Carbon dioxide
Carbon dioxide is a colorless gas made of one carbon atom and two oxygen atoms. It is a normal part of air, respiration, photosynthesis, oceans, volcanoes, combustion, and the carbon cycle, but rising atmospheric carbon dioxide from human activity is also a central driver of modern climate change.
What carbon dioxide is
Carbon dioxide, or CO2, is a chemical compound made from one carbon atom bonded to two oxygen atoms. At ordinary temperatures and pressures, it is a colorless gas. It is present in Earth's atmosphere in small amounts compared with nitrogen and oxygen, but it has outsized importance because it moves through living systems, oceans, rocks, industry, and climate.
Molecule and properties
A carbon dioxide molecule is linear: oxygen-carbon-oxygen. The molecule is not combustible, and carbon dioxide can displace oxygen in enclosed spaces. When cooled and compressed, it can become liquid; at normal atmospheric pressure, solid carbon dioxide is dry ice, which sublimates directly from solid to gas rather than melting into liquid.
In living systems
Animals, plants, fungi, and many microbes release carbon dioxide during respiration as they break down carbon-rich molecules for energy. Plants, algae, and cyanobacteria also use carbon dioxide in photosynthesis, turning it into organic carbon compounds with energy from light. This exchange is one of the reasons CO2 is central to food webs and the carbon cycle.
Natural and human sources
Natural sources of carbon dioxide include respiration, decay, wildfires, ocean exchange, volcanic activity, and chemical weathering. Human activities add large extra flows, especially by burning coal, oil, and natural gas, producing cement, clearing forests, and burning biomass. Those added flows matter because they exceed what natural sinks can quickly absorb.
Carbon dioxide and climate
Carbon dioxide is a greenhouse gas. It absorbs some infrared radiation emitted by Earth's surface and atmosphere, then re-emits energy in different directions. This heat-trapping role is natural and necessary, but increasing atmospheric CO2 strengthens the greenhouse effect. Because CO2 can remain in the climate system for a long time, emissions today affect warming far into the future.
Oceans and acidity
Oceans absorb a large amount of carbon dioxide from the atmosphere. Dissolved CO2 reacts with seawater to form carbonic acid and related carbonate chemistry changes. This process is a major part of the carbon cycle, but extra CO2 can lower ocean pH and reduce carbonate ion availability, which affects corals, shell-forming organisms, and marine ecosystems.
Indoor air
Indoors, carbon dioxide often rises because people exhale it. CO2 is not a complete measure of indoor air quality, but it can be a useful indicator of occupancy and ventilation in many people-dominated spaces. High readings can signal that outdoor-air ventilation is low relative to the number of occupants, though other pollutants need their own controls and measurements.
Uses and safety
Carbon dioxide is used in carbonated beverages, fire extinguishers, welding shielding gas, greenhouses, refrigeration, dry ice, chemical production, and some industrial processes. Safety depends on concentration and setting. CO2 is not poisonous in the same way as carbon monoxide, but high levels can displace oxygen and cause headache, dizziness, unconsciousness, or death in confined spaces.
Why it matters
Carbon dioxide matters because it connects chemistry to everyday life, biology, industry, health, and climate. The same molecule is plant food, a respiration product, a useful industrial gas, a ventilation clue, an ocean-chemistry driver, and the main long-lived greenhouse gas added by human activity. Understanding CO2 helps separate normal natural cycles from the rapid changes caused by extra emissions.