Carbon dioxide (CO₂) is a trace gas in Earth’s atmosphere. It is also found in large quantities dissolved in the world’s oceans. It is a byproduct of cellular respiration and is an essential component of photosynthesis—plants, algae, and certain types of bacteria remove it from the air in the process of carbon fixation.

Carbon dioxide is also a greenhouse gas produced as a byproduct of human activities. Burning fossil fuels—coal, oil, and natural gas—is the number one source of global CO₂ emissions. In 2009, the world got more than 80 percent of its energy from fossil fuels. Sixteen countries got 99 percent or more of their energy from fossil fuels. Electricity, heat production, and transportation are the biggest sources of global CO₂ emissions. Broken down by fuel type, the single largest source of global CO₂ emissions is the consumption of coal, followed by petroleum, then natural gas.

CO₂, like other greenhouse gases, is found naturally in Earth’s atmosphere. Scientists believe that the concentration of CO₂ in Earth’s atmosphere remained relatively stable for thousands of years at roughly 280 parts per million (ppm). However, since the Industrial Revolution in the 18th century, human activity has significantly increased the atmospheric concentration. Today, the concentration of CO₂ in the atmosphere stands at about 390 ppm—an increase of over 30 percent.

This map layer shows average annual CO₂ emissions per capita in metric tons for each country from 2006-2010. The data come from the United States Energy Information Administration.