Is there an Environmental Urban Kuznets Curve? The case of polluting emissions due to daily mobility in 37 cities. (In French)

The Environmental Kuznets Curve (EKC) has given rise to a flourishing literature since the beginning of the 90’s. The EKC postulates an inverted U-shaped relationship between income and polluting emissions: there would be a level of wealth beyond which polluting emissions would decrease. Surprisingly, this issue has rarely been applied to the cities. Yet we consider such a question as a pertinent one. This article aims at analyzing the Urban EKC (UEKC) hypothesis. It tests it with a sample of 37 cities in the world. Previous studies on the UEKC hypothesis are very scarce. They are the ground for us, to define a specific methodological posture. First, we use polluting emissions per capita instead of pollutants concentrations: thus we control for the influence of urban size. Second, we only take in account pollutants due to a unique source, which is daily mobility. This makes the explanation of the income-polluting emissions relation easier, as our comments are based on a specific, well constituted literature about factors of daily mobility. We expose the theoretical mechanisms by which the UEKC due to daily mobility could be validated. The impact of income on polluting emissions is threefold : behavioural, with a direct effect and an indirect one ; technical (the environmental efficiency of the vehicles increases) ; political (planning authorities wish to evolve towards a « sustainable mobility »). The empirical part of the paper is a test of the UEKC on a sample of 37 cities in the world. We present three important results. First, the estimation of quadratic regressions gives an inverted U-shaped relationship for most of the pollutants, which doesn’t permit to invalidate the UEKC hypothesis. Second, we show that the explanation of such curves is linked to two sets of factors: individual behaviours (e.g. modal choice) and collective choices (e.g. transit supply). Third, we discuss the validity of the UEKC hypothesis, that is we seek to explain the level of polluting emissions. As many factors are entangled, we use a principal components analysis to show that the influence of income may in fact reflect the influence of both urban form and consumers’ habits on polluting emissions due to daily mobility.