Abstract
The degree of physical-biogeochemical equilibration of the climate system determines for how long global warming will continue after anthropogenic CO2 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.
Highlights
Transient global warming due to greenhouse gas radiative forcing is substantially reduced by ocean heat uptake
All ensembles show that the realized warming fraction (RWF) is lower for models with higher equilibrium climate sensitivities (ECS)
This is a confirmation of analytical considerations (Hansen et al 1984) and results from earlier model generations (Raper et al 2002, Winton et al 2010). We have reproduced this influence of the equilibrium climate sensitivity (ECS) on the RWF in new Bern3D-LPX simulations using a simple global feedback tuning parameter
Summary
Transient global warming due to greenhouse gas radiative forcing is substantially reduced by ocean heat uptake. The realized warming fraction (RWF) (Stouffer 2004, Solomon et al 2009) is an important policyrelevant quantity, because models with a lower RWF indicate that global warming may continue for centuries after greenhouse gas emissions cease (Solomon et al 2009, Matthews and Zickfeld 2012, Frölicher et al 2014, Frölicher and Paynter 2015, Ehlert and Zickfeld 2017) This continued warming is commonly referred to as zero emission warming commitment (ZEC) and may strongly influence long-term climate change mitigation policies. Frölicher and Paynter (2015) show a strong anticorrelation between the RWF and the ZEC in Earth system models (ESMs) from the Climate Model Intercomparison Project phase 5 (CMIP5), and a weaker but consistent anticorrelation in ESMs of Intermediate Complexity (EMICs). The ZEC is influenced by the state of biogeochemical equilibration, Ehlert and Zickfeld (2017) show that the influence of the physical equilibration (measured by the RWF) is dominant
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