National income estimates derived from the use of accounting methods based on the SNA (such as GDP, NNP and NNP.) are said to be static. That is, they measure the flow of aggregate income or expenditure at a point in time.

If this is the case, what specific condition has to be stipulated to make an environmentally adjusted net national product (NNP.) an indicator of sustainability? Explain why a sustainability indicator that is based on NNP, can only meet the condition for weak sustainability (i.e., why it cannot be an used as an indicator of strong sustainability).