Certain of the earth's resources, such as oil and coal, are non-renewable and sooner or later will be completely exhausted. Although other resources, such as plant crops, cattle, fish and timber, renew themselves and can be regularly cropped to provide the food, clothing and shelter essential to human survival, it is not so clearly realized that these resources are renewable only to the extent that their use is rationally planned and managed. There are limits to the extent to which we can draw on these resources; if these limits are are exceeded, this will destroy the capacity of resource renewal.
At the current rate of consumption, oil will run out in about 30 years' time, tin, cadmium, lead and zinc in 40 years, copper, antimony and nickel in about 70 years. Most current utilization of aquatic animals, of the wild plants and animals of the land, of forests and of grazing lands is not sustainable.