Greenhouse Gases - Overview

Written by Colin O'Dowd. Posted in Greenhouse Gases

Greenhouse Gases

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are tri-atomic molecules like water vapor, carbon dioxide, nitrous oxide, and ozone. Although not tri-atomic, methane is also an important greenhouse gas.

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Carbon dioxide (CO2): Fossil fuel use is the primary source of CO2.  CO2 can also be emitted from direct human-induced impacts on forestry and other land use, such as through deforestation, land clearing for agriculture, and degradation of soils. Likewise, land can also remove CO2 from the atmosphere through reforestation, improvement of soils, and other activities.


 Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C rather than the present average of 15 °C. Water vapour is regarded as a natural gas/vapour that we have no real control over. This vapour is the dominant greenhouse gas and contributes to a natural greenhouse effect of the order of 33 K or 155 W m-2. Water vapour concentrations, by volume in dry air are of the order of <0.5 %, CO2 vapour concentration, by volume in dry air is of the order of 0.035% and CH4 vapour is of the order of 0.00017%.


Why is there so much concern over the CO2 greenhouse gas effect?

Clearly, the natural effect is stronger than the anthropogenic effect. This brings us from a seriously uncomfortable zone to a comfortable zone but the comfort zone is perhaps quite precarious. The current radiative forcing of anthropogenic greenhouse gases is of the order of 4 W m-2 and has a temperature increase of the order of 0.6K. This increase is driven mainly by CO2 emissions and is set to increase by about 3K over the next hundred years. This will change the climate significantly. 

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Radiative Forcing & Global Warming Potential

Radiative Forcing: change in radiation flux resulting from a unit increase in the atmospheric concentration of a given species (Wm-2 ppb-1) – see IPCC for complete definition of Radiative Forcing.


Global Warming Potential (GWP) is defined as the ratio of the time integrated radiative forcing from instantaneous release of one kilogram of a substance relative to that of one kilogram of carbon dioxide (CO2) and can be defined as:

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Three factors influence the GWP of a GHG:

  1. The adsorption bands and their uniqueness – if there is overlap with other adsorption banks, it is less important; Depends on molecule structure: polyatomic molecules or higher;
  2. The chemical lifetime of the molecule;
  3.  Mixing ratio - concentration

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Current CO2 levels at Mace Head:

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