Alain Ayong Le Kama

Sustainable growth, renewable resources and pollution

Abstract Our intention is to present a growth model with an environmental resource which has its own regeneration process. The stock of this resource serves as a source of utility and an input to production. We also intend to introduce a negative externality caused by a pollution flow which we assume to be proportional to production. In the context of this model, it is shown that, by using the utility level of the Green Golden Rule as a generalization of the Ramseys bliss point for solving an optimal growth problems with a zero discount rate, an optimal path converges to the Green Golden Rule configuration.

Optimal Emission Policy Under the Risk of Irreversible Pollution

We consider an optimal consumption and pollution problem that has two important features. Environmental damages due to economic activities may be irreversible and the level at which the degradation becomes irreversible is unknown. Particular attention is paid to the situation where agents are relatively impatient and/or do not care a lot about the environment and/or Nature regenerates at low rate. We show that the optimal policy of the uncertain problem drives the economy in the long run toward a steady state while, when ignoring irreversibility, the economy follows a balanced growth path accompanied by a perpetual decrease in environmental quality and consumption, both asymptotically converging toward zero. Therefore, accounting for the risk of irreversibility induces more conservative decisions regarding consumption and polluting emissions. In general, however, we cannot rule out situations where the economy will optimally follow an irreversible path and consequently, will also be left, in the long run, with an irreversibly degraded environment.

A NOTE ON THE CONSEQUENCES OF AN ENDOGENOUS DISCOUNTING DEPENDING ON THE ENVIRONMENTAL QUALITY

Our intention is to study, in the framework of a very simple optimal growth model, the consequences on the optimal paths followed by consumption and the environmental quality of an endogenous discounting. Consumption directly comes from the use of environmental services and so is a direct cause of environmental degradation. The environment is valued both as a source of consumption and as an amenity. For a sustainability concern, we introduce an endogenous discount rate growing with the environmental quality, and compare the optimal growth paths with the ones obtained in the usual case of exogenous and constant discounting. We show that the convergence of the environmental quality towards a steady state occurs only for a very special configuration of the parameters in the exogenous discounting case, while it occurs generically in the endogenous discounting one. This happens for a utility discount rate becoming suficiently high when the environmental quality is high and suficiently low when the environmental quality is poor. In this case then, endogenous discounting with a positive marginal discount rate allows us to avoid the depletion of the environment.

Optimal Carbon Capture and Storage Policies

The IPCC recommends the use of carbon capture and sequestration (CCS) technologies in order to achieve the Kyoto environmental goals. This paper sheds light on this issue by assessing the optimal strategy regarding the long-term use of CCS technologies. The aim is to analyze the optimal CCS policy when the sequestration rate is endogenous, being therefore one specific tool of the environmental policy. We develop a simple growth model to identify the main driving forces that should determine the optimal CCS policy. We show that, under some conditions on the cost of extractions, CCS may be a long-term solution to curb carbon emissions. We also show that over time the social planner will choose to decrease the rate of capture and sequestration. We then derive the decentralized equilibrium outcome by considering the programs of the fossil resource-holder and of the representative consumer. Finally, we determine the optimal environmental policy, i.e. the carbon tax scheme, as well as the dynamics of the fossil fuel price needed to implement it.

Preservation and exogenous uncertain future preferences

We extend the Beltratti, Chichilnisky and Heals (1993) and (1998) continuous-time stochastic dynamic framework to analyze the optimal depletion of an asset whose consumption is irreversible, in the face of uncertainty about future preferences. Their model is rather general and so the results are general qualitative theorems. We show that in some interesting cases it is possible to solve their model analytically. The cases involve constant elasticity utility functions and the assumption of a Poisson process for the evolution of preferences.

Water Conservation Versus Soil Salinity Control

This paper tackles the increasingly significant problem of irrigation-induced soil salinity within a groundwater management model. Irrigation can result not only in heavier salt concentrations but also in the removal of salt from the soil through return flows. Given these contradictory observations, we are interested in the effects on soil salt concentration if irrigation efficiency is improved. We develop a model of salt concentration patterns in both soil and groundwater. We introduce a negative externality to the production process by assuming that soil degradation due to higher soil salinity affects total factor productivity. Within this framework, we show that in the presence of this externality, increasing irrigation efficiency can lead to higher or lower soil salt concentration, depending on the social cost of transferring salt from one reservoir to another.