A carbon cycle model based environmental optimization method is proposed that minimizes the maximum and steady state atmospheric CO2 concentration. The proposed method is applied to a fossil to biomass energy transition problem. The optimization results indicate that a gradual change is more effective than immediate or delayed step changes, and that afforestation is essential in addition to reforestation. From these results, it is suggested that, in order to avoid a huge carbon debt, fossil fuels should be used as a complement until biomass resources are increased to an optimum level by afforestation. Furthermore, it is predicted that using biomass instead of fossils cannot fully recover the initial state, even if supported by intensive afforestation. The misleading concept of carbon neutrality of biomass is also clarified using the proposed model, which shows that biomass is not a preferable alternative to fossil fuels. Nonetheless, the proposed method is applicable to optimal energy utilization of fossil and biomass resources.