This paper reexamines the Poyago-Theotoky model and provides additional investigation that was conducted under a corrected environmental damage parameter. As new findings, we obtain the following. First, social welfare under a time-consistent emission tax (emission subsidy) policy is always welfare-enhancing rather than the case of laissez-faire. Second, if the environmental damage parameter is sufficiently small, then the equilibrium emission tax rate is invariably negative. It is therefore an emission subsidy. Moreover, total emissions under the emission subsidy become smaller than those under laissez-faire if the damage parameter is sufficiently small, and if the R&D cost is low. However, total emissions under the emission subsidy become greater than those under laissez-faire if the damage parameter is sufficiently small, and if the R&D cost is high.