Carbon dioxide is a colorless, odorless gas made of one carbon atom and two oxygen atoms. It is essential to life. Plants use it in photosynthesis. Animals exhale it. It also traps heat in the atmosphere. Without CO2, Earth would be far colder and likely frozen.

Human activity has pushed CO2 levels far beyond natural ranges. Burning coal, oil, and gas releases carbon that was stored underground for millions of years. Deforestation removes trees that would absorb it. Atmospheric concentrations have risen from about 280 parts per million in 1750 to over 420 today. That is higher than at any point in the past 800,000 years.

CO2 is not the only greenhouse gas, but it is the most important one humans emit. It is also long-lived. A molecule released today can stay in the atmosphere for centuries. That means today's emissions commit the planet to warming for generations. Reducing them quickly is the core challenge of climate policy.

Key facts

• Chemical formula: CO2
• Produced by combustion and respiration
• Absorbed by plants and oceans
• Current atmospheric level above 420 ppm
• Long atmospheric lifetime