Carbon dioxide emissions are good indicators for evaluating environmental pollution. The aim of this study is to test for the existence of an environmental Kuznets curve (EKC) for CO2 using the pooled mean group (PMG) estimation method for different income group of countries namely high, middle, low income countries, and global (all group countries) for the period of 1960 to 2010. Our PMG estimation results support that inverted U-shape of the EKC is valid for the high-income, low-income, and global cases but not for middle-income countries. This suggests that as income for those that have the inverted U-shape EKC increases environment is being affected poorly at the beginning and then quality of environment improves. The middle-income countries that have linear positive relationship between pollution and income do not have environmentally friendly technologies. Therefore these countries did not show sufficient improvement in the use of clean energy and energy efficiency, and have still effect on global warming.
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