Recommended temperature metrics for carbon budget estimates, model evaluation and climate policy

Abstract

Recent estimates of the amount of carbon dioxide that can still be emitted while achieving the Paris Agreement temperature goals are larger than previously thought. One potential reason for these larger estimates may be the different temperature metrics used to estimate the observed global mean warming for the historical period, as they affect the size of the remaining carbon budget. Here we explain the reasons behind these remaining carbon budget increases, and discuss how methodological choices of the global mean temperature metric and the reference period influence estimates of the remaining carbon budget. We argue that the choice of the temperature metric should depend on the domain of application. For scientific estimates of total or remaining carbon budgets, globally averaged surface air temperature estimates should be used consistently for the past and the future. However, when used to inform the achievement of the Paris Agreement goal, a temperature metric consistent with the science that was underlying and directly informed the Paris Agreement should be applied. The resulting remaining carbon budgets should be calculated using the appropriate metric or adjusted to reflect these differences among temperature metrics. Transparency and understanding of the implications of such choices are crucial to providing useful information that can bridge the science–policy gap.

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Fig. 1: Schematic representation of the effects of updating the baseline with respect to the cumulative CO2 emissions and temperature change.
Fig. 2: Contributions to differences in recent observed and modelled warming.
Fig. 3: Differences in ocean and sea ice coverage in CMIP5 models, and related differences between GBST and GSAT metrics, under different future emission scenarios.

Data availability

The Cowtan and Way32 GBST datasets with different SST reconstructions are available at: https://www-users.york.ac.uk/~kdc3/papers/coverage2013/. The HadCRUT4.6 data is available at: https://www.metoffice.gov.uk/hadobs/hadcrut4/. GISTEMPv4 is available at: https://data.giss.nasa.gov/gistemp/. COBE-SST2 and ERSSTv5 data is provided by the NOAA/OAR/ESRL PSD via https://www.esrl.noaa.gov/psd/data/gridded/. ERA-Interim is available at: https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim. MERRA2 was downloaded from: https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/. CMIP5 and CMIP6 model output is available at: http://pcmdi9.llnl.gov/. CESM1 pacemaker experiments are available at: https://www.earthsystemgrid.org/.

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Acknowledgements

We are thankful for the discussions at the workshop on carbon budgets, co-organized by J.R. and attended by K.B.T., N.P.G., H.M.D. and J.R., with the support of the Global Carbon Project, the CRESCENDO project, Stanford University, the University of Melbourne and Simon Fraser University. We thank E. Bush and A. Schurer for helpful insights. We thank K. Cowtan for providing data and the computer code for blending SAT and SST estimates. We thank I. Bethke, G. Foster, C. K. Folland, M. Huber, Y. Kosaka, J. L. Lean, K. Rypdal and A. Schmidt for providing data used in Fig. 2. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. The US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support for CMIP and led the development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. K.B.T, C-F.S. and J.R. were supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 820829 (CONSTRAIN project). K.B.T. was also supported by the UK NERC-funded SMURPHs project (grant no. NE/N006143/1). C.F.S. and P.P. acknowledge support from the German Federal Ministry of Education and Research (grant no. 01LN1711A).

Author information

C.-F.S. initiated the study. K.B.T. wrote the manuscript with substantial input from C.-F.S., J.R., M.B.S., H.D.M. and N.P.G. Figure 2 was produced by M.B.S., Fig. 3 by P.P. and the remaining figures by K.B.T., with suggestions from other authors. All authors participated in manuscript editing and revisions.

Correspondence to Katarzyna B. Tokarska.

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Tokarska, K.B., Schleussner, C., Rogelj, J. et al. Recommended temperature metrics for carbon budget estimates, model evaluation and climate policy. Nat. Geosci. 12, 964–971 (2019). https://doi.org/10.1038/s41561-019-0493-5

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