The atmosphere, biosphere and ocean surface layers contain approximately equal amounts of carbon (the deep ocean contains over 50 times as much). The flux of carbon between these pools is dominated by the process of photosynthesis which is itself influenced by the CO2 concentration of the atmosphere. Over the last 200 years deforestation and now fossil fuel combustion have added CO2 to the atmosphere to increase the concentration by 27%—half the increase occurred over the last 30 years. The resulting greenhouse effect has been accompanied by an increase in the global average temperature of about 0.5°; other 'greenhouse gases' (N2O1 O3, CFCs, CH4) are also contributing to the warming. Present uncertainties with estimating carbon flows result from the disputed contribution of deforestation, the level of increased plant productivity and carbon storage due to CO2 fertilization, and problems with estimating net primary production. Uncertainties in estimating future carbon balances arise from the extent of fossil fuel use especially by USA, USSR and China, the degree of control of greenhouse gas emissions, the CO2 fertilization effect on plant productivity and vegetation responses (including moisture availability) and the rate of change of the climatic factors resulting from changing carbon fluxes.