Biological CO2 fixations were assessed from an economical view point. Three methods are perspective as the countermeasure of global warming. The first is afforestation. It can store CO2 by 200-300 ton-C/ha as organic substances in timbers and soils. The cost of CO2 fixation is decided from the cost of plantation, being 700-1, 000 yen/ton-C in developping countries. The second is microbial CO2 fixation using chlorella sp. The fixation products can be used as feed for livestock. The replacement of feed crop with chlorella can reduce Green House effects Gases (GHGs) emission by cutting the GHGs from crop fields during cultivation.In this case, the cost for the fixation would be free, since the fixation products are merchandise. The third is a utilization of biological pumping in the ocean to decrease atmospheric CO2 concentration, i. e., the enhanced growths of the phvtoplanktons in the ocean and then, transfer of atmospheric CO2 to the deep ocean. If the iron fertilization were effective for increase of some ocean productivities, the cost for CO2 fixation would be around 100yen/ton-C.
The feasibility of biological CO2 fixation can be assessed by the comparison of their costs with that of physical methods and political methods. The cost would be 25, 000yen/ton-C for CO2 recovery from stack gases and its ocean dumping as a physical method. For carbon taxation to stabilize CO2 emission increase to the level of 1988, it would cost 240, 000yen/ton-C. The biological CO2 fixation is the most economical method among the three and therefore, the most feasible for global warming.
