Carbon dioxide

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Carbon dioxide (CO₂) is a colorless, odorless, incombustible gas that is vital to life on Earth. It is a naturally occurring chemical compound that is present in the atmosphere. Carbon dioxide exists in the Earth's atmosphere at a concentration of approximately 0.04 percent (400 parts per million) by volume. It is produced from human and animal respiration, volcanoes, hot springs, and geysers, and the burning of coal, petroleum, and natural gas, which are commonly known as fossil fuels.

Background

The above diagram shows carbon movements between land, the atmosphere, and oceans (in billions of tons per year). Yellow numbers indicate natural fluctuations, red numbers indicate human contributions, and white numbers represent stored carbon. The diagram does not include the volcanic and tectonic activity that sequesters and releases carbon (image from NASA).

Roughly all of the Earth's atmosphere is made up of five gases: nitrogen, oxygen, water vapor, argon, and carbon dioxide. Carbon dioxide is the fourth-most abundant compound in the atmosphere at 0.04 percent. Nitrogen and oxygen are the most abundant compounds at 78 percent and 20.9 percent, respectively. Meanwhile, carbon dioxide is the most abundant gas in the atmospheres of Mars and Venus.

The global carbon cycle is the exchange of carbon dioxide between the Earth's atmosphere, land, oceans, and fossil fuels, all of which emit and absorb CO₂. For example, plants and some microbes use carbon dioxide in photosynthesis, the process by which these organisms create food. These organisms then emit oxygen as a byproduct of photosynthesis. Forests and parts of the ocean, which support photosynthetic organisms, act as carbon sinks that remove carbon dioxide from the atmosphere through photosynthesis. The early atmosphere of Earth had significantly higher carbon dioxide levels compared to now and contained nearly no oxygen. The emergence of photosynthetic organisms like plants led to increased oxygen levels in the atmosphere, allowing the development of oxygen-breathing organisms such as plants, animals, and human beings.

Two significant natural sources of carbon dioxide are volcanos and wildfires. Respiration, the process by which organisms convert food into energy, releases carbon dioxide. The combustion of coal, petroleum, and natural gas, including internal combustion engines, also emit carbon dioxide.

Common products that use pressurized carbon dioxide include bicycle tires, life jackets, and paintball guns. Carbon dioxide is used to produce soda pop and is created by yeast during fermentation in the production of alcoholic beverages. Because it is inflammable, carbon dioxide is used in some fire extinguishers.

Historical concentration of carbon dioxide

Scientists have attempted to calculate the historical concentration of carbon dioxide in the atmosphere in the distant past as a comparison to recent concentrations. According to the GEOCARB model, which is a well-known geochemical model developed by scientist Robert Berner at Yale University to measure the geologic carbon cycle, carbon dioxide levels in the Ordovician period (420 million to 450 million years ago) were greater than 4,000 parts per million (ppm). Carbon dioxide levels in the Jurassic-Cretaceous periods (132 million to 151 million years ago) were 2,000 ppm. According to Berner, however, there are uncertainties in the model because the periods occurred so long ago, and the model does not take into account short-term carbon dioxide fluctuations during those periods.

Between 1750 and 2015, the atmospheric concentration of carbon dioxide rose from approximately 275-280 parts per million (ppm) to approximately 400 ppm. The historical estimates are based on measurements of air in polar ice sheets. Another estimate of atmospheric carbon dioxide comes from the Mauna Loa Observatory in Hawaii. In 1960, the atmospheric concentration of carbon dioxide was approximately 316 ppm. In 2015, the concentration was approximately 400 ppm. The chart below details the estimated concentration of carbon dioxide each year since 1960.

Carbon dioxide and greenhouse gases

The five naturally present greenhouse gases are water vapor, ozone, carbon dioxide, methane, and nitrous oxide. As a greenhouse gas, carbon dioxide captures radiant energy in the atmosphere. This phenomenon, known as the greenhouse effect, occurs when radiation that is attempting to enter into space is trapped in the atmosphere. Without carbon dioxide and other greenhouse gases, Earth would be much colder and inhospitable to life.

The most important greenhouse gas is water vapor because it is the largest contributor to the greenhouse effect. According to the American Chemical Society, water vapor accounts for approximately 60 percent of the warming caused by the greenhouse effect. The global average concentration of water vapor, however, is difficult to measure because it varies between seasons and between one place and another, though a warmer atmosphere will likely contain more water vapor.

The table below shows an estimate of the tropospheric concentration of the four other greenhouse gases in the pre-industrial era (pre-1750) and the estimate of the tropospheric concentration of these gases measured in April 2016. The figures come from the Carbon Dioxide Information Analysis Center at the U.S. Department of Energy and were calculated using data from the United Nations' Intergovernmental Panel on Climate Change (IPCC). The carbon dioxide concentration given is the average for the year 2015 from the U.S. National Oceanic and Atmospheric Administration (NOAA).

