Ignoring environmental externalities
Environmental costs arise either through the damage done as a consequence of resource exploitation or through the effort expended to redress the damage. Damage costs may also be imposed in the process of development, through the destruction of certain types of renewable resources such as: large-scale loss of tropical forests, soil degradation due to salinization, and imperfect cultivation of sub-marginal lands.
Some 30 million square kilometres (19% of the earth's land surface) are threatened with desertification, and consequently with huge economic and human losses. From 1970-1980, the economic cost of pollution damage in developed countries varied between 3 and 5% of GNP.
The costing of the damage due to the irrational use of natural resources and/or pollution is not complete. Environmental damage is often selective and unequally distributed in time and space and among societies. Many of the physical, biological, and socio-economic consequences of large development projects are inadequately known, and some cannot be assessed. Examples of this are landscape or historic monuments threatened with irreversible change. Even if all the consequences could be enumerated and their likelihood assessed, placing a price tag on them would pose further difficulties.