CO2

Carbon dioxide, or CO2, is a very common, naturally occurring molecule that contains two oxygen atoms and one carbon atom. In everyday conditions on Earth, carbon dioxide is a commonly occurring gas that is all around us. It is colorless, odorless, is naturally present in Earth's atmosphere and is an important part of Earth's carbon cycle. All humans and animals exhale carbon dioxide when they breathe, and plants absorb it during a process called photosynthesis in order to grow.

CO2 is called a greenhouse gas (GHG) because as part of Earth's atmosphere CO2 traps the energy from the sun and keeps the world at a livable temperature. But increases in atmospheric CO2 associated with human activities can pose problems. For example, on the one hand, burning of fossil fuels releases extra CO2 into the atmosphere (along with other greenhouse gases), and, on the other, destruction of forested areas causes less CO2 to be absorbed by trees, etc. Both cases lead to too much energy or heat being trapped in our atmosphere. This extra energy causes increasing climatic instability, which results in major changes in weather patterns.

(Source: Global CCS Institute)