Greenhouse gas

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Greenhouse gases are heat-trapping gases that absorb infrared radiation in the Earth's atmosphere and retransmits to the Earth's surface, which warms the globe to allow for carbon-based life. These gases include water vapor, methane, carbon dioxide, nitrous oxide, ozone, chlorofluorocarbons, and hydrofluorocarbons. All of these gases are naturally present in the atmosphere, though concentrations fluctuate based on natural and human-caused activity. Sources of greenhouse gases include oceans, volcanoes, wildfires, agricultural activities, plant growth and decay, and the combustion of coal, oil, and natural gas.^[1]

Background[edit]

Greenhouse gases are naturally present chemical compounds in the Earth's atmosphere. The greenhouse gases most present in Earth's atmosphere are carbon dioxide, methane, nitrous oxide, and water vapor. Greenhouse gases contribute to the greenhouse effect, which occurs when the Earth's surface and atmosphere absorbs solar energy and reradiates the energy back into space. A portion of the absorbed energy is emitted by land and oceans, absorbed by the Earth's atmosphere, and reradiated back to the Earth. Greenhouse gases can absorb and reradiate heat because they are composed of three or more atoms, which are held together loosely and vibrate after they absorb heat. These atoms release heat that is then re-absorbed by other greenhouse gas molecules. A portion of this heat remains in the atmosphere or returns to warm the Earth's surface. The greenhouse effect is one of several climate forcings, which are the major drivers of Earth's climate, alongside solar activity, volcanic eruptions, and aerosols. Greenhouse gases in the atmosphere keep the average global temperature at 59º Fahrenheit (F), or 15º Celsius (C). Without these gases, the global average temperature would be around 0º F, or -18º C, which would make Earth generally inhospitable for carbon-based life.^[2][3][4]

The image below shows how the greenhouse effect is produced.

Illustration of the greenhouse effect

Greenhouse gas concentrations[edit]

Earth's atmosphere is composed of different gases in different amounts. The atmosphere is more or less permanently composed of nitrogen (78 percent), oxygen (21 percent), and argon (0.9 percent). Trace gases, including greenhouse gases, account for the remaining 0.1 percent. As of April 2016, carbon dioxide accounted for 0.04 percent of the atmosphere's chemical composition. Water vapor, the largest contributor to the greenhouse effect, varies in concentration between seasons and at different altitudes of the Earth's lower atmosphere. Thus, it is difficult to measure precisely, though a warmer atmosphere in general contains more water vapor. The atmosphere also contains trace amounts of human-made greenhouse gases, such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).^[5][6][7][8]

The table below shows an estimate of the tropospheric (the lowest region of the atmosphere, with a height of between 3.7–6.2 miles) concentration of the five naturally occurring greenhouse gases (excluding water vapor) 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).^[7]

Types of greenhouse gas[edit]

• Water vapor is the most abundant greenhouse gas in the atmosphere and the largest contributor to the greenhouse effect. The atmosphere collects water vapor as water evaporates from the Earth’s surface. The rate of evaporation depends on temperature of the oceans and the air. As more water is collected in the atmosphere, it condenses and falls as snow or rain within 1-2 weeks. Water vapor is vital to cloud formation, and clouds can either trap or reflect heat, which may result either in warming or cooling effects.^[9][10]

• 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 and is produced from human and animal respiration, volcanoes, hot springs, and geysers, and the burning of coal, petroleum, and natural gas. CO₂ fluctuates in and out of the atmosphere through land, oceans, and biological organisms as part of the global carbon cycle. As of 2015, CO₂ concentration in the Earth's atmosphere was approximately 0.04 percent (400 parts per million) by volume. Between 1750 and 2015, the atmospheric concentration of carbon dioxide rose from approximately 275-280 parts per million (ppm) to approximately 400 ppm.^[9][11][12]

• Methane (CH₄) a colorless, odorless, nontoxic, and flammable gas that is the main component of natural gas. It is a naturally occurring gas emitted from sources such as wetlands, swamps, oceans, sediments, wildfires, and volcanoes. Human-caused sources of methane include landfills, livestock cultivation, natural gas extraction, and other industrial activities. Though methane is approximately 100 times more powerful at trapping heat than carbon dioxide, its lifetime in the atmosphere is between 10-12 years, compared to between 20–200 years for carbon dioxide.^[9][13]

• Nitrous oxide (N₂O) is a colorless, odorless naturally occurring gas and is used to produce anesthesia and as a fuel additive to increase oxygen during combustion. Nitrous oxide can also be used to make fertilizer, fibers, and other synthetic products. Natural sources of nitrous oxide include microbial processes in soils and oceans. Nitrous oxide's lifetime in the atmosphere is approximately 114 years.^[9][14][15]

• Ozone is a colorless, inorganic gas that exists at ground level and in the Earth’s upper atmosphere. Naturally occurring ozone in the stratosphere (a layer from six to 30 miles above the Earth’s surface) shields the Earth from the sun’s ultraviolet radiation. Ground-level ozone is formed in the troposphere (the lowest layer of the Earth’s atmosphere) when nitrogen oxide and volatile organic compounds react in sunlight to produce smog.^[9][16]

• Hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) are synthetic chemicals used as refrigerants, cleaning solvents, and aerosol propellants. These gases can have atmospheric lifetimes of up to hundreds of years and are removed when they are destroyed by sunlight.^[9][17]

Greenhouse gases and global warming[edit]

Scientists study the impact higher greenhouse gas concentrations may have on global warming, defined as a rise in global average temperature, and this warming's . One metric, equilibrium climate sensitivity (ECS), looks at global average temperature changes in relation to higher carbon dioxide (CO₂) concentrations. Carbon dioxide is an important greenhouse gas because it remains in the atmosphere longer than other GHGs. 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.^[18]

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."^[19][20]

• 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.^[21]

• 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.^[22]

• 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.^[23]

Climate feedbacks[edit]

In addition, scientists observe other factors that may influence the greenhouse effect and other climate forcings. These factors are feedbacks, which are processes that can either accelerate or reduce the warming effects of climate forcings like the greenhouse effect. A positive feedback can increase warming, while a negative feedback can reduce warming. The following positive and negative feedbacks can affect the impacts of the greenhouse effect on warming.^[24][25]

• Clouds: Clouds reflect back into space approximately one-third of all sunlight that hits the Earth's atmosphere. Increased cloudiness caused by more water in the atmosphere can limit the amount of sunlight from hitting the Earth's surface, resulting in less absorbed heat and less warming.
• Precipitation: A warmer atmosphere that holds more water can increase precipitation, though not in all regions. Changes in precipitation patterns can result in more water available for plants, which remove carbon dioxide (a greenhouse gas, also known as CO₂) from the atmosphere. Increased plant growth could result in more CO₂ absorption from the atmosphere and thus less warming.
• Forested areas: As negative feedbacks, forests, trees, and other plants remove CO₂ from the atmosphere and thus can reduce warming that may be the result of increased CO₂ in the atmosphere. However, forests and plants may also be cut down so that land is cleared for agriculture or other uses, which results in less CO₂ absorption.
• Ice: Compared to ocean surfaces, which are dark and absorb heat more quickly, ice is white and thus quickly and more easily reflects sunlight. More ice and glaciers result in greater heat reflection and thus less warming. However, greater sea ice and glacier melting reduces this reflection and allows the ocean to absorb more heat and accelerate warming. This process is known as the ice albedo feedback.

See also[edit]

• 

  Greenhouse effect

• 

  Ozone

• 

  Methane

• 

  Carbon dioxide

Footnotes[edit]

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