Following Roberts and Grimes (1997), we study the temporal instability of the environmental Kuznets curve for a panel of 51 countries over the period 1970-2008. Our results show that: (1) an inverted U-shape curve appears in the early 1980s; (2) the environmental efficiency increases over time as the coefficient of carbon dioxide (CO2) emissions per unit of primary energy consumption decreases; (3) there is a rising trend of the turning point income that goes from 10,000 USD in the early 1980s to almost 20,000 USD in 2008.