Greenhouse Effect

Temperature-regulating atmospheric gases, called "greenhouse gases" or GHGs, form a blanket around the earth and trap heat from the sun within the earth’s atmosphere.  This process keeps the planet warm and habitable.  The "greenhouse effect" is primarily a function of the concentration of water vapor, carbon dioxide (CO₂), and other trace gases in the atmosphere that absorb the terrestrial radiation leaving the surface of the Earth.  Changes in the atmospheric concentrations of these greenhouse gases can alter the balance of energy transfers between the atmosphere, space, land, and the oceans.

Climate models from the Intergovernmental Panel on Climate Change (IPCC), as well as, models from other scientific bodies, indicate that global concentrations of GHGs have been rising steadily over the past 100 years.  As atmospheric concentrations of GHGs increase, the greenhouse blanket gets thicker.  This causes heat to be trapped in the lower layers of the atmosphere and may cause global average temperatures to rise. 

Greenhouse Gases and Global Warming Potential

Naturally occurring GHGs include water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃).   CO₂, CH₄, and N₂O are  emitted and removed  continuously from the atmosphere by natural processes on earth.   Anthropogenous, or human-produced, activities can cause additional quantities of these and other GHGs to be emitted or removed, thereby changing the global average atmospheric concentrations.

The principal GHGs that enter the atmosphere because of human activities are:

• Carbon Dioxide (CO₂): CO₂ enters the atmosphere through the burning of fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products.   It is also as a result of other chemical reactions (e.g., cement manufacturing).  CO₂ is removed from the atmosphere when it is absorbed by plants as part of the biological carbon cycle.

• Methane (CH₄): CH₄ is emitted during the production and transport of coal, natural gas, and oil.  CH₄ emissions also result from livestock and other agricultural practices , as well as, by the decay of organic waste in municipal solid waste landfills.

•  Nitrous Oxide (N₂O): N₂O is emitted during agricultural and industrial activities, as well as, during combustion of fossil fuels and solid waste.

•  Fluorinated Gases:  A classification of three types of manmade/synthetic, powerful greenhouse gases  including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆).   Fluorinated gases, such as sulfur hexafluoride, are sometimes used as substitutes for ozone-depleting substances (i.e. CFCs, HCFCs, and halons).  Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are halocarbons that contain chlorine, while halocarbons that contain bromine are referred to as bromofluorocarbons (i.e., halons).   The other fluorine-containing halogenated substances—HFCs, PFCs, and SF₆ are typically emitted in smaller quantities.  They do not deplete stratospheric ozone, but because they are potent GHGs, they are sometimes referred to as High Global Warming Potential gases (“High GWP gases”).

Recognizing the problem of potential global climate change, the World Meteorological Organization (WMO) and the United Nations Environment Programme  (UNEP) established the Intergovernmental Panel on Climate Change  (IPCC) in 1988.  It is open to all members of the UN and WMO.  The role of the IPCC is to assess potential impacts (i.e., scientific, technical and socio-economic) and develop options for adapting and mitigating human-induced climate change on a comprehensive, objective, and  transparent basis.

The IPCC developed the Global Warming Potential (GWP) concept to compare the ability of each GHG to trap heat in the atmosphere relative to another gas.  The GWP of a GHG is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas (IPCC 2001).  Direct radiative effects occur when the gas itself is a GHG.  The reference gas used is CO₂, and therefore GWP-weighted emissions are measured in teragrams of CO₂ equivalent (Tg CO₂ Eq).  The relationship between gigagrams (Gg) of a gas and Tg CO₂ Eq can be expressed as follows:

where,
            Tg CO₂  Eq. = Teragrams of Carbon Dioxide Equivalents
            Gg = Gigagrams (equivalent to a thousand metric tons)
            GWP = Global Warming Potential
            Tg = Teragrams

Because of the sheer size of a teragram (one million metric tons), metric tons of CO₂ equivalent (MT CO₂ Eq) are generally used as the reporting unit of GHG.  A metric ton is equal to 1.1023 short tons, or 2,204.6 pounds. 

The GWP depends on both the efficiency of the molecule as a GHG and its atmospheric lifetime.  GWP is measured relative to the same mass of CO₂ and evaluated for a specific timescale (20, 100, or 500 years).  Thus, if a molecule has a high GWP on a short time scale (20 years, for example) but has only a short lifetime, it will have a large GWP on a 20 year scale but a small one on a 100 year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO₂ its GWP will increase with time.

The IPCC also estimates the atmospheric lifetime of GHGs.  The atmospheric lifetime describes how long it takes to restore the system to equilibrium following a small increase in the concentration of the gas in the atmosphere.  Individual molecules may interchange with other reservoirs such as soil, the oceans, and biological systems, but the mean lifetime refers to decaying away the excess.

Some examples of the six primary types of GHGs follow:

Note that a substance's GWP depends on the timespan over which the potential is calculated.  A gas which is quickly removed from the atmosphere may initially have a large effect, but then the effect is minimized.  Thus methane, has a potential of 23 over 100 years, but 62 over 20 years; conversely sulfur hexafluoride has a GWP of 22,000 over 100 years but 15,100 over 20 years (IPCC TAR).  The GWP value depends on how the gas concentration decays over time in the atmosphere.  This is often not precisely known and hence the values should not be considered exact.  Therefore, when  a GWP  is quoted, it is important to reference the calculation.

The GHGs with relatively long atmospheric lifetimes (e.g., CO₂, CH₄, N₂O, HFCs, PFCs, and SF₆) tend to be evenly distributed throughout the atmosphere, and consequently global average concentrations can be determined.  However, the short-lived GHGs such as water vapor, carbon monoxide, tropospheric ozone, ozone precursors (e.g., NOx, and NMVOCs), and tropospheric aerosols (e.g., SO₂ products and carbonaceous particles),  vary regionally so it is difficult to quantify their global radiative forcing impacts.  No GWP values are attributed to these gases which are short-lived and spatially inhomogeneous in the atmosphere.

The IPCC published a first assessment report in 1990, a supplementary report in 1992, a second assessment report (SAR) in 1995, and a third assessment report (TAR) in 2001.  The fourth assessment report (AR4) CLIMATE CHANGE 2007, was published by the IPCC in 2008.  While the TAR and AR4 provide updated or revised GWP values, the GHG GWP values and atmospheric lifetimes established by the SAR are associated with the Kyoto Protocol and are therefore generally referenced in reporting protocols.