Abstract
A plausible simulation of the global energy balance is a first-order requirement for a credible climate model. Here I investigate the representation of the global energy balance in 40 state-of-the-art global climate models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6). In the CMIP6 multi-model mean, the magnitudes of the energy balance components are often in better agreement with recent reference estimates compared to earlier model generations on a global mean basis. However, the inter-model spread in the representation of many of the components remains substantial, often on the order of 10–20 Wm−2 globally, except for aspects of the shortwave clear-sky budgets, which are now more consistently simulated by the CMIP6 models. The substantial inter-model spread in the simulated global mean latent heat fluxes in the CMIP6 models, exceeding 20% (18 Wm−2), further implies also large discrepancies in their representation of the global water balance. From a historic perspective of model development over the past decades, the largest adjustments in the magnitudes of the simulated present-day global mean energy balance components occurred in the shortwave atmospheric clear-sky absorption and the surface downward longwave radiation. Both components were gradually adjusted upwards over several model generations, on the order of 10 Wm−2, to reach 73 and 344 Wm−2, respectively in the CMIP6 multi-model means. Thereby, CMIP6 has become the first model generation that largely remediates long-standing model deficiencies related to an overestimation in surface downward shortwave and compensational underestimation in downward longwave radiation in its multi-model mean.
Highlights
The global energy balance fundamentally constrains the energy content of Earth’s climate system as well as its internal distribution
The global mean shortwave radiation budget under cloud-free conditions in Coupled Model Intercomparison Project phase 6 (CMIP6) is in remarkable agreement with recent reference estimates, in its multi-model mean which is within 1 Wm−2 of the reference values for the total (TOA), atmosphere and surface absorption, and in the majority of the individual models which are in close agreement with these references
The global energy budget components of up to 40 newly available GCMs participating in CMIP6 have been assessed both under all-sky and clear-sky conditions, covering total climate system (TOA), surface and atmospheric budgets
Summary
The global energy balance fundamentally constrains the energy content of Earth’s climate system as well as its internal distribution. The model-output variables under consideration for this study are the shortwave and longwave radiative fluxes at the surface and the TOA under both all-sky and clear-sky conditions, as well as the non-radiative fluxes of surface sensible and latent heat They stem from the “historical all forcings” experiments of CMIP6, which aim at simulating the climate evolution since preindustrial times as realistic as possibly, considering all major natural and anthropogenic forcings, namely changes in solar output, atmospheric greenhouse gases, aerosol loadings (tropospheric and stratospheric volcanic), and land use (Eyring et al 2016). After decades of large discrepancies in published reference estimates for the global surface energy budget components, the abovementioned recent independent approaches provide estimates that converge to within a few Wm−2 on a global mean basis (Wild 2017). This increases the confidence in these references and enhances their usefulness as guidance in the assessment of the CMIP6 global mean energy budget components as discussed in the following
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