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
strongly, on the assumption that monetary values for environmental factors cannot be assigned unless people make economic decisions about these factors