Understanding Climate Model Forcing
In past the energetic input to the climate system, called the radiative forcing, has not been well-known, making it hard to understand whether model projections differ because the models are forced differently or because the inherent climate response is uncertain.
By establishing cost-effective methods for determining the radiative forcing each climate model experiences, and ensuring that all models provide these calculations, it will be possible to far more cleanly disentagne variability in forcing from variability in climate sensitivity in the next generation of coordinated climate experiments.
The next generation of coordinated climate model experiements, CMIP6, is organized around three questions. The phrasing of one of these defining questions - "How does the Earth system respond to forcing?" - suggests that forcing is always well-known, yet the radiative forcing to which this question refers has historically been uncertain in coordinated experiments even as understanding of how best to infer radiative forcing has evolved. The Radiative Forcing Model Intercomparison Project endorsed by CMIP6 seeks to provide a foundation for answering the question through three related activities: (i) accurate characterization of the effective radiative forcing relative to a near pre-industrial baseline, and careful diagnosis of the components of this forcing; (ii) assessment of the absolute accuracy of clear-sky radiative transfer parameterizations against reference models on the global scales relevant for climate modeling; and (iii) identification of robust model responses to tightly-specified aerosol radiative forcing from 1850 to present. In past months the protocols for RFMIP have been finalized and published, with special attention paid to accurate and efficient methods for the characterization of effective radiative forcing.