Net zero-emission pathways reduce the physical and economic risks of climate change

Articles https://doi.org/10.1038/s41558-021-01218-z Net zero-emission pathways reduce the physical and economic risks of climate change Laurent Drouet 1 ✉, Valentina Bosetti 1,2, Simone A. Padoan3, Lara Aleluia Reis 1, Christoph Bertram 4, Francesco Dalla Longa 5, Jacques Després 6, Johannes Emmerling 1, Florian Fosse 6, Kostas Fragkiadakis 7, Stefan Frank 8, Oliver Fricko 8, Shinichiro Fujimori 8,9,10, Mathijs Harmsen 11,12, Volker Krey 8, Ken Oshiro 9, Larissa P. Nogueira 5, Leonidas Paroussos 7, Franziska Piontek 4, Keywan Riahi 8,13, Pedro R. R. Rochedo 14, Roberto Schaeffer 14, Jun’ya Takakura 10, Kaj-Ivar van der Wijst 11,12, Bob van der Zwaan 5,15,16, Detlef van Vuuren 11,12, Zoi Vrontisi 7, Matthias Weitzel 6, Behnam Zakeri 8 and Massimo Tavoni 1,17 Mitigation pathways exploring end-of-century temperature targets often entail temperature overshoot. Little is known about the additional climate risks generated by overshooting temperature. Here we assessed the benefits of limiting overshoot. We computed the probabilistic impacts for different warming targets and overshoot levels on the basis of an ensemble of integrated assessment models. We explored both physical and macroeconomic impacts, including persistent and non-persistent climate impacts. We found that temperature overshooting affects the likelihood of many critical physical impacts, such as those associated with heat extremes. Limiting overshoot reduces risk in the right tail of the distribution, in particular for low-temperature targets where larger overshoots arise as a way to lower short-term mitigation costs. We also showed how, after mid-century, overshoot leads to both higher mitigation costs and economic losses from the additional impacts. The study highlights the need to include climate risk analysis in low-carbon pathways. M ultiple mitigation trajectories are consistent with climate stabilization1, which may lead to different climate change risks2,3. One important feature of the pathways is the extent to which temperature is allowed to temporarily exceed a given target, commonly known as ‘overshoot’. Given historical emissions, stringent long-term temperature targets, such as limiting the temperature increase to 1.5 °C in 2100, often entail temporary temperature exceedance to be compensated by net negative carbon emissions in the second half of the century4. These pathways are the outcome of Integrated Assessment Models (IAMs) constrained to meet fixed-year targets, often for 21005–7. The extent of overshoot is a function of many variables defining how rapidly human systems can be transformed, including socioeconomic and technological progress variables. For example, the assumptions of bioenergy technologies with carbon dioxide capture and geologic storage vary substantially across models8. It is also rooted in the choice of normative parameters. For example, time discounting consistent with proper consideration of future generations reduces overshoot and reliance on carbon dioxide removal9. Finally, the overshoot might depend on the way scenarios are designed and executed10. To overcome some of the limitations of end-of-century target scenarios, a scenario design has been recently proposed. It caps the peak temperature reached during the century, limiting ‘net zero’ carbon emissions11. One reason for the temperature overshoot is that, usually, cost-minimizing emission pathways don’t account for the climate benefits associated with different temperature trajectories. Detailed process IAMs, such as those providing input to the IPCC12, are tools primarily designed for mitigation analysis. As such, they don’t take into account that overshoot trajectories lead to worse heat extremes than no-overshoot trajectories13. Benefit–cost IAMs include climate impacts, but lack mitigation strategy details and focus solely on monetary impacts14. Thus, their capacity to evaluate the full trade-offs implied by different intertemporal mitigation trajectories compliant with given climate stabilization targets is limited. Still, recent benefit–cost studies have highlighted the economic inequality repercussions in low-temperature cases15. Here we combined mitigation pathways with a postprocessing analysis of both physical and economic climate impacts, employing advanced statistical approaches. We used a large set of scenarios generated by a multimodel ensemble of nine leading detailed process IAMs, which explore end-of-century budget scenarios (where overshoot is allowed) versus net zero emission constrained budget scenarios. RFF-CMCC European Institute of Economics and the Environment, Centro Euro-Mediterraneo sui Cambiamenti Climatici, Milan, Italy. 2Department of Economics and IGIER, Bocconi University, Milan, Italy. 3Department of Decision Sciences, Bocconi University of Milan and Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC), Milan, Italy. 4 Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam, Germany. 5TNO Energy Transition, Amsterdam, the Netherlands. 6European Commission, Joint Research Centre (JRC), Seville, Spain. 7E3Modelling, Athens, Greece. 8International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria. 9Department of Environmental Engineering, Kyoto University, Kyoto, Japan. 10National Institute for Environmental Studies (NIES), Tsukuba, Japan. 11PBL Netherlands Environmental Assessment Agency, The Hague, the Netherlands. 12Copernicus Institute for Sustainable Development, Utrecht University, Utrecht, the Netherlands. 13Graz University of Technology, Graz, Austria. 14CENERGIA/COPPE, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. 15University of Amsterdam, Amsterdam, the Netherlands. 16 Johns Hopkins University, Bologna, Italy. 17Department of Management, Economics and Industrial Engineering, Politecnico di Milano, Milan, Italy. ✉e-mail: 1 1070 Nature Climate Change | VOL 11 | December 2021 | 1070–1076 | www.nature.com/natureclimatechange Articles NaTurE CLiMaTE CHangE a b AIM/CGE V2.2 COFFEE 1.1 MESSAGEix−GLOBIOM_1.1 40 0.16 °C CO2 emissions (GtCO2) 0.09 °C 0.05 °C 1.0 600 GtCO2 500 GtCO2 700 GtCO2 20 End of century 0 Net zero –20 2025 POLES ENGAGE TIAM−ECN 1.1 WITCH 5.0 End of century 0.13 °C 0.09 °C 0.04 °C 1.5 Net zero 1.0 600 GtCO2 2050 2075 2100 c Non−CO2 emissions (GtCO2e) Global mean temperature increase (°C) 1.5 End of century 15 Net zero C 10 M 5 W A P 800 GtCO2 800 GtCO2 0 2025 2050 2075 2100 2025 2050 2075 2100 2025 2050 2075 2100 2025 2050 2075 2100 Fig. 1 | Influence of emission target formulation on the temperature and emission projections across models. a, The global mean temperature increase for an illustrative selection of model scenario combinations, leading to a similar temperature in 2100, likely 1.5 °C. Each subpanel displays two scenarios for the same amount of cumulative emissions. The NZ design is in blue, and the EOC de (...truncated)