The past composition of the atmosphere is gradually being elucidated with increasing precision. But advances in the understanding of the ecological and evolutionary implications of atmospheric changes are not keeping pace. A graphic model is presented here to indicate the complexity of interacting forces resulting from a change in atmospheric carbon dioxide level, and it can be seen that any resultant ecological outcome will be influenced by a series of (sometimes opposing) factors. This is particularly true of the hydrological consequences of atmospheric change and hydrology will undoubtedly prove a major influence on vegetation.
Changes in the levels of both atmospheric carbon dioxide and oxygen have had profound effects on the evolution of plant life by modifying the selective pressures favouring alternative systems of carbon assimilation. These photosynthetic strategies are also closely related to local hydrology. More ecological and physiological information is clearly needed before the biological consequences of Variations in the atmosphere of the past can be appreciated.