This paper explores the impact of U.S. emission controls and fuel economy improvements on the global warming potential (GWP) of new light-duty vehicles. Fuel economy improvements have reduced the GWP of both passenger cars and light-duty trucks by lowering the per mile emissions of carbon dioxide (CO2). Further GWP reductions have been achieved by emission standards for criteria pollutants: carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx).
The GWP of a criteria pollutant was calculated by multiplying the emission rate by a relative global warming factor to obtain a CO2 equivalent emission rate. Both CO2 and criteria pollutant emission rates per vehicle have decreased substantially for new light-duty vehicles over the period from 1968 to 1991. Over that period, the GWP from CO2 was reduced by almost 50% in new vehicles by improving fuel economy. In that same time period, the GWP from criteria pollutants from new vehicles was reduced with emission controls by from 80% to 90%, depending on the global warming time frame of interest. Consequently, total reductions in the GWP of new passenger cars and light-duty trucks have been on the order of 55 to 75 percent compared to precontrol (before 1968) new vehicles.
However, the reduction in GWP caused by emission control of criteria pollutants has been larger than the reduction caused by improved fuel economy (i.e., reduced C02). The contribution of criteria pollutants to the GWP of precontrol new vehicles was substantial, but their contribution has been reduced significantly due to U.S. emission controls. As a result, the contribution of criteria pollutants to global warming is now much less than the contribution of CO2 from fuel consumption.