Climate change will entail large societal and ecological impacts. Therefore, we need to be able to accurately predict the evolution of climate. An important requirement for this is good knowledge on the carbon cycle, including how much carbon dioxide is taken up by vegetation (photosynthesis). Along with carbon dioxide, plants take up the trace gas carbonyl sulphide (COS) in their leaves. Therefore, COS offers a way of quantifying photosynthesis. In this research we will combine available COS observations with simple modelling tools and numerical optimisation techniques. This will increase our knowledge of the coupled COS-CO2 budgets including improved estimates of photosynthesis.