CO2 is a scientific abbreviation for carbon dioxide. CO2 is a greenhouse gas that holds heat in the atmosphere, causing the temperature of the earth to rise (the greenhouse effect).

CO2 is released during the combustion of biomass such as wood and plant waste and fossil fuels: oil, gas and coal. Much less CO2 is released by the combustion of natural gas than by the combustion of oil and coal:  30% less CO2 is released compared to oil, and 50% less CO2 compared to coal.