پیشرفت فن آوری و توسعه پایدار: در مورد اثر جهش چی؟

Sustainability concepts that rest on the idea of resource- or energy-efficiency improvements due to technological progress tend to overestimate the potential saving effects because they frequently ignore the behavioral responses evoked by technological improvements. Efficiency improvements also affect the demand for resources and energy, and often an increase in efficiency by 1% will cause a reduction in resource use that is far below 1% or, sometimes, it can even cause an increase in resource use. This phenomenon is commonly labeled the rebound effect, which is well-known among energy economists, but never attracted much attention in ecological economics. The paper starts with the traditional neoclassical analysis of the rebound effect in a partial equilibrium framework that concentrates on the demand of one particular energy service such as mobility or room temperature. It also provides an overview of some of the main empirical studies based on this model that mostly confirm the existence of the rebound effect, but are controversial about its actual importance. However, we have to go beyond the neoclassical single-service model in order to take care of the variety of possible feedback affecting energy use. The paper presents two important expansions of the single-service model in order to show the potential relevance of the rebound effect to ecological economics. First, it is shown that in a multi-services model it proves to be difficult to make general statements about the relevance of the rebound effect. In this case, the overall effect of an increase in energy efficiency on total energy use depends on the on the assumptions about the substitutability between the services considered and the direction of the income effect. Second, the paper also tries to take care of the fact that changes in resource use or energy use are frequently just ‘side-effects’ of other forms of technological progress. Especially technological change of a time-saving nature can have a large influence on energy use as many time-saving devices (for example, faster modes of transport) require an increase in energy consumption that is frequently reinforced by a ‘rebound effect with respect to time’. This effect will be especially strong when wages are high and, at the same time, energy prices are low, as is currently the case in most industrialized countries. Consequently, the paper also provides a strong argument for the introduction of energy taxes.

Many concepts of sustainable development emphasize the importance of efficiency improvements by technological progress. Technology is supposed to help us in promoting a society where it is possible to keep our present standard of life or even increase it while at the same time using less resources and especially less energy (see, for example, von Weizsäcker et al., 1997). These concepts rest on the idea that an increase in efficiency by 1% will, more or less, also lead to a decrease in resource use by 1%. However, this is usually not the case because technological improvements evoke behavioral responses. Often an increase in efficiency by 1% will cause a reduction in resource use that is far below 1% or, sometimes, it can even cause an increase in resource use. Among energy economists this phenomenon is known as the rebound effect1, which, however has not been studied in detail by ecological economists. The exact definition of the rebound effect varies in the literature. Sometimes, the term is used in a very general sense as a description of the functioning of market economies where increases in efficiency are frequently overcompensated by ‘growth effects’ (for example, Radermacher, 1997). An increase in the ratio of GDP to energy or to resource use does not necessarily lead to a decrease in energy or resource use because, at the same time, there will also be growth in economic activities. In this general sense the rebound effect describes increases in resource or energy efficiency that do not result in a corresponding decrease in energy or resource use. However, from this broad perspective, the growth in economic activities is not necessarily connected to efficiency improvements due to new technologies. The growth effects may be due to structural changes and a general growth tendency of market economies, which immediately leads into the discussion whether a continuously growing economy ever can be sustainable (Binswanger, 1995 and Daly, 1996) and whether the growth tendency may be reversed. Although these are important topics, they will not be further elaborated in this paper as, here, we are interested in the more specific question how efficiency improvements due to technological progress affect the demand for resources and energy. So far, the rebound effect has been mainly associated with energy use and the question how energy efficiency improvements affect energy consumption. Energy economists (especially Khazzoom, 1980, Khazzoom, 1987, Brookes, 1990 and Wirl, 1997) have come up with precise definitions of the rebound effect, which can easily be applied to resource use in general. This narrow but precise definition of the rebound effect is based on the following considerations. If technological progress makes equipment more energy efficient, less energy is needed to produce the same amount of product or service. However, the amount of product or service usually does not stay the same. Because the equipment becomes more energy efficient, the cost per unit of product or service that is produced with this equipment falls which, in turn, increases the demand for the product or the service. If energy efficiency of a car is increased by technological innovations, the 100 km can be driven with less fuel and, therefore, at lower cost. Consequently, people may drive more and longer distances because mobility (for example expressed in passenger km) has become cheaper. Several empirical studies of the 1980s and 1990s confirm the existence of the narrowly defined rebound effect with respect to improvements of energy efficiency in heating systems and insulation as well as with respect to transport activities. Generally, economists seem to agree that there exists a rebound effect, but they disagree about its actual importance. The disagreement, to a large degree, is the result of the very strong assumptions of the single-service model, from which the rebound effect is derived. It neglects the various possibilities of substitution effects among services as well as the income effect, and econometric studies based on the single-service model, therefore, can be misleading. In this paper we will argue that the rebound effect is indeed of empirical relevance and that behavioral responses evoked by efficiency increases can have a large impact on energy use even when we go beyond the single-service model in order to take care of the variety of the possible feedbacks affecting energy use. The paper also tries to take care of the fact that changes in resource use or energy use are frequently just ‘side-effects’ of other forms of technological progress. Especially technological change of a time-saving nature can have a large influence on energy use as many time-saving devices (e.g. faster modes of transport) require an increase in energy consumption. An effect that, as will be shown, is reinforced by a frequently strong rebound effect, not with respect to energy but with respect to time, which adds a new perspective to the discussion of the rebound effect. This ‘rebound effect with respect to time’ will be especially strong when wages are high and, at the same time, energy prices are low, as is currently the case in most industrialized countries. High wages, which represent the opportunity costs of time, in combination with low energy prices encourage the increasing use of time-saving but energy-intensive devices leading to an overall increase in energy use as people constantly try to ‘save’ time. The paper is organized as follows. Section 2 gives a precise definition of the rebound effect in a single-service model and also discusses the limits of this approach. Section 3 provides a summary of the main empirical studies which are largely based on the single-service model. In Section 4 we will outline the possible income and substitution effects that can be of considerable importance once we go from a single-service model to a multi-services model. Section 5 presents a further perspective on the rebound effect by considering time-saving innovations and their impact on energy use. Finally, Section 6 summarizes the main arguments.

