Carbon dioxide (CO2) is an odorless gas that is highly important to life on Earth. CO2 is also known as a greenhouse gas; an excessive concentration can disrupt the natural regulation of temperature in the atmosphere and lead to global warming. The concentration of CO2 has especially increased as a result of the industrial revolution and exponential growth in manufacturing activities around the world. Deforestation, agriculture and fossil fuel use are the primary sources of CO2. According to the most recent data from 2012 provided by the U.S. Energy Information Administration, the top five countries that produce the most CO2 are China, the United States, India, Russia and Japan.

China

China is the largest emitter of carbon dioxide gas in the world with 8.1 billion metric tons in 2012. The primary source of CO2 emissions in China is coal burning. About 66% of the total energy derived in China comes from coal alone, and since coal is rich in carbon, burning it in China's power and industrial plants and boilers releases large amounts of CO2 into the atmosphere.

Also, China is one of the largest importers of oil, which contributes to large CO2 emissions through the country's use of motor vehicles. China plans to reduce its dependence on coal and decrease overall pollution in large cities in the future by generating more electricity using nuclear, renewable energy sources and natural gas.

The US

The U.S. is the second largest emitter of CO2, with approximately 5.27 billion metric tons of carbon dioxide emission in 2012. The largest source of CO2 emissions in the U.S. comes from power generation, transportation and industry. Even though the U.S. government undertook significant efforts to reduce the reliance on coal for electricity generation, about 90% of energy-related CO2 emissions still comes from coal, which accounts for 31% of total CO2 emissions.

Also, the U.S. economy is heavily reliant on the transportation sector, which burns petroleum for trucks, ships, trains and planes. U.S. consumers especially depend on their cars as their primary means of transportation, and this also contributes to the CO2 footprint through gasoline and diesel. Petroleum alone accounts for 43% of the total CO2 emissions.

Another large contributor to CO2 emissions in the U.S. is industry, which burns fossil fuels for energy. Also, the U.S. chemical sector uses various chemical reactions necessary to produce goods from raw materials, which in the process, emit CO2.

India

India is the third largest emitter of CO2 in the world; it produced about 1.83 billion metric tons of CO2 in 2012. As the Indian economy set out on a path toward urbanization and industrialization, the consumption of solid fuels, such as coal, skyrocketed. Coal consumption in India almost doubled from 420 million metric tons in 2004 to 800 million metric tons in 2014. Coal mines are abundant in India, and coal is generally cheaper in the country than imported oil and gas. Given these trends, the Indian economy is likely to increase its reliance on coal as the main source of energy for electricity generation and powering its heavy industry. India's CO2 footprint is bound to go up in the future.

The Russian Federation

Russia is the fourth largest contributor toward CO2 emissions in the world with 1.78 billion metric tons in 2012. Russia has one of the largest natural gas deposits in the world, and natural gas is the primary source of energy and power generation in the country, which contributes approximately 50% of the total carbon dioxide emissions. Coal, which is widely used in chemical and other basic material industries and for power generation in Russia, is the second largest contributor of Russia's CO2 emissions.

Japan

Japan is the fifth largest emitter of CO2 worldwide, producing 1.26 billion metric tons of carbon dioxide in 2012. Japan is heavily reliant on burning natural gas and coal to generate electricity for its population and various industries. After the nuclear reactors in Fukushima were shut down in 2011, dependence on fossil fuels increased even more. As Japan prepares to reopen its nuclear power plants, its CO2 footprint may stabilize in the future.