Climate targets are designed to inform policies that would limit the magnitude and impacts of climate change caused by anthropogenic emissions of greenhouse gases and other substances. The target that is currently recognized by most world governments1 places a limit of two degrees Celsius on the global mean warming since preindustrial times. This would require large sustained reductions in carbon dioxide emissions during the twenty-first century and beyond2,3,4. Such a global temperature target, however, is not sufficient to control many other quantities, such as transient sea level rise5, ocean acidification6,7 and net primary production on land8,9. Here, using an Earth system model of intermediate complexity (EMIC) in an observation-informed Bayesian approach, we show that allowable carbon emissions are substantially reduced when multiple climate targets are set. We take into account uncertainties in physical and carbon cycle model parameters, radiative efficiencies10, climate sensitivity11 and carbon cycle feedbacks12,13 along with a large set of observational constraints. Within this framework, we explore a broad range of economically feasible greenhouse gas scenarios from the integrated assessment community14,15,16,17 to determine the likelihood of meeting a combination of specific global and regional targets under various assumptions. For any given likelihood of meeting a set of such targets, the allowable cumulative emissions are greatly reduced from those inferred from the temperature target alone. Therefore, temperature targets alone are unable to comprehensively limit the risks from anthropogenic emissions.
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We thank T. Kaminski for providing the Global Atmospheric Tracer Model (TM2) transport matrices, J. M. Lyman, H. Keith, and S. Gerber for providing observational data sets, R. Spahni, R. Roth, S. Ritz, B. Stocker and K. Strassmann for discussions and help with the model code, and K. Bieri for the IT support. We are grateful to the modelling teams participating in the EMF-21, International Institute for Applied Systems Analysis (IIASA) GGI, and AME projects for providing scenario data, and to the EMIC AR5 and coupled model intercomparison (CMIP5) projects for providing model forcing data sets. This project was funded by the Swiss National Science Foundation and the European Project on Ocean Acidification (EPOCA 211384) and the European Project CARBOCHANGE (264879) which both received funding from the European Commission’s Seventh Framework Programme (FP7/20072013).
The authors declare no competing financial interests.
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Steinacher, M., Joos, F. & Stocker, T. Allowable carbon emissions lowered by multiple climate targets. Nature 499, 197–201 (2013). https://doi.org/10.1038/nature12269
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