In 2002, California adopted legislation [California Bill AB 1493] which required the California Air Resources Board (ARB) to implement regulations to control emissions of greenhouse gases (GHG) from motor vehicles in California. The regulation was developed by the ARB in 2004, and became effective from 1 January 2006.

The standards phase-in over the period of 2009 to 2016, as shown in Table 1. The average reduction of greenhouse gases from new California cars and light trucks will be about 22% in 2012 and about 30% in 2016, compared to model year 2004 vehicles (however, in absolute terms GHG emissions would still increase due to projected growth in vehicle population and miles traveled).

Table 1
California Fleet Average GHG Emission Standards
Time FrameYearGHG Standard, g CO2/mi (g CO2/km)CAFE Equivalent, mpg (l/100 km)
PC/LDT1LDT2PC/LDT1LDT2
Near Term 2009323 (201)439 (274)27.6 (8.52)20.3 (11.59)
2010301 (188)420 (262)29.6 (7.95)21.2 (11.10)
2011267 (166)390 (243)33.3 (7.06)22.8 (10.32)
2012233 (145)361 (225)38.2 (6.16)24.7 (9.52)
Medium Term 2013227 (142)355 (221)39.2 (6.00)25.1 (9.37)
2014222 (138)350 (218)40.1 (5.87)25.4 (9.26)
2015213 (133)341 (213)41.8 (5.63)26.1 (9.01)
2016205 (128)332 (207)43.4 (5.42)26.8 (8.78)

The GHG standards are incorporated into the California low emission vehicle (LEV) legislation. There are two fleet average GHG requirements: (1) for passenger car/light-duty truck 1 (PC/LDT1) category, which includes all passenger cars and light-duty trucks below 3,750 lbs equivalent test weight (ETW); and (2) for light-duty truck 2 (LDT2) category, including light trucks between 3,751 lbs ETW and 8,500 lbs gross vehicle weight (GVW). In addition, medium-duty passenger vehicles (MDPVs) from 8,500 to 10,000 lbs GVW are included in the LDT2 category for GHG emission standards.

The GHG standards are defined in grams per mile of CO2-equivalent emissions (g CO2/mi in Table 1), calculated from the following formula:

CO2-Equivalent = CO2 + 296 × N2O + 23 × CH4 - AC Allowances

A manufacturer may use N2O = 0.006 g/mi in lieu of measuring N2O exhaust emissions. The AC emission allowances are determined based on the design of the air conditioning system (with higher allowances for more leak-free and energy-efficient systems). Two sets of CO2 values are determined: (1) city values measured over the FTP test, and (2) highway values over the HWFET cycle. In the calculation of average emission for a manufacturer, the city values are taken with a weight factor of 55%, and the highway values with a weight of 45%. Additional adjustment factors and special methods are used for calculation in vehicles fueled by alternative fuels and in ZEV vehicles.

The regulation also includes GHG emission credits for manufacturers who have emissions below the standards. Credits can be earned for reductions in GHG emissions achieved in model years 2000-2008 (i.e., prior to the date the regulation becomes effective) and during the phase-in period. Accumulated credits can be used to offset compliance shortfalls up to one year after the end of the phase-in at full value, or in the second and third years after the end of the phase-in at a discounted rate.

The California GHG emission standards have been challenged in court by the automobile industry on the basis that they are a veiled form of fuel economy regulation, and only the federal government has the authority to regulate fuel economy under the CAFE legislation. California lawmakers have been arguing that they regulate vehicle emissions—as permitted under the Clean Air Act—not fuel economy.

The California greenhouse gas emission regulation has been adopted by a number of other states.