Cointegrating Polynomial Regressions With Power Law Trends: Environmental Kuznets Curve or Omitted Time Effects?

The environmental Kuznets curve predicts an inverted U-shaped relationship between air pollution and economic growth. Current analyses frequently employ models that restrict nonlinearities in the data to be explained by economic growth only. We propose a Global Trend Augmented Cointegrating Polynomial Regression (GTACPR) to allow for nonlinearities in time and economic growth. The theoretical properties of the GTACPR are established. Empirically, a single global trend accurately captures all nonlinearities for all the countries studied, leading to a linear relationship between GDP and CO2. This suggests that the environmental improvement of the last years is due to factors different from GDP.