Estimate of greenhouse gas concentrations since pre-1750 (as of April 2016)
Greenhouse gas	Pre-1750 tropospheric concentration†	Current tropospheric concentration (measured in April 2016)‡	Percentage change
Carbon dioxide	280 parts per million (ppm)	399.52 ppm	42.6%
Methane	722 parts per billion (ppb)	1,834 ppb	154.0%
Nitrous oxide	270 parts per billion (ppb)	328 ppb	21.4%
Tropospheric ozone	237 parts per billion (ppb)	337 ppb	42.1%
†Pre-industrial concentrations of carbon dioxide, methane, and nitrous oxide come from the IPCC's 2013 report. The IPCC assumed that the concentrations of these gases from prior to 1750 are practically uninfluenced by human activities. ‡Because atmospheric concentrations of most gases tend to vary over the course of a year, figures given represent averages over a specific 12-month period for all gases except tropospheric ozone, for which the most recent total amount has been more broadly estimated. Source: Carbon Dioxide Information Analysis Center, "Recent greenhouse gas concentrations," April 2016

Carbon dioxide and global warming

Scientists study the relationship between carbon dioxide concentrations in the atmosphere and global temperatures to determine how higher CO₂ levels may contribute or have contributed to global warming (defined as a rise in global average temperature). Equilibrium climate sensitivity (ECS) is a metric to calculate global average temperature changes due to higher CO₂ concentrations. Specifically, scientists use ECS to determine how the global average temperature (which is calculated from various temperature measurements around the globe) may respond to a doubling of CO₂ in the atmosphere, taking into account multiple climatic factors. ECS is calculated using computer-generated simulations known as climate models.

The discussion over carbon dioxide and global warming focuses on the precise role of CO₂ emissions in global temperatures compared to other climatic factors. Below are summaries of different scientific views on the role of CO₂ in global warming:

• According to its 2007 assessment on global warming and climate change, the United Nations' Intergovernmental Panel on Climate Change (IPCC), whose stated objective is to provide "the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change", argued that equilibrium climate sensitivity would be between 2°C (Celsius) and 4.5°C with a best estimate of roughly 3°C by the year 2100. In its 2014 report, the IPCC reduced its low-end estimate of climate sensitivity to between 1.5°C and 4.5°C. The 2014 report concluded that the increase in "global atmospheric concentrations of CO₂ [carbon dioxide], methane (CH₄) and nitrous oxide (N₂O)" from human activities, particularly the use of coal, oil, and natural gas, is responsible for "most of the observed increase in global average temperatures since the mid-20th century."

• According to David Titley, director of the Center for Solutions to Weather and Climate Risk at Pennsylvania State University, the warming effect of human-caused CO₂ emissions from 1750 to 2011 accounted for approximately 42 percent of the total warming influence from human-caused emissions during this period. Titley further argued that the reduced rate of warming observed between 1998 and 2013 did not alter the view that human-caused CO₂ emissions were the dominant driver of global warming; that climate models predict short periods of warming and cooling; that the reduced rate of warming occurred alongside continued near-surface warming of the atmosphere; and that the reduced rate of warming will likely become more rapid over time due to existing CO₂ concentrations in the atmosphere.

• According to Judith Curry, former Chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, discrepancies exist between climate model predictions of global temperatures and observational data. Curry argued that climate models did not predict the reduced rate of warming between 1998 and 2013 and that equilibrium climate sensitivity (the effect on global temperatures by doubling CO₂ concentrations) may be around or below 2°C, which is lower than estimated by the IPCC. Curry further argued that uncertainties in the role of water vapor, cloud formation, and solar variation, which can affect the rate of warming, must be better understood to determine the precise role of human-caused CO₂ emissions on future warming.

• According to the Nongovernmental International Panel on Climate Change, a panel of scientists and scholars who "are not predisposed to believe climate change is caused by human greenhouse gas emissions", climate models used by the IPCC overestimate the climate's sensitivity to CO₂ emissions by excluding or poorly modeling factors that may moderate or reduce global warming, such as low-level cloud formation, the cooling effects of natural and industrial-caused aerosols, and ocean emissions of dimethyl sulfide (an organic sulfur compound that contributes to cloud formation). In its 2015 report, the panel argued that solar activity in the 20th century was strong enough to account for rising global temperatures and that solar activity may have an equal or greater effect on the atmosphere than CO₂.

Recent news

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Carbon dioxide - Google News

See also

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  Greenhouse gas

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  Climate change

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  Methane

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  Opposing views of climate change theory