The Earth's greenhouse effect.Source: Ocean World, Texas A and M University

Greenhouse gases (GHGs) are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. Common GHGs in the Earth's atmosphere include water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons.

Greenhouse gases, mainly water vapor, essential determinants of temperature of the Earth. The greenhouse effect they produce help to heats the Earth to temperatures far above the minus 454 degrees Fahrenheit (3 degrees Kelvin) of deep space, and thus makes life possible.

Human activities such as energy use and land use change have significant impact upon the levels of greenhouse gases in the atmosphere, which in turn affects the Earth's temperature. The 2007 assessment report compiled by the Intergovernmental Panel on Climate Change (IPCC) observed that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century".

Emissions versus concentrations

A key distinction is that between emissions and concentrations of greenhouse gases.  Emissions refer to the quantity released to the atmosphere and are measured in mass units per unit time. For example, human activity released about 8.2 billion metric tons of carbon to the atmosphere in 2006.  Concentration refers to amount of greenhouse gas per unit volume is the atmosphere and is measured in units such as parts per million volume (ppmv). For example, the concentration of CO₂ in the atmosphere in 2009 is close to 390 ppm. Concentrations are important from a climate perspective because they determine the amount of thermal energy that the atmosphere absorbs.

Natural and anthropogenic sources of GHGs

Coal-fired power plants are a major source of greenhouse emissions. Source: USEPA

Greenhouse gases find their way to the atmosphere from natural and anthropogenic (human) sources. The main sources of greenhouse gases from human activity are:

• the combustion of carbon-based of fossil fuels that release CO₂
• deforestation and other land use changes that  CO₂
• livestock enteric fermentation and manure management, paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions that release methane
• use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
• agricultural activities, especially the use of fertilizers, that release nitrous oxide (N₂O).

Global warming potentials

The impact of a greenhouse gas on climate is measured by its global warming potential (GWP). It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by definition 1). The GWP depends (1) the absorption of infrared radiation by a given gas, (2)  what portion of the electromagnetic spectrum the gas absorbs infrared radiation, and (3) the atmospheric lifetime of the gas. The GWP represents the combined effect of the differing times these gases remain in the atmosphere and their relative effectiveness in absorbing outgoing thermal infrared radiation. A high GWP correlates with a large infrared absorption and a long atmospheric lifetime.The GWPs for the major greenhouse gases are listed below:

• Carbon dioxide (CO₂):                1
• Methane (CH₄):                       21
• Nitrous oxide (N₂O):               310
• CFCs:                        3,800-8,100
• HCFCs:                          90-1,800
• HFCs:                         140-11,700

Radiative forcing

The NOAA Annual Greenhouse Index. Source: NOAA

Note that GWPs are warming potential per unit mass.  The total contribution of a gas to warming also depends on the amount of gas in the atmosphere and its lifetime. To capture all of these effects, scientists use the concept of radiative forcing for quantitative comparisons of the strength of different agents in causing climate change. Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. The word "radiative" signifies that the factors affect the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. Positive forcing tends to warm the surface while negative forcing tends to cool it. Forcing values are expressed in watts per square meter (Wm^-2). For example, an increase in radiative forcing of +1 Watt per square meter is like shining one small holiday tree light bulb over every square meter of the Earth.

Scientists at the National Oceanic and Atmospheric Administration have incorporated these concepts into the Annual Greenhouse Gas Index (AGGI): the change in annual average total radiative forcing by all the long-lived greenhouse gases. CO₂ dominates the total forcing with methane and the CFCs becoming relatively smaller contributors to the total forcing over time. The five major greenhouse gases account for about 97% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 3% is contributed by the 10 minor halogen gases. Most of the observed increase is related to CO₂. For the year 2007, the AGGI was 1.24 (an increase in total radiative forcing of 24% since 1990). The increase in CO₂ forcing alone since 1990 was about 34%.

The role of water vapor

Water vapor is the principle absorber of outgoing thermal radiation. In terms of mass, water vapor is far more prevalent (about 0.3% of atmospheric mass, compared to about 0.06% for CO2), and so constitutes about 80% of all greenhouse gases by mass (~90% by volume).  Water vapor accounts for between 36% and 66% of the total greenhouse effect.

Although water vapor is an important energy absorber, most scientists view changes in water vapor as a feedback from climate change, rather than as a forcing or cause of climate change. The basis for this argument is that water has a relatively short residence time the atmosphere of about 10 days, compared to decades to centuries for CO₂.  This means that when surface temperatures change (whether from CO₂,  solar forcing,  or volcanoes etc.), water vapor adjusts quickly to reflect that.

Trends in GHG emissions

Since 1751 about 329 PgC have been released to the atmosphere from the combustion of fossil fuels and the production of cement. Half of these emissions have occurred since the mid 1970s. The 2006 global fossil-fuel carbon emission estimate, 7.5 PgC, represents an all-time high and a 3.2% increase from 2005. Coal and oil account for 77% of carbon emissions, while natural gas accounts for another 19%.

Trends in GHG concentrations

The long run temerpature and CO2 record as revealed by Antarctic ice core data. Source: Luthi et al. (2008)

Deep ice cores drilled in Antarctica reveal Earth's long run history of green house gas concentrations. Scientists have drilled 3,270m into the Antarctic ice, which equates to drilling nearly 900,000 years back in time. Gas bubbles trapped as the ice formed yield important evidence of the mixture of gases present in the atmosphere at that time, and of temperature. The ice core data reveal that CO₂ levels are substantially higher now than at any time in the last 800,000 years. Carbon dioxide concentrations over the 800,000 range from 172 to 300 ppm. The current value of about 390 ppmv is well beyond any level observed in the pre-industrial period.

The same ice cores reveal that that the concentration of methane (CH₄), another important greenhouse gas, is also substantially higher now than at any time in the last 800,000 years. In recent decades, however, the the rate of increase in CH₄ emissions has slowed considerably.

The ice core data also reveal the Earth's natural climate rhythm over the last 800,000 years. When carbon dioxide changed there was always an accompanying climate change. Higher concentrations of greenhouse gases are associated with high temperatures, and vice versa. This connection is one reason that scientists conclude that the recent warming of the Earth is due in large part to the record high level of green gas concentrations.

The recent historical trend in CO₂ concentration is revealed in what is known as the Mauna Loa curve.  The curve is also known as the "Keeling curve", named for Charles D. Keeling (1928-2005), an American pioneer in the monitoring of carbon dioxide concentrations in the atmosphere.  Since 1958, the concentration of CO₂ in the atmosphere has been measured daily at Mauna Loa Observatory, Hawaii. Mauna Loa Observatory is located on the Island of Hawaii at an elevation of 3,397 meters above  sea level on the northern flank of Mauna Loa volcano. The Mauna Loa record shows a 23% increase in the mean annual concentration, from 316 parts per million by volume (ppmv) of dry air in 1959 to nearly 390 ppmv in 2009.

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