Carbon Dioxide 101

WHAT IS CARBON DIOXIDE?

Carbon Dioxide molecule — Depiction of a carbon dioxide molecule.

Carbon dioxide (commonly abbreviated as CO₂) is a clear gas composed of one atom of carbon (C) and two atoms of oxygen (O). Carbon dioxide is one of many molecules where carbon is commonly found on the Earth. It does not burn, and in standard temperature and pressure conditions it is stable, inert, and non-toxic. Carbon dioxide occurs naturally in small amounts (about 0.04 percent) in the Earth's atmosphere.

HUMANS BREATHE IT

Carbon dioxide is a minor part of the air that humans breathe. It is also a byproduct of our body’s metabolism and is subsequently exhaled from the lungs.

PLANTS NEED IT

Despite the minor amount of CO₂ in the air, it is essential to plant life and is a key part of the global carbon cycle. Plants take in CO₂, break down the CO₂ into carbon and oxygen, release the oxygen to the atmosphere, and then retain the carbon to live and grow. When the plant dies or is burned, the carbon recombines with O₂ in the air and CO₂ is formed again, completing the cycle.

Myth: The presence of CO₂ in the atmosphere is bad and only comes from the burning of fossil fuels.

Reality: Carbon dioxide is derived from both natural and anthropogenic sources, is essential to plant life, and is a key part of the Earth’s carbon lifecycle.

WHAT IS THE CARBON CYCLE?

The carbon cycle is the process through which carbon is cycled through the air, ground, plants, animals, and fossil fuels. People and animals inhale oxygen from the air and exhale carbon dioxide (CO₂), while plants absorb CO₂ for photosynthesis and emit oxygen back into the atmosphere. Carbon dioxide is also exchanged between the atmosphere and the oceans. This natural system of processes keeps CO₂ levels in the atmosphere stable over time. The figure below depicts the carbon cycle by showing how carbon moves between land, the atmosphere, and the ocean through various natural- and human-initiated processes. On land, carbon is contained within formations, the soil, plants, and animals. When these decompose, the carbon can be emitted to the atmosphere as CO₂. Once in the atmosphere, the carbon can then be absorbed by the oceans or by a land/ocean-based plant or shell-bearing animal. It is important to note that only a small amount of the Earth’s carbon moves through the carbon cycle each year.

WHAT IS THE GREENHOUSE EFFECT?

The greenhouse effect is used to describe the phenomenon whereby the Earth's atmosphere traps solar radiation, caused by the presence of gases, such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and water vapor (H₂O). Collectively, these gases are referred to as greenhouse gases (GHGs). The greenhouse effect gets its name from the process that actually occurs in a greenhouse. In a greenhouse, short wavelength visible sunlight shines through the glass panes and warms the air and the plants inside. The radiation emitted from the heated objects inside the greenhouse are of longer wavelength and therefore are unable to pass through the glass barrier, maintaining a warm temperature in the greenhouse.

The Earth's natural greenhouse effect acts similarly. Sunlight that enters the atmosphere is either reflected, absorbed, or simply passes through. The sunlight that passes through the atmosphere is either absorbed by the Earth's surface or reflected back into space. The Earth's surface heats up after absorbing this sunlight, and emits long wavelength radiation back into the atmosphere. Some of this radiation passes through the atmosphere and into space, but the rest of it is either reflected back to the Earth’s surface or absorbed by atmospheric GHGs that re-radiate longer wavelengths back to Earth's surface. These GHGs trap the sun’s energy within the atmosphere, warming the planet.

GHGs can be compared to the glass panes in the greenhouse example, since they trap indirect heat from the sun. Carbon dioxide and other GHGs help create and maintain the natural greenhouse effect that keeps Earth hospitable to life. GHGs do not have a negative effect when present in natural amounts; in fact, the Earth’s average temperatures would be much cooler without them.

Myth: GHGs like CO₂ should be completely removed from the atmosphere.

Reality: GHGs like CO₂, within certain concentration ranges, help to maintain a global temperature hospitable to life.

WHAT ARE THE PRIMARY SOURCES OF CO₂?

There are both natural carbon dioxide (CO₂) sources and man-made (anthropogenic) CO₂ sources.

NATURAL CO₂ SOURCES

Natural CO₂ sources account for the majority of CO₂ released into the atmosphere. Oceans provide the greatest annual amount of CO₂ of any natural or anthropogenic source. Other sources of natural CO₂include animal and plant respiration, decomposition of organic matter, forest fires, and emissions from volcanic eruptions. There are also naturally occurring CO₂ deposits found in formation layers within the Earth’s crust that could serve as CO₂ sources.

ANTHROPOGENIC CO₂ SOURCES

Anthropogenic CO₂ sources are part of our everyday activities and include those from power generation, transportation, industrial sources, chemical production, petroleum production, and agricultural practices. Many of these source types burn fossil fuels (coal, oil, and natural gas), with CO₂ emissions as a byproduct. Of these CO₂ sources, electric power generation contributes the greatest amount of anthropogenic CO₂ to the atmosphere.

Myth: Carbon dioxide comes only from anthropogenic sources, especially from the burning of fossil fuels.

Reality: Carbon dioxide comes from both natural and anthropogenic sources; natural sources are predominant.

WHAT ARE PEOPLE DOING NOW TO MANAGE CO₂?

Monitoring well at the an NETL-sponsored CO2 storage project. — Monitoring well at the an NETL-sponsored
CO2 storage project.

A combined portfolio of carbon management options is being implemented to reduce current emission levels of carbon dioxide (CO₂) associated with energy production while maintaining energy security and building the technologies and knowledge base needed to mitigate carbon emissions. The U.S. portfolio includes:

Use fuels with reduced carbon emission intensity – renewables, nuclear, and natural gas.
Adopt more efficient technologies on both the energy demand and supply sides.
Develop and implement carbon capture and storage (CCS) technology.

CCS is the separation and capture of CO₂ from industrial processes followed by the transportation and safe, permanent storage in deep underground geologic formations. CCS is a viable management option for anthropogenic CO₂ because numerous studies have shown that it can account for up to 55 percent of the emissions mitigation needed to stabilize and ultimately reduce concentrations of CO₂ in the atmosphere. Since 1997, the U.S. Department of Energy's (DOE) Carbon Storage Program has significantly advanced CCS through a diverse portfolio of applied research projects that include industry, academia, and other research facilities, as well as through collaborative research through the National Laboratory Network, including the National Energy Technology Laboratory’s (NETL) Research and Innovation Center (RIC). There are numerous projects across the country where CO₂ is being injected into suitable deep formations to demonstrate the process of it being stored safely. There is also the potential to develop approaches that can convert captured CO₂ into useful products, such as a fuel, chemicals, or plastics.

Myth: Nothing is being done about managing CO₂ emissions from large-scale anthropogenic sources.

Reality: Advances in carbon capture and storage (CCS) as a CO₂ mitigation option are occurring through research and development (R&D) efforts in the United States and around the world.