A carbon dioxide molecule is made of one carbon atom and two oxygen atoms and has a molecular formula of CO2. It is a colorless, nearly odorless gas that is naturally present in the Earth's atmosphere. The gas is used by plants as part of the photosynthetic cycle.
Carbon dioxide in the atmosphere is minimal, accounting for three of every 10,000 parts of atmospheric volume. The gas absorbs radiant energy from the sun. When the Earth was being formed, CO2 helped create the atmosphere, which warmed the planet. The greenhouse effect is an accelerated version of how CO2 normally works. More of the sun's energy is trapped, increasing the Earth's atmospheric temperature at a faster rate.
Photosynthesis in plants shows the direct relationship between carbon dioxide and oxygen. Carbon dioxide is pulled into the plants through pores called stomata. The gas travels to cells rich with chlorophyll, the substance that makes plants look green. The energy of the sun, along with the chlorophyll, creates carbohydrates and oxygen from the carbon dioxide. The carbohydrates are used and stored by the plants and the oxygen is released into the air.
When humans breathe air, they use the oxygen and exhale the carbon dioxide. With fewer plants, humans have less oxygen to breathe. That is why when visiting a forest or the sea, which has algae and seaweeds that produce oxygen, the air seems so much fresher.