Hanf, Peter M. Haugan, Gabriele Hegerl, Hermann Held, Peter Hoffmann, Lars Kutzbach, Heena Patel, Simone Pulver, Ingrid van Putten, Simone Rödder, Delf Rothe, Jana Sillmann, Stefanie Trümper, Detlef van Vuuren

Citation

Engels, Anita; Jochem Marotzke; Eduardo Gonçalves Gresse; Andres López-Rivera; Anna Pagnone; Jan Wilkens (eds.); 2023. Hamburg Climate Futures Outlook 2023. The plausibility of a 1.5°C limit to global warming - Social drivers and physical processes. Cluster of Excellence Climate, Climate Change, and Society (CLICCS). Hamburg, Germany.