The Green Technocrat

I’ve been talking about Global Warming Potential (GWP) a lot lately, particularly in regards to methane emissions from livestock.  What I have not addressed in any of my previous posts is what exactly GWP is, how it is calculated, and why some gases, such as methane, have a higher GWP than others?  To clear that up for anyone who is interested here is a quick description of what GWP is and what factors are used to calculate it.

Because different gases absorb different amounts infrared radiation (and thus different amounts of energy), and because different substances have different atmospheric lifetimes, i.e., remain in the atmosphere for different lengths of time depending on their chemical structure, different gases will trap different amounts of heat.

Phew! That was quite a mouthful.  Basically all I am getting at is that not all gases are the same (big surprise) and because of that a need exists to be able to compare different greenhouse gases to one another.  Global Warming Potential is a scale that meets this need by allowing us to compare the ability of a gas to trap heat to the most abundant greenhouse gas of all, carbon dioxide (which by definition has a GWP of 1).

The GWP of a given gas depends on three factors:

• Absorption of infrared radiation (heat)
• Atmospheric lifetime (how long it hangs out in the atmosphere)
• Spectral location of its absorbing wavelengths

The first two are pretty straight forward, if a gas absorbs more heat than CO2, or if it absorbs less heat per mole (amount), but stays in the atmosphere longer than CO2, and thus compensates for this difference in absorption then the gas will have a higher GWP than CO2.  But what about the third factor, spectral location of absorbing wavelengths?  Here is where things get a little trickier.

The spectral location of the absorbing wavelengths is basically the wavelength(s) of infrared radiation (heat) that a given gas absorbs efficiently.  However, even if a particular gas efficiently absorbs infrared radiation at shorter wavelengths (i.e. higher energy) this does not necessarily mean that it will have a higher GWP.  If theses wavelengths are already easily absorbed by the atmosphere simply because of it’s molecular makeup then the fact that a gas absorbs these wavelengths has little effect on boosting the gas’s GWP.  Rather, the spectral location of absorbing wavelengths will have the most dramatic effect on a gas’s GWP when the gas absorbs wavelengths that are not easily absorbed by the atmosphere and would normally pass through it and back into space.

And because I am way to lazy (and probably incompetent) to figure out the formula for calculating GWP on my own here it is straight from Wikipedia (with a few comments added).

“The GWP 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 (i.e the ratio between your gas of interest and CO2):

where TH is the time horizon over which the calculation is considered; a_x is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm^-2 kg^-1) and [x(t)] is the time-dependent decay in abundance of the substance following an instantaneous release of it at time t=0. The denominator contains the corresponding quantities for the reference gas (i.e. CO₂). The radiative efficiencies a_x and a_r are not necessarily constant over time. While the absorption of infrared radiation by many greenhouse gases varies linearly with their abundance, a few important ones display non-linear behavior for current and likely future abundances (e.g., CO₂, CH₄, and N₂O). For those gases, the relative radiative forcing will depend upon abundance and hence upon the future scenario adopted.”

The Intergovernmental Panel on Climate Change (IPCC) provides the generally accepted values for GWP, which changed slightly between 1996 and 2001. An exact definition of how GWP is calculated is to be found in the IPCC’s 2001 Third Assessment Report.”