365 Days of Climate Awareness 319 – Global Warming Potential

Global warming potential (GWP) is the analytical method for comparing different greenhouse gases. They differ both in their absorptive capacity, known as “radiative efficiency”, and in their residence time, known as lifetime, in the atmosphere. The combination of those two yields the gas’ total potential for warming the planet. In these calculations as in other measures, water vapor, itself a very powerful greenhouse gas, is not included because it is not considered “well-mixed”. That is to say, concentrations of H₂O vapor vary very widely in time and space around the world due to weather and climatic conditions.

Atmospheric concentration of carbon dioxide.

Global warming potential is treated as a scalar: that is to say, a number without units, because all greenhouse gases are compared to carbon dioxide. CO₂, by definition, has a GWP of 1. Since the gases have vastly different lifetimes: carbon dioxide, roughly 120 years, methane roughly 10.4, and CFCs less than that, their warming potential is averaged into a 100-year span. A few principle gases, using this 100-year average, are listed below. (For concentrations: ppm = parts per million; ppb = parts per billion; ppt = parts per trillion.)

1)     

Atmospheric concentration of methane.

1)  Carbon dioxide (CO₂): GWP = 1; lifetime ~120 yrs (but can vary widely); 410 ppm

2)      2) Methane (CH₄): GWP = 28; lifetime = 10.5 yrs; 1,800 ppb

3)      3) Nitrous oxide (N₂O): GWP = 265; lifetime = 132 yrs; 380 ppb

4)      4) Various chlorofluorocarbons (CFCs): 4,660-13,900; lifetimes 16-500 yrs; 1-100 ppt

5)      5) Sulfur hexafluoride (SF₆): GWP = 23,500; lifetime = 3,200 yrs; 10 ppt

Atmospheric concentration of nitrous oxide.

Different time frames (20 years, 200 years) are used by different agencies around the world, but the concept is the same: amount of energy absorbed by a given mass of greenhouse gas during that time, given the average solar input of roughly 1.366 kW/sq m. With this statistical tool scientists can directly compare the impacts of various gases, though for high-resolution climate modeling, the different gases with their distinct behaviors are accounted for separately.

Atmospheric concentrations of fluorocarbons and other chemicals.

Tomorrow: introduction to general circulation models.

Be brave, be steadfast, and be well.