Carbon dioxide

Carbon dioxide is a chemical compound with the chemical formula CO₂. It consists of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. At room temperature, carbon dioxide is a gas that is usually odorless and clear. It plays a very important role in life on Earth because it is part of the carbon cycle, providing carbon for plants, animals, and other living things.

In the air, carbon dioxide acts like a blanket, letting in sunlight but also trapping heat. This is why it is called a greenhouse gas. Human activities, especially burning fossil fuels, have increased the amount of carbon dioxide in the atmosphere. This increase is one of the main reasons for climate change, which affects weather patterns and ecosystems around the world.

Carbon dioxide is also involved in many natural processes. Plants, algae, and cyanobacteria use it along with water and sunlight to make food through photosynthesis, releasing oxygen into the air. When animals and plants use energy, they break down food and release carbon dioxide back into the air through respiration. This constant 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, where it helps control the water’s ability to hold certain chemicals. Most of the carbon dioxide in water stays as gas and does not turn into acid.

Biological role

Carbon dioxide plays an important role in living things. It is created when organisms, like plants, animals, and fungi, break down sugars and fats for energy. This process is called cellular respiration.

Plants use carbon dioxide in a process called photosynthesis. They take in carbon dioxide and water to make sugars, which they use for food. They also release oxygen as a by-product. This process helps plants grow and provides 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 travels 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 that is found in Earth's atmosphere. It plays an important role in the planet's climate by trapping heat and helping to keep the Earth warm enough to support life.

Oceans

Main articles: Carbon cycle and Ocean acidification

Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100

Carbon dioxide also dissolves in the oceans, where it helps form different compounds. The oceans can hold a lot more carbon dioxide than the air, acting like a big storage area for this gas. This process is important for balancing carbon in the environment.

Ocean acidification

When carbon dioxide dissolves in seawater, it creates substances that can change the ocean's chemistry. This process, called ocean acidification, can affect marine life and ecosystems.

Hydrothermal vents

In some deep parts of the ocean, such as around underwater volcanoes, carbon dioxide can come out of the Earth’s surface through openings called hydrothermal vents. These places can sometimes release almost pure liquid carbon dioxide.