* Gas: This is the most common state of carbon dioxide on Earth. It makes up about 0.04% of the atmosphere.

* Dissolved in water: Carbon dioxide dissolves in water, forming carbonic acid (H2CO3). This is a major component of the carbon cycle, as it contributes to ocean acidification.

* Solid: Carbon dioxide can also exist as a solid, known as dry ice. This occurs at very low temperatures and high pressures. While not common on Earth's surface, it does occur naturally in polar regions and is used in various industrial applications.

So, while carbon dioxide is primarily a gas, it exists in other states as well, each playing a role in the Earth's complex systems.