2.6.1 The Fixed Stock Paradigm
While putting down the input rule for environmental sustainability a restriction has been imposed that the rate of harvesting renewable natural capital be less than, or equal to, the sustainable yield. However, with non-renewable resources, there cannot be any sustainable yield. The resource will be depleted so long as the use rate, - the 'harvest', is positive. This means their supply at any particular point in time is a fixed stock that can only diminish with use. It is therefore necessary that in the case of non-renewable resources, instead of seeking to find an optimal rate of use, attention should be restricted towards finding the optimal rate of depletion of the resource (Pearce and Turner 1990).

Some attempts have been made to estimate the time in future up to which the corresponding projected demands can be met from the known reserves. At recent levels of global production, for example, the 310 million tonne of world copper reserves would last about 35 years (Tilton 1996). However, the term reserve, by definition, means the quantity of economically exploitable mineral under the given technology. Such estimations fail to take into account the influence of new discoveries, new technologies and changing prices on the mineral stock over time. Nevertheless, if current situation is any indication the spiralling increase in demand (due to rising per capita income and growing population) for raw materials, particularly minerals, cannot be offset by new discoveries alone. While technology may allow a great quantum of geological resources to be turned into mineable reserves by allowing lower grade, more remote and more difficult-to-process deposits, the environmental wastes to be generated per unit of output will, without doubt, rise significantly. This is evident from the fact that copper produced from 0.3% ore generates at least ten times more waste-rock per tonne of ore than copper from 3.0% ore (Tilton 1996).

On the demand side, modern technology has broadened the scope and option for meeting a particular need and in the process often increasing the substitutability. Recent evidences suggest that advancement of technology has more than offset the depletion of mineral reserves through consumption and left them progressively less scarce (Pearse 1991). The case of copper is a good example. It would have been extremely costly, if not materially impossible, to provide the quantity of copper that would have been required to maintain today's level of communication through the available technology of 1940s. Fortunately, the metal has virtually been replaced for long distance communication.

The proponents of fixed stock paradigms quite rightly suggest that if the cost of environmental degradation is not internalised in near future, prevailing market mechanisms will definitely lead to over production of exhaustible resources through deployment of excessively polluting technologies. Such a situation if allowed to continue will soon result in crossing the carrying capacity of environment at the production sites in particular and the global environment in general.

2.6.2 The Opportunity Cost Paradigm
Historic evidences suggest that the neo-Malthusian concern over possible scarcity of land and natural resources due to excessive increase in population and expansion of economies is largely misplaced. It is mostly the economists who support the opportunity cost paradigm. According to them the limited stock of land and natural resource, in the world does not prevent continuing expansion of production and improvement in material standards of living. They question the validity of reserves as an appropriate measure of ultimate stock and argue that the total available stock of minerals invariably results in an exceedingly long life expectancy. For example, the 1.5x1015 tonne of copper available in the earth's crust would last for some 161 million years (Tilton 1991, 1996). Moreover, with the notable exception of fossil fuels, many so-called non-renewable resources are not destroyed when consumed.

The possible recycle and re-use of such commodities depend on the cost of recycling vis-à-vis intake of mined resources. Scarcity is therefore only economic and not due to physical availability. Economists further suggest that,- much before the exhaustion of physical stock, either rises in production cost or availability of substitutes would cause a cessation of demand.

Many development thinkers therefore reject the fixed stock paradigm and propose an opportunity cost paradigm that stresses the heterogeneity of exhaustible resources. Exploitation is prioritised based on ease of access, the ease of working and the richness of a deposit. With the exhaustion of low-cap high grade deposits mining gradually shift to 'difficult-to-mine' low grade deposits with a corresponding rise in cost, which in turn causes the demand to fall. However, not all exhaustible resources would be subjected to scarcity due to over exploitation and cost rise. The cost increasing effects of depletion may often be more than offset by the cost reducing effects of new discoveries, new technology and other developments. The relevant costs are the full opportunity cost of finding and producing mineral commodities, including any external environmental cost (Tilton 1996). Studies (Barnett and Morse 1963) reveal that capital and labour costs for producing minerals have actually reduced consistently. Proponents of the opportunity cost paradigm believe that any price rise (as an effect of scarcity) would set in motion a series of self correcting activities making the system quite robust. These self-correcting activities may also take into account environmental cost if public policies allow internalisation of such costs.