Carbon dioxide

Carbon dioxide is a chemical compound with the chemical formula CO₂. It has one carbon atom covalently double bonded to two oxygen atoms. At room temperature, carbon dioxide is a gas that has no smell and is clear. It is very important for life on Earth because it is part of the carbon cycle. It gives carbon to plants, animals, and other living things.

In the air, carbon dioxide acts like a blanket. It lets in sunlight but also traps heat. This is why it is called a greenhouse gas. Human activities, like burning fossil fuels, have added more carbon dioxide to the air. This increase helps cause climate change. Climate change changes weather patterns and ecosystems around the world.

Carbon dioxide is part of many natural processes. Plants, algae, and cyanobacteria use it with water and sunlight to make food through photosynthesis. This releases oxygen into the air. When animals and plants use energy, they break down food and release carbon dioxide into the air through respiration. This cycle helps keep Earth’s environment balanced.

Chemical and physical properties

Carbon dioxide, or CO₂, is a molecule made of one carbon atom linked to two oxygen atoms. At room temperature, it is a gas that has no color or smell when found in small amounts.

When carbon dioxide mixes with water, it forms a substance called carbonic acid. This weak acid changes how water behaves, especially in the ocean. Most of the carbon dioxide in water stays as gas and does not turn into acid.

Biological role

Carbon dioxide is important for living things. It is made when plants, animals, and fungi break down sugars and fats to get energy. This is called cellular respiration.

Plants use carbon dioxide in a process called photosynthesis. They take in carbon dioxide and water to make sugars for food. They also release oxygen as a by-product. This helps plants grow and gives food for other animals.

Typical CO₂ concentration effects
Concentration	Note
280 ppm	Pre-industrial levels
421 ppm	Current (May 2022) levels
~1121 ppm	ASHRAE recommendation for indoor air
5,000 ppm	USA 8h exposure limit
10,000 ppm	Cognitive impairment, Canada's long term exposure limit
10,000-20,000 ppm	Drowsiness
20,000-50,000 ppm	Headaches, sleepiness; poor concentration, loss of attention, slight nausea also possible

Human physiology

The human body makes about 2.3 pounds (1 kilogram) of carbon dioxide each day. This gas moves through the veins and is breathed out through the lungs, which lowers the amount in the arteries.

Carbon dioxide in the blood is carried in three ways: most becomes bicarbonate ions, some dissolves in blood fluid, and a small amount attaches to hemoglobin in red blood cells. Hemoglobin helps carry both oxygen and carbon dioxide but in different ways. This process helps move carbon dioxide from tissues to the lungs.

Reference ranges or averages for partial pressures of carbon dioxide (abbreviated pCO₂)
Blood compartment	(kPa)	(mm Hg)
Venous blood carbon dioxide	5.5–6.8	41–51
Alveolar pulmonary gas pressures	4.8	36
Arterial blood carbon dioxide	4.7–6.0	35–45

Concentrations and role in the environment

Atmosphere

Further information: Carbon cycle

Annual CO2 flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since the 1960s. Units in equivalent gigatonnes carbon per year.

Carbon dioxide is a gas in the air around us. It helps keep Earth warm by trapping heat. This makes Earth a place where life can thrive.

Oceans

Main articles: Carbon cycle and Ocean acidification

Carbon dioxide can dissolve in the ocean. It helps form different compounds there. The ocean can hold much more carbon dioxide than the air, acting like a big storage space for the gas. This helps balance carbon in nature.

Ocean acidification

When carbon dioxide mixes with seawater, it changes the ocean's chemistry. This process, called ocean acidification, can affect sea creatures and their homes.

Hydrothermal vents

In deep ocean areas, near underwater volcanoes, carbon dioxide can escape from the Earth through openings called hydrothermal vents. Sometimes, these places release almost pure liquid carbon dioxide.