nonlinMIP contribution to CMIP6: model intercomparison project for non-linear mechanisms: physical basis, experimental design and analysis principles (v1.0)

Article, Other literature type English OPEN
Good, Peter ; Andrews, Timothy ; Chadwick, Robin ; Dufresne, Jean-Louis ; Gregory, Jonathan M. ; Lowe, Jason A. ; Schaller, Nathalie ; Shiogama, Hideo (2016)
  • Publisher: Copernicus Publications
  • Journal: (issn: 1991-9603, eissn: 1991-9603)
  • Related identifiers: doi: 10.5194/gmd-9-4019-2016
  • Subject: QE1-996.5 | Geology

nonlinMIP provides experiments that account for state-dependent regional and global climate responses. The experiments have two main applications: (1) to focus understanding of responses to CO<sub>2</sub> forcing on states relevant to specific policy or scientific questions (e.g. change under low-forcing scenarios, the benefits of mitigation, or from past cold climates to the present day), or (2) to understand the state dependence (non-linearity) of climate change – i.e. why doubling the forcing may not double the response. State dependence (non-linearity) of responses can be large at regional scales, with important implications for understanding mechanisms and for general circulation model (GCM) emulation techniques (e.g. energy balance models and pattern-scaling methods). However, these processes are hard to explore using traditional experiments, which explains why they have had so little attention in previous studies. Some single model studies have established novel analysis principles and some physical mechanisms. There is now a need to explore robustness and uncertainty in such mechanisms across a range of models (point 2 above), and, more broadly, to focus work on understanding the response to CO<sub>2</sub> on climate states relevant to specific policy/science questions (point 1).<br><br> nonlinMIP addresses this using a simple, small set of CO<sub>2</sub>-forced experiments that are able to separate linear and non-linear mechanisms cleanly, with a good signal-to-noise ratio – while being demonstrably traceable to realistic transient scenarios. The design builds on the CMIP5 (Coupled Model Intercomparison Project Phase 5) and CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) protocols, and is centred around a suite of instantaneous atmospheric CO<sub>2</sub> change experiments, with a ramp-up–ramp-down experiment to test traceability to gradual forcing scenarios. In all cases the models are intended to be used with CO<sub>2</sub> concentrations rather than CO<sub>2</sub> emissions as the input. The understanding gained will help interpret the spread in policy-relevant scenario projections.<br><br> Here we outline the basic physical principles behind nonlinMIP, and the method of establishing traceability from abruptCO<sub>2</sub> to gradual forcing experiments, before detailing the experimental design, and finally some analysis principles. The test of traceability from abruptCO<sub>2</sub> to transient experiments is recommended as a standard analysis within the CMIP5 and CMIP6 DECK protocols.
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