Carbon dioxide is a colorless, odorless gas that occurs naturally in small quantities in Earth's atmosphere. The earth's oceans, soil, plants and animals release CO2. The formula for carbon dioxide is CO2. The CO2 molecule contains 2 oxygen atoms that each share 2 electrons with a carbon atom to form 2 carbon-oxygen double bonds. The atoms are arranged like this (OHT). This is called a "linear molecule". Carbon dioxide is commonly found as a gas and is never liquid. It sublimates into a solid known as "dry ice" which is used as a substitute for regular ice as it is much colder and does not melt. Humans and animals exhale carbon dioxide, often called a greenhouse gas, as a waste product. Plants absorb this CO2 and use it to produce food. This is called photosynthesis. During this process oxygen is released which is then breathed in by humans and animals. This procedure is repeated over and over and a natural balance is achieved. However, this natural balance is disrupted by human activity. Every year, people around the world emit more than 5.5 billion tons of CO2 into the atmosphere. 75% of this is caused by the use of fossil fuels. These fuels are continually burned to run factories, power plants and vehicles. The main sources of CO2 emissions are electricity utilities, residential buildings, industry and transport. The remaining 25% is caused by the destruction of the world's forests. The reason for this is that there are fewer trees and plants that absorb CO2 but there are just as many, if not more, humans and animals that exhale it. The amount of CO2 in a planet's atmosphere affects the planet's temperature. . As more and more CO2 accumulates in the atmosphere, less heat can escape and the planet becomes hotter. CO2 traps solar radiation like a greenhouse. This is called global warming or the greenhouse effect. Global warming is becoming a serious problem and CO2 is the main cause. The earth is now hotter than it has been in thousands of years. The amount of CO2 deposited in the Earth's atmosphere by human activities is expected to double by 2050. It could quadruple in the future as developing countries, such as China, are eager to improve their standards of living.