توسعه پایدار اقتصادی و محیط زیست: نظریه و شواهد

The relationship between growth and pollution is studied through a vintage capital model, where new technologies are more environmentally friendly. We find that once the optimal scrapping age of technologies is reached, an economy may achieve two possible cases of sustainable development, one in which pollution falls and another in which it stabilizes, or a catastrophic outcome, where environmental quality reaches its lower bound. The outcome will depend on countries’ investment path and their propensity to innovate in environmentally clean technologies, both of which are likely to differ across economies. Empirical results using long time series for a number of developed and developing countries indeed confirm heterogenous experiences in the pollution-output relationship.

Since the seminal paper by Grossman and Krueger (1995) there has been considerable academic interest in the relationship between economic development and environmental pollution. Importantly the authors have shown empirically that the link between these follows an inverted U-shaped pattern, now commonly referred to as the Environmental Kuznets Curve (EKC). This suggests that lower income regions are ‘too poor to be green’, but as countries become richer they will naturally reduce their generation of pollution, either through reducing production (the so-called scale effect), or by switching to cleaner technologies (referred to as the technique effect) or finally by switching to cleaner sectors (i.e. the composition effect). Several recent studies, however, have put the existence and the exact shape of an EKC into question (Stern, 2004). In view of the recent policy developments, resolving this issue seems of particular importance. More precisely, the recent Kyoto Protocol has set reduction targets for pollutant emissions to which developed countries are expected to commit themselves to, but from which developing countries are at the first instance exempt. This would suggest that policymakers are of the view that wealth on its own does not result in a – possibly sufficient – reduction in pollution, a stance which as of date has not yet been substantiated in the academic literature.

The continuing debate concerning the link between economic development and pollution, in particular with regard to the possible existence of an EKC, demonstrates that this issue is still of considerable interest to both academics and policymakers. However, while there is an abundance of empirical studies, generally generating mixed results, theoretical investigations have remained relatively scarce. In the present contribution, we shed new light on this debate by providing parameter conditions for an EKC. Our theoretical model, which is particularly adapted to the kind of issue treated here since it allows for the possibility for newer technologies to be cleaner, points towards several major conclusions. First, an important distinction is made between pollution and environmental quality. While in general the EKC literature considers these two measures to be roughly the inverse of each other, we take account of nature's self regeneration capacity. This has important consequences for the interpretation of results in empirical studies using data on pollution given that stable pollution levels are not necessarily incompatible with sustainable economic development. Second, we are able to derive explicit conditions for the existence of a bell-shaped EKC in terms of countries' investment growth rates, and the rate at which their technologies improve in terms of environmental friendliness. Since these two parameters are in reality likely to be very different from country to country, it is thus important from an empirical point of view to examine countries individually, unless one can explicitly control for all the determinants of these two variables. Last, when studying the relationship between pollution and output our model demonstrates that it is important to distinguish between periods before and after the optimal scrapping age has been reached. While the link is always increasing before the stationary state, thereafter it will depend on countries' investment rates and willingness to improve technologies in terms of environmental friendliness.