According to the Netherlands Environmental Assessment Agency, by 2006 China overtook the United States for the top spot in carbon dioxide emissions. The United States now ranks second in that category.
According to the World Resources Institute, five countries (the United States, China, Russia, Japan, and India) contribute more than fifty percent of the world’s total carbon dioxide emissions. The United States ranks first, contributing 22% of the total.
Note: see updated information regarding this ranking here.