The problem that environmental economics aims to remedy is the following: people making economic decisions cannot readily compare environmental factors, such as clean air and the survival of endangered species, with other costs and benefits  As environmental economists recognize, solving this problem requires assigning monetary values to environmental factors  But monetary values result from people comparing costs and benefits in order to arrive at economic decisions
Thus, environmental economics is stymied by what motivates it
at best weakly, because the passage fails to establish that economic decision-makers do not by and large take adequate account of environmental factors