Analogt tittar du bara på metan när det är som allra värst, i ditt fall de första 12 åren sammanlagda(*), och i princip bortser från att metangas försvinner snabbare ur atmosfären.The GWP uses radiant energy (like that emanating from a household radiator) as the unit of measure, and it takes account of two factors influencing the effect of these gases: their potency and their lifetimes in the atmosphere. Molecule for molecule, some gases are much more effective than others at emitting heat back toward the surface of the earth, in a process called radiative "forcing." And although all additions from human emission are removed eventually by one or another natural process, some are gone within a few years while others remain for centuries. The GWP calculation assumes an additional kilogram of a particular gas is added to the atmosphere today. This kilogram's worth of molecules exerts some radiative forcing (measured in Watts) in the first instant, a slightly lower forcing in the next (because some molecules have by then been removed), and so on. These effects are added up over time to yield the "integrated radiative forcing" (measured in Watt-years) of the original emission. It's like evaluating a baseball or cricket player who gets many hits or runs in his first year, but whose performance declines with age. The number of hits or runs in a year is analogous to the instantaneous forcing; the career total is the integrated value.
Angående CO2-utsläpp från diesel kontra CNG. Jag har bara sett siffror 20-25% reduktion av CO2 jämfört CNG-bensin. Ungefär samma siffror som för diesel - bensin.
(*)För att få fram GWP-faktorn så delas denna sammanlagda växthuseffekt med motsvarande beräkning för koldioxid. På så vis får med en dalande faktor med tiden, men tar hänsyn till de första årens höga växthuseffekt.