  A dynamic model is constructed that generalises the Hartwick and Van Long
(2020) endogenous discounting setup by introducing externalities and asks what
implications this has for optimal natural resource extraction with constant
consumption. It is shown that a modified form of the Hotelling and Hartwick
rule holds in which the externality component of price is a specific function
of the instantaneous user costs and cross price elasticities. It is
demonstrated that the externality adjusted marginal user cost of remaining
natural reserves is equal to the marginal user cost of extracted resources
invested in human-made reproducible capital. This lends itself to a discrete
form with a readily intuitive economic interpretation that illuminates the
stepwise impact of externality pricing on optimal extraction schedules.
