"Following Grimes and Roberts (1997), this paper studies the temporal instability of the environmental Kuznets curve for a sample of 51 countries over the period 1970-2008. Results show that: (1) an inverted U-shape curve appears in the early 1980s; (2) environmental efficiency increases over time as the coefficient of carbon dioxide (CO2) emissions per unit of primary energy consumption decreases; (3) there is an upward trend of the turning point, which goes from $10,000 in the early 1980s to almost $20,000 in 2008. JEL Codes: C21, O13, O44, Q40, Q56"