Feasibility of peak temperature targets in light of institutional constraints

nature climate change Article https://doi.org/10.1038/s41558-024-02073-4 Feasibility of peak temperature targets in light of institutional constraints Received: 28 February 2024 Accepted: 20 June 2024 Published online: xx xx xxxx Check for updates Christoph Bertram 1,2 , Elina Brutschin 3, Laurent Drouet 4,5, Gunnar Luderer 2,6, Bas van Ruijven 3, Lara Aleluia Reis 4,5, Luiz Bernardo Baptista 7, Harmen-Sytze de Boer 8, Ryna Cui 1, Vassilis Daioglou 8,9, Florian Fosse 10, Dimitris Fragkiadakis 11, Oliver Fricko 3, Shinichiro Fujimori 3,12,13, Nate Hultman 1, Gokul Iyer 1,14, Kimon Keramidas 10,15, Volker Krey 3, Elmar Kriegler 2,16, Robin D. Lamboll 17, Rahel Mandaroux 2, Pedro Rochedo 18, Joeri Rogelj 3,17, Roberto Schaeffer 7, Diego Silva 13, Isabela Tagomori 8, Detlef van Vuuren 8,9, Zoi Vrontisi 11 & Keywan Riahi 3,19 Despite faster-than-expected progress in clean energy technology deployment, global annual CO2 emissions have increased from 2020 to 2023. The feasibility of limiting warming to 1.5 °C is therefore questioned. Here we present a model intercomparison study that accounts for emissions trends until 2023 and compares cost-effective scenarios to alternative scenarios with institutional, geophysical and technological feasibility constraints and enablers informed by previous literature. Our results show that the most ambitious mitigation trajectories with updated climate information still manage to limit peak warming to below 1.6 °C (‘low overshoot’) with around 50% likelihood. However, feasibility constraints, especially in the institutional dimension, decrease this maximum likelihood considerably to 5–45%. Accelerated energy demand transformation can reduce costs for staying below 2 °C but have only a limited impact on further increasing the likelihood of limiting warming to 1.6 °C. Our study helps to establish a new benchmark of mitigation scenarios that goes beyond the dominant cost-effective scenario design. Global temperature rise is expected to peak around the time when global CO2 emissions reach net-zero levels1,2. Reaching global net-zero CO2 emissions quickly while limiting cumulative emissions therefore lies at the core of achieving the long-term goal of the Paris Agreement3,4. The level of peak temperature and the speed at which it is reached determines the adaptation needs for infrastructure and natural systems5. The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6)6 assessed a large number of scenarios and categorized them based on various metrics, including their projected peak temperature, and found a relatively large number (97) of scenarios still limiting warming to 1.5 °C with no or limited overshoot, defined as peak temperature below 1.6 °C with >50% likelihood. However, the feasibility A full list of affiliations appears at the end of the paper. Nature Climate Change of these have been questioned7,8, and recent emissions increases from 2020 to 20239 have underscored those doubts. In addition, since AR6, continued measurements and advances in climate science have led to a downward correction of remaining carbon budgets for a given peak temperature target10. Furthermore, the understanding of the feasibility of near-term deployment of different mitigation options has improved with continued deployment (or lack thereof). Studies looking into feasibility aspects8,11–13 have also highlighted the difficulty of fast emissions reductions as part of dedicated climate policies, especially in countries that lack the governance and institutional capabilities to enforce regulation in other policy domains (such as taxation or environmental regulation). e-mail: Article https://doi.org/10.1038/s41558-024-02073-4 Table 1 | Treatment of five feasibility dimensions taken from IPCC AR6 in this study Feasibility dimension Geophysical Scenario name components ‘Tech’ Use in study Variables Technological Combined constraint Limit on yearly bioenergy potential, cumulative geological storage of carbon Growth of carbon capture and storage (CCS), nuclear, solar, wind and gas electricity+ generation (maximum deployment levels at different time horizons or annual rate of change of market shares) Institutional Socio-cultural Economic ‘Institutional’ ‘Enablers’ Policy design constraints with three subcases (optimistic, default and pessimistic) (Fig. 1) Enabling factor via reduced constraint Result metric Regional carbon price differentiation and capping of carbon prices (and capping of emissions reduction in default and pessimistic case) Behavioural changes reducing energy demand, especially for high-income activities and countries; reduced constraint on electrification Carbon prices Details and motivation Methods and Supplementary Information sections 2–4. Our study thus explores the feasibility of ambitious peak temperature targets in the Paris Agreement target range, in light of the current state of knowledge, taking into account the observed emissions rebound after the COVID-19 pandemic14 and the improved understanding of feasibility8,11–13 along five relevant dimensions (Table 1): geophysical, technological, institutional, socio-cultural and economic. Using eight state-of-the-art global multi-regional process-based integrated assessment models (IAMs), we explore a set of 20 scenarios (Methods and Supplementary Table 1), including both the cost-effective settings that dominate the IPCC scenario assessments and scenarios with harmonized variation of explicit feasibility considerations (Table 1). The choice for this treatment is informed by previous studies and the participating models’ capabilities and is not fully comprehensive in the sense that additional variables and aspects15 could also be assessed. However, we use a more systematic approach than previous studies’16,17 scenarios and also assess the impact with and without regional differentiation, both of which have been identified as crucial missing pieces in previous studies18–21. We explore the impact of explicit consideration of feasibility constraints on six scenarios that limit peak warming to less than 2 °C with more than 66% likelihood (defined as a 1,000 Gt CO2 carbon budget from 2018) and additionally explore the lowest end of achievable peak temperature under variation of feasibility assumptions in 14 additional scenarios. Complementary to other studies looking at the role of short-lived climate forcers22 or individual energy sectors or technologies23–26, we focus here on total CO2 emissions and especially on the energy sector (details of the modelling in Methods and Supplementary Information sections 2–4). Thus, we only evaluate the warming implication via the link with cumulative CO2 emissions. The models used in this analysis do include other greenhouse gases— including methane (CH4), which is very important for understanding the trajectory of peak temperatures27. However, due to the lack of available evidence regarding the levels of C (...truncated)