What is the optimum level of atmospheric carbon dioxide for maximum forest, crop and pasture growth, without compromising human health?

Evidence suggests that historical atmospheric carbon dioxide levels were markedly higher than at present, and that these higher levels were associated with very high plant productivity – such as during the carboniferous age. Even today, greenhouse producers pump carbon dioxide into their production units to facilitate plant growth, suggesting our current atmosphere is carbon dioxide poor. A sensible approach to atmospheric carbon dioxide management requires a clear understanding of the optimum levels necessary for plant productivity witout compromising human health.