Shown here are four representations chemists use for carbon dioxide. In the colored models, carbon is light gray and oxygen is red. Click on image for full size (25 Kb GIF) Windows to the Universe original artwork by Randy Russell.

Carbon dioxide is a colorless and non-flammable gas at normal temperature and pressure. Although much less abundant than nitrogen and oxygen in Earth's atmosphere, carbon dioxide is an important constituent of our planet's atmosphere. A molecule of carbon dioxide (CO₂) is made up of one carbon atom and two oxygen atoms.

Carbon dioxide is an important greenhouse gas that helps to trap heat in our atmosphere through the greenhouse effect. Without it, our planet would be inhospitably cold. However, a gradual increase in CO₂ concentrations in Earth's atmosphere is helping to drive global warming, threatening to disrupt our planet's climate as average global temperatures gradually rise.

Carbon dioxide is the fourth most abundant component of dry air. It has a concentration of 380 ppmv (parts per million by volume) in Earth's atmosphere. Scientists estimate that before human industrial activity, CO₂ concentration was around 270 ppmv. Carbon dioxide levels in our atmosphere have thus risen about 40% since the onset of human industrialization, and are expected to play a troubling role in raising global temperature. Atmospheric carbon dioxide concentrations have varied substantially in the pre-human history of our planet, and have had profound impacts on global temperatures in the past.

Carbon dioxide plays a key role in Earth's carbon cycle, the set of processes that cycle carbon in many forms throughout our environment. Volcanic outgassing and wildfires are two significant natural sources of CO₂ in Earth's atmosphere. Respiration, the process by which organisms liberate energy from food, emits carbon dioxide. When you exhale, it is carbon dioxide (amongst other gases) that you breathe out. Combustion, whether in the guise of wildfires, as a result of slash-and-burn agricultural practices, or in internal combustion engines, produces carbon dioxide.

Photosynthesis, the biochemical process by which plants and some microbes create food, uses up carbon dioxide. Photosynthetic organisms combine CO₂ and water (H₂O) to produce carbohydrates (such as sugars) and emit oxygen as a byproduct. Places such as forests and areas of the ocean that support photosynthetic microbes therefore act as massive carbon "sinks", removing carbon dioxide from the atmosphere via photosynthesis. Earth's early atmosphere had much higher CO₂ levels and almost no oxygen; the rise of photosynthetic organisms led to an increase in oxygen which enabled the development of oxygen-breathing creatures such as us!

Burning generates CO₂, although incomplete combustion due to limited oxygen supply or an excess of carbon can also produce carbon monoxide (CO). Carbon monoxide, a dangerous pollutant, eventually oxidizes to carbon dioxide.

Small canisters containing pressurized CO₂ are used to inflate bicycle tires and life jackets and to power paintball guns. The "fizz" in soda pop is supplied by carbon dioxide. Carbon dioxide is also released by yeast during fermentation, giving beer its head and making champagne bubbly. Because it is not flammable, CO₂ is used in some fire extinguishers. Carbon dioxide forms a weak acid, called carbonic acid (H₂CO₃), when dissolved in water.

Carbon dioxide is the most abundant gas in the atmospheres of Mars and Venus. Solid, frozen carbon dioxide is called "dry ice". The polar ice caps of Mars are a mixture of normal water ice and dry ice. Liquid CO₂ only forms at pressures higher than about 5 times the atmospheric pressure on Earth at sea level, so in many situations dry ice does not melt into a liquid form. Instead, it goes directly from a solid state to a gaseous state in a process called sublimation.

Here are four different ways chemists use to show a molecule of carbon dioxide. In the colored molecule models, carbon is light gray and oxygen is red. Click on image for full size (25 Kb GIF) Windows to the Universe original artwork by Randy Russell.

Carbon dioxide (CO₂) is a kind of gas. There isn't that much carbon dioxide in Earth's atmosphere, but it is still very important. Carbon dioxide is a greenhouse gas. That means it helps trap heat coming from the Sun in our atmosphere through the greenhouse effect. Without carbon dioxide in our air, the Earth would be very cold.

Where does the carbon dioxide in our air come from? When humans and other animals breathe, we take in oxygen and breathe out carbon dioxide. We use the oxygen to get energy from food - this is called respiration.

Burning things also makes lots of carbon dioxide. Natural fires like forest fires make carbon dioxide. Things that humans burn make CO₂, too. The smokestacks of factories that burn coal give off carbon dioxide. The engines of cars, trucks, and buses also pump carbon dioxide into the air. They sometimes give off another gas, carbon monoxide, too.

Earth isn't the only place where carbon dioxide is important. Most of the atmosphere of Venus is CO₂. The atmosphere of Mars is also mostly carbon dioxide. If carbon dioxide gets really cold, it can freeze into a solid. This kind of ice is called "dry ice". The polar ice caps on Mars are partly made of dry ice!

Here are four different ways chemists use to show a molecule of carbon dioxide. In the colored molecule models, carbon is light gray and oxygen is red. Click on image for full size (25 Kb GIF) Windows to the Universe original artwork by Randy Russell.

Carbon dioxide (CO₂) is a kind of gas. There isn't that much carbon dioxide in Earth's atmosphere, but it is still very important. Carbon dioxide is a greenhouse gas. That means it helps trap heat coming from the Sun in our atmosphere through the greenhouse effect. Without carbon dioxide in our air, the Earth would be very cold.

Where does the carbon dioxide in our air come from? When humans and other animals breathe, we take in oxygen and breathe out carbon dioxide. We use the oxygen to get energy from food - this is called respiration.

Burning things also makes lots of carbon dioxide. Natural fires like forest fires make carbon dioxide. Things that humans burn make CO₂, too. The smokestacks of factories that burn coal give off carbon dioxide. The engines of cars, trucks, and buses also pump carbon dioxide into the air. They sometimes give off another gas, carbon monoxide, too.

Earth isn't the only place where carbon dioxide is important. Most of the atmosphere of Venus is CO₂. The atmosphere of Mars is also mostly carbon dioxide. If carbon dioxide gets really cold, it can freeze into a solid. This kind of ice is called "dry ice". The polar ice caps on Mars are partly made of dry ice!