02 Apr 2024
 | 02 Apr 2024
Status: this preprint is open for discussion.

Equilibrium global warming – a scaling perspective

Peter O. Passenier

Abstract. In the field of climate-change research a lot of effort is devoted to the ‘narrowing down’ of uncertainties in the estimation of the (fast-feedback) Equilibrium climate sensitivity (ECS), the mean global warming as a result of a doubling of the CO2 concentration in the atmosphere, in order to improve the predictability of the Earth climate system to determine required future greenhouse-gas mitigation targets. A recent update of this quantity was provided by Hansen et al. (2023), reporting a value of 4.8 °C ± 1.2 °C for doubled CO2. This outcome is based on a variety of paleo-climate information to overcome limitations of the present, mainly model-based, “best estimate” of about 3 °C (IPCC AR6, 2021). Applying the formal framework of feedback analysis, originating in electrical engineering and control systems, the present study sets out to explore possible consequences of this high-end ECS update for the long-term Earth system sensitivity (ESS), taking into account ‘slow’ feedbacks by ice sheets and trace gases in a warming world (according to the recent Hansen study for today’s GHGs concentrations in the atmosphere likely leading to 10 °C equilibrium global warming). As a result, principal scaling relations between variations in the fast-feedback ECS and slow-feedback ESS are derived, primarily focusing on a better mechanistic understanding of interactions in (besides merely improving the predictability of) the Earth climate system. These scaling relations may be applied to determine the equilibrium global warming eventually to be expected for a specified CO2 amount in the atmosphere. As an illustration, implications for the current geopolitical approach, aiming at 1.5 or at most 2 degrees Celsius of global warming as required by the Paris agreement—while we already seem to be on a 10 degrees track because of warming in the pipeline—are analyzed.

Peter O. Passenier

Status: open

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Peter O. Passenier
Peter O. Passenier


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Short summary
In a recent study of Hansen et al. (2023), an update of the fast-feedback ECS was provided, reporting a value of 4.8 °C for doubled CO2. Applying the framework of feedback analysis, originating in electrical engineering, the present study explores possible consequences of this ECS update for the long-term ESS, taking into account ‘slow’ feedbacks by ice sheets and trace gases in a warming world. Implications for the Paris agreement, aiming at a maximum of 2 °C of global warming are analyzed.