Based on the arguments presented in this paper, the rebound effect with respect to households is indeed a relevant phenomenon that is too often neglected in the discussion of sustainable development. Sustainability concepts that rest on the idea of resource or energy improvements due to technological progress tend to overestimate the actual saving effects because they ignore the behavioral responses evoked by technological improvements that lead to the rebound effect. Of course, resource-saving technological change is a necessary condition for a decrease in resource intensity of economic activities. However, it is not a sufficient condition because of the rebound effect with respect to energy as well as, and probably even more important, the rebound effect with respect to time. In the most simple case, one can analyze how an increase in energy efficiency in the production of a single service changes the demand for that service, provided that the only relevant input used for producing the service is energy. This rather strong assumption underlies the single-service model that traditionally has been used by energy economists and which is restated in Section 2 The single-service model provides an accurate description of the rebound effect as long as substitution between different services and the income effect are of negligible quantity. However, this assumption is not always valid, and, in this case, the estimated rebound effects based on the single-service models may under- or overestimate the actual feedback in energy use induced by higher energy efficiency. Especially Lovins (1988) argued along these lines by stating that the rebound effect was more likely to be an income effect rather than a price effect. Consequently, the single-service model would be misleading and one would have to consider the income and substitution effects between different services. In a multi-services model, as shown in Section 4, it is difficult to make general statements about the relevance of the rebound effect. The overall effect of an increase in energy efficiency on total energy use depends on the particular services considered as well as on the assumptions about the substitutability between these services and the direction of the income effect. But there is no evidence that would question the relevance of the rebound effect even in a multi-services model. On the contrary, the feedback will even be stronger than suggested by the single- service model if substitutability between services is high and if the demand for energy services increases with income (positive income effect). However, the traditional single-service model of the rebound effect may also prove to be too restrictive in another respect as it concentrates on energy as the only relevant input in the production of a service. Especially time is a further important input to the household production of services and time-saving innovations can exert a large influence on energy use of households. Therefore, the paper offers a further perspective on the rebound effect (Section 5), where the focus is on time-saving innovations. Time-saving devices usually require more energy as is most evident from transport where an increase in the efficiency of time use (faster modes of transport) tends to be associated with a larger input of energy. If there is a time-saving innovation in the production of a particular service (for example, mobility), it can be produced with less time and at the same time households will demand more of this service (a rebound effect with respect to time) and substitute it for other services that are more time intensive but usually less energy intensive. Therefore, the overall effect of time-saving technological progress will be an increase in energy use. All perspectives on the rebound effect presented in the paper support the claim that resource-saving or energy-saving technological progress will not be sufficient in order to make the economy sustainable because of the induced feedback in energy or resource demand. But the traditionally used single-service model of the rebound effect does not capture the full range of these feedbacks. Moreover, energy use is also strongly influenced by innovations of a time-saving nature. The higher are the opportunity costs of time (wage rate) and the lower are energy prices, the more these innovations tend to increase the overall energy use. Therefore, an ecological tax reform that would increase the relative price of energy in comparison to the wage rate would be an important step to actually reduce energy consumption in the household sector as it dampens the ‘rebound effect with respect to energy’ (as described in 2 and 4) as well as the ‘rebound effect with respect to time’ as long as time-saving devices are energy intensive.