We propose a new simple climate modelling (SCM) framework for making fast climate projections and for interpreting global climate model (GCM) projections. This SCM is derived from CO_2 step GCM experiments (a core integration in the fifth Climate Model Intercomparison Project CMIP5), and is similar to linear impulse-response theory. Its results are therefore traceable to GCM physics and closely related to a commonly-used method of analysing GCMs. Its formulation needs no tuning and permits clear validation. After discussing theoretical properties, we show that this SCM can work well, using the HadCM3 GCM for global temperature, precipitation, heat uptake and outgoing radiation under a range of forcing scenarios. The validation reveals interesting deviations from impulse-response linearity in surface temperature, radiative response and precipitation. These non-linearities emerge not just in climate feedback (as previously reported), but also in the rapid adjustment to forcing change. Some commonality is found between mechanisms by which temperature and precipitation depart from impulse-response linearity. Our results suggest that in support of the 4xCO_2 experiments within CMIP5, additional 2xCO_2 integrations from other GCMs could be very valuable. We also show how our framework can help understand time-dependent scenario projections, and suggest that this approach could be used as part of a process where GCM understanding and SCM formulation develop together.
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