The realized warming fraction: a multi-model sensitivity study

Abstract

The degree of physical-biogeochemical equilibration of the climate system determines for how long global warming will continue after anthropogenic CO 2 emissions have ceased. The physical part of this equilibration process is quantified by the realized warming fraction (RWF), but RWF estimates differ strongly between different climate models. Here we analyze the RWF spread and its physical causes in three model ensembles: 1. an ensemble of comprehensive climate models, 2. an ensemble of reduced-complexity models, and 3. an observationally constrained parameter ensemble of the Bern3D-LPX reduced-complexity model. We show that RWF is generally lower in models with higher equilibrium climate sensitivity. The RWF uncertainty from applying different extrapolation methods for climate sensitivity is substantial, but smaller than the inter-model spread in the three ensembles. We decompose the inter-model spread of RWF using a diagnostic global energy balance model, to compare the spread contribution by the climate sensitivity to contributions by other physical quantities: the efficiency and efficacy of ocean heat uptake, and the effective radiative forcing. In the ensembles of the comprehensive climate models and the Bern3D-LPX model, the spread of the RWF is mostly determined by the spread of the climate sensitivity; for the reduced-complexity models, the spread contribution by the ocean heat uptake efficiency is dominant. Compared to the comprehensive models, the reduced-complexity models have a lower range of climate sensitivities and lower, more unitary ocean heat uptake efficacies, resulting in higher RWF. However, by tuning such models to higher climate sensitivities, they can also achieve RWF values in the lower range of comprehensive models, as demonstrated for Bern3D-LPX. This suggests that reduced-complexity models remain useful tools for future climate change projections, but should employ a range of climate sensitivity tunings to account for the uncertainty in both the long-term warming and the RWF.

Evaluation of CNRM Earth System Model, CNRM‐ESM2‐1: Role of Earth System Processes in Present‐Day and Future Climate

“…The realized warming fraction (RWF), that is, the TCR‐to‐ECS ratio, is higher in CNRM‐ESM2‐1 than in CNRM‐CM6‐1, suggesting that more warming has being expressed in the former. This behavior agrees with the findings of Pfister and Stocker () based on the previous generation of Earth system models, suggesting that high‐ECS models display relatively low RWF and hence a stronger warming commitment.…”

Changes in Local and Global Climate Feedbacks in the Absence of Interactive Clouds: Southern Ocean–Climate Interactions in Two Intermediate-Complexity Models

“…An efficacy of « . 1 is diagnosed for most comprehensive models (e.g., Winton et al 2010), while « is close to 1 for most EMICs except for the two studied here, which are more consistent with the comprehensive models (Pfister and Stocker 2018). Assuming a constant efficacy « exceeding 1 can account for the fact that the model output lies below the dashed line in Fig.…”

“…Out of the 14 EMICs considered in that study, 8 lack interactive clouds (Eby et al 2013). In only two out of these eight models, l becomes substantially less negative with time (Pfister and Stocker 2018;Fig. S2 therein): in the Bern3D-LPX model (Ritz et al 2011) and the LOVECLIM model (Goosse et al 2010).…”

“…To investigate how much this Arctic melting contributes to the increase in global mean shortwave feedback (Fig. 7c), we have performed simulations in which the albedo was fixed in one hemisphere, and allowed to evolve freely in the other hemisphere [as detailed in Pfister and Stocker (2017)]. In this way, we have found that about 0.1 W m 22 K 21 of the l SW increase during the first 500 years can be attributed to Arctic contributions, while the remaining increase (including the full increase after 500 years) is due to Southern Ocean contributions.…”

“…In this study, we investigate the causes of the time dependence in the global mean feedback in two Earth system models of intermediate complexity (EMICs). These models were selected based on our earlier multimodel energy balance analysis (Pfister and Stocker 2018). Out of the 14 EMICs considered in that study, 8 lack interactive clouds (Eby et al 2013).…”