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The equilibrium sensitivity of the Earth's temperature to radiation changes

Abstract

The Earth's climate is changing rapidly as a result of anthropogenic carbon emissions, and damaging impacts are expected to increase with warming. To prevent these and limit long-term global surface warming to, for example, 2 °C, a level of stabilization or of peak atmospheric CO2 concentrations needs to be set. Climate sensitivity, the global equilibrium surface warming after a doubling of atmospheric CO2 concentration, can help with the translation of atmospheric CO2 levels to warming. Various observations favour a climate sensitivity value of about 3 °C, with a likely range of about 2–4.5 °C. However, the physics of the response and uncertainties in forcing lead to fundamental difficulties in ruling out higher values. The quest to determine climate sensitivity has now been going on for decades, with disturbingly little progress in narrowing the large uncertainty range. However, in the process, fascinating new insights into the climate system and into policy aspects regarding mitigation have been gained. The well-constrained lower limit of climate sensitivity and the transient rate of warming already provide useful information for policy makers. But the upper limit of climate sensitivity will be more difficult to quantify.

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Figure 1: The concept of radiative forcing, feedbacks and climate sensitivity.
Figure 2: Relation between amplifying feedbacks f and climate sensitivity S.
Figure 3: Distributions and ranges for climate sensitivity from different lines of evidence.
Figure 4: The observed global warming provides only a weak constraint on climate sensitivity.
Figure 5: Relation between atmospheric equivalent CO2 concentration chosen for stabilization and key impacts associated with equilibrium global temperature increase.
Figure 6: Allowed emissions for a stabilization of atmospheric CO2 at 450 p.p.m. as shown in Fig. 5.

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Acknowledgements

The International Detection and Attribution Working Group (IDAG) acknowledges support from the US Department of Energy's Office of Science, Office of Biological and Environmental Research and the National Oceanic and Atmospheric Administration's Climate Program Office.

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Knutti, R., Hegerl, G. The equilibrium sensitivity of the Earth's temperature to radiation changes. Nature Geosci 1, 735–743 (2008). https://doi.org/10.1038/ngeo337

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