The production and use of fuels for transportation also results in emissions of other important GHGs besides CO2, including methane (CH4) and nitrous oxide (N2O). These emissions can be significant, especially for some types of vehicles and fuels. Furthermore, other aspects of transportation, such as the use of refrigerants for automotive air conditioners, also cause significant releases of GHGs. While smaller in quantity, these emissions are important because of the relatively high “global warming potential” values of these gases (i.e., on a 100-year assessment basis a CH4 molecule has about 23 times the effect of a CO2 molecule, and an N2O molecule has about 296 times the effect of a CO2 molecule). For conventional vehicles, these non-CO2 GHG emissions can contribute approximately a quarter of the value of overall vehicle emissions, but for alternative fuel vehicles the contribution can be much higher or lower, in the range of 1 to 57 percent.