On the concept of green national income

The present paper generalizes the Weitzman/Hartwick approach to national income accounting. We first establish a close general relation between the current value Hamiltonian of an optimal control problem and the optimal value of the objective integral if the time argument does not enter the constraints and enters the objective function only as a discount factor. This result is applied to a simple economic model covering most models found in the literature on national income accounting involving pollution and non-renewable resources. We critically review the usefulness of net national product as a welfare indicator and as an indicator for sustainability.