To understand how global warming can be kept well below 2 degrees Celsius and even 1.5 degrees Celsius, climate policy uses scenarios that describe how society could reduce its greenhouse gas emissions. However, current scenarios have a key weakness: they typically focus on reaching specific climate goals in 2100. This choice may encourage risky pathways that delay action, reach higher-than-acceptable mid-century warming, and rely on net removal of carbon dioxide thereafter to undo their initial shortfall in reductions of emissions. Here we draw on insights from physical science to propose a scenario framework that focuses on capping global warming at a specific maximum level with either temperature stabilization or reversal thereafter. The ambition of climate action until carbon neutrality determines peak warming, and can be followed by a variety of long-term states with different sustainability implications. The approach proposed here closely mirrors the intentions of the United Nations Paris Agreement, and makes questions of intergenerational equity into explicit design choices.
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The MESSAGEix modelling framework61, including its macroeconomic module MACRO, is available under an APACHE 2.0 open-source license at http://github.com/iiasa/message_ix. Data can be analysed online via a dedicated scenario explorer instance at https://data.ene.iiasa.ac.at/paris-lttg-explorer; further analytical codes for producing figures are not available.
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We thank the International Institute for Applied Systems Analysis (IIASA) for hosting and maintaining the IPCC AR5 Scenario Database at https://tntcat.iiasa.ac.at/AR5DB/; O. Fricko for feedback and analysis during the explorative stages of the project; S. Frank and P. Havlík for supplying the MESSAGEix framework with GLOBIOM land-use data; and J. Cook for expert feedback.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Peer review information Nature thanks Nathan Gillett and Katharine Mach for their contribution to the peer review of this work.
Extended data figures and tables
a, Emission price trajectories applied to non-CO2 greenhouse gas emissions in the default case and the two sensitivity cases. In line with the scope of emissions covered under the UNFCCC, emissions from aerosol or aerosol precursor species such as black carbon or sulphur dioxide are not explicitly subjected to a carbon price. b–g, Resulting emissions of CO2 and internally consistent evolutions of a selection of non-CO2 emissions. h–j, Effect of non-CO2 sensitivity cases on decadal rate of temperature change in the period 2090–2100. Note that the sensitivity case that assumes zero penalty on non-CO2 emissions is extremely unlikely in light of recent efforts that specifically target reductions in methane and fluorinated gas emissions. Emissions of non-CO2 gases are translated into CO2 equivalence using global warming potentials over a 100-year time horizon as reported in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change74.
Extended Data Fig. 3 Illustration of system configurations and of contributions of CDR technologies to achieve net zero CO2 emissions.
Corresponding system configurations are shown for all cases shown in Fig. 3. The four levels of net negative CO2 emissions to be achieved by the end of the century are for identification purposes only and are not visible on this figure as they will only be achieved after the point of reaching net zero CO2 emissions. In the cases selected here, net zero CO2 emissions are reached in 2050. Five illustrative system variations are shown per level (labelled A–E, defined in Extended Data Tables 3, 4). NA, scenarios did not solve under the imposed CDR and bioenergy constraints (Extended Data Table 4). Fossil fuel and industry CCS contributions (white hatched areas) represent CO2 that is generated but not emitted to the atmosphere. Net negative CO2 emissions are the sum of gross positive CO2 emissions from energy and industrial sources and gross positive land-use CO2 emissions, and are zero by design in this time step. Gross negative CO2 emissions comprise gross negative land-use CO2 emissions and CDR through BECCS. The combined size of all bars per scenario gives an indication of the overall size of the remaining CO2 producing system by the end of the century. Because the timing of CDR upscaling and amount of CDR at the time of reaching global net zero CO2 emissions was not explicitly varied in the set of illustrative scenarios developed for this study, it would be wrong to interpret the narrow degree of variation and general agreement across scenarios as a robust feature. Variations could be explored through additional dedicated studies (Extended Data Table 2).
Extended Data Fig. 4 Illustration of scenario variation and differences between the scenario logic presented in this study and an end-of-century scenario approach.
a, Pink-to-red, scenarios created with the scenario logic presented in this paper; blue dashed, scenarios created with an end-of-century scenario approach (see labelling). b, For a given amount of cumulative CO2 emissions all scenarios result in a similar amount of temperature increase by 2100 (crosses and diamonds), but different levels of maximum (peak) warming (circles and squares). c, Replication of Fig. 3 showing how stable emissions levels in the second half of the century can be achieved by a variety of system configurations with different amounts of CDR. Note that to achieve a scenario that limits global mean temperature rise in 2100 to 1.5 °C, the standard end-of-century scenario approach would suggest net negative CO2 emissions of about 15 Gt CO2 per year in 2100, while the scenario logic presented in this paper allows the construction of scenarios that achieve that temperature in 2100 with zero to about 5 Gt CO2 per year of net negative CO2 emissions, and a variety of gross CDR contributions.
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Rogelj, J., Huppmann, D., Krey, V. et al. A new scenario logic for the Paris Agreement long-term temperature goal. Nature 573, 357–363 (2019). https://doi.org/10.1038/s41586-019-1541-4
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