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Background: A key current issue in studies of global CO2 is the cause of the recent pronounced changes in the timing (phase) and amplitude of its annual cycle. Aim: To use a sensitivity analysis in order to identify the most influential parameters in a new four-box diffusion model of global CO2 transport. Methods: A new diffusion model of global atmospheric transport is developed, implemented by optimisation and used to assess the effects of oceanic, terrestrial and anthropogenic CO2 fluxes on the annual CO2 cycle. As the terrestrial phenology of living organisms in the temperate zones is identified as the central process of concern an improved phenological model for the temperate zones is proposed and used in up-scaling from plant to planet. Results: The diffusion model is found to be able to mimic many of the features observed by the GLOBALVIEW-CO2 network, including the global trend, latitudinal gradient, and annual waveform. Conclusions: It is concluded that, out of five rival hypotheses considered at the outset, the springtime phenology of terrestrial biomes is the dominant controller of recent changes to the annual CO2 cycle.