Background/Question/Methods   The nature and magnitude of feedbacks involving atmospheric carbon dioxide concentration and global climate are not easily inferred from the still-limited direct experimental evidence. In a global-scale geophysical approach, I consider various sources of evidence on time scales from seasonal to glacial-interglacial cycles, using a synthesis of atmospheric and ice-core measurements and modelling of the carbon cycle.

Results/Conclusions   There is evidence for a negative feedback in the climate-carbon system due to persistent, ecosystem-level carbon dioxide fertilization, and a positive feedback primarily due to the effects of temperature on the rate of soil organic matter decomposition. Global modelling is broadly consistent with the available evidence, but many quantitative questions remain. The consequences of interactions between the nitrogen and carbon cycles, in particular, are not yet on a firm quantitative basis and call for a new strategy in the evaluation of large-scale models.