Although there is little controversy about global warming, there is still a debate regarding whether global warming is mainly due to anthropogenic greenhouse gas emissions. Many researchers strongly believe that global warming is mainly due to greenhouse gas emissions. However, other scientists argue that the standard models overstate the importance of CO2 emissions. We propose a reduced-form regression-based test. With the temperature and CO2 emissions data from the U.S., we find little evidence in support of the notion that recent global warming is mainly due to CO2 emissions. Our results, therefore, call for more research on the causes of recent global warming.