Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2012 / S25 / 3 / 5 / Solution

The real carbon cycle has multiple reservoirs and adjustment times, rather than one universally exponential removal process. Air-sea exchange, ocean mixing and carbonate chemistry give a long-lived atmospheric tail; uptake also changes with temperature, circulation and carbon loading. For example, a warming ocean can absorb less additional carbon dioxide, so a fixed $b_0$ omits a potentially positive climate-carbon feedback. Terrestrial vegetation and soil responses provide further examples. One missing feature is the dependence of carbon uptake on the changing state of the ocean and climate.