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Committed warming inferred from observations


Due to the lifetime of CO2, the thermal inertia of the oceans1,2, and the temporary impacts of short-lived aerosols3,4,5 and reactive greenhouse gases6, the Earth’s climate is not equilibrated with anthropogenic forcing. As a result, even if fossil-fuel emissions were to suddenly cease, some level of committed warming is expected due to past emissions as studied previously using climate models6,7,8,9,10,11. Here, we provide an observational-based quantification of this committed warming using the instrument record of global-mean warming12, recently improved estimates of Earth’s energy imbalance13, and estimates of radiative forcing from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change14. Compared with pre-industrial levels, we find a committed warming of 1.5 K (0.9–3.6, 5th–95th percentile) at equilibrium, and of 1.3 K (0.9–2.3) within this century. However, when assuming that ocean carbon uptake cancels remnant greenhouse gas-induced warming on centennial timescales, committed warming is reduced to 1.1 K (0.7–1.8). In the latter case there is a 13% risk that committed warming already exceeds the 1.5 K target set in Paris15. Regular updates of these observationally constrained committed warming estimates, although simplistic, can provide transparent guidance as uncertainty regarding transient climate sensitivity inevitably narrows16 and the understanding of the limitations of the framework11,17,18,19,20,21 is advanced.

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Figure 1: Probabilities of transient climate response (TCR) and equilibrium climate sensitivity (ECS).
Figure 2: Estimates of committed warming under five different sets of assumptions.
Figure 3: Commitment as a function of transient climate response.

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The work of T.M. is supported by the Max-Planck-Gesellschaft (MPG). R.P. is supported by the Regional and Global Climate Modeling Program of the US Department of Energy under grant DE-SC0012549 and by the National Science Foundation under grant ATM-1138394. The original motivation for this study arose at a preparation meeting for the IPCC special report on the 1.5 degree target (SR1.5) arranged by C. Textor and R. von Kuhlmann on behalf of the Federal Ministry for Education and Research in Germany (BMBF). The study benefited from comments and input from A. Dessler, J. Gregory, N. Lewis, V. Brovkin and P. Lanschützer.

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The original idea for this study was conceived by T.M. R.P. and T.M. developed the methodology and wrote the manuscript.

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Correspondence to Thorsten Mauritsen.

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The authors declare no competing financial interests.

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Mauritsen, T., Pincus, R. Committed warming inferred from observations. Nature Clim Change 7, 652–655 (2017).

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