A primitive economic model with classical population theory is constructed in order to examine the greenhouse effect on the sustainability of human population as well as the environmental tax when the sustainability is in danger. The conclusion of this paper is that when the negative effect is small, the tax can guarantee the sustainability, where the effective tax rate interval for the sustainability shrinks as the negative effect rises. When the negative effect exceeds the critical level, however, the environmental tax cannot guarantee the sustainability of human population. Thus, the remedial measure to reduce the greenhouse effect other than the environmental tax is needed for the sustainability.