The main components of automobile exhaust are carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 (CO₂) and water vapor (H₂O). Carbon dioxide is the most important anthropogenic greenhouse gas

Greenhouse gas

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 (GHG) and the most significant Greenhouse Gas

Greenhouse gas

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 emitted in the U.S. (with 82-84% of all U.S. emissions). Increases in carbon dioxide concentration are due primarily to fossil fuel

Fossil fuel

Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

 use and land-use change.

The CO₂ emission standards can be referred to the fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 or to the vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

 :

• The European Union
  European Union
  The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
  
   is moving towards fuel mandatory CO₂ standards and USA has reflected it in the Greenhouse Gas Score.

• There are information, but no mandatory, standards for CO₂ limits in vehicle (engine) emissions, excepting some state regulations (i.e. California). See hybrid vehicles.

CO₂ mass emission in vehicles is measured in g/km (g/mi

Mile

A mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...

 * 0.621371192 = g/km).

Low Carbon Fuel Standard

Transportation accounts for forty percent of California's annual greenhouse gas emissions, and California relies on petroleum-based fuels for 96 percent of its transportation needs.

The Californian Governor Arnold Schwarzenegger

Arnold Schwarzenegger

Arnold Alois Schwarzenegger is an Austrian-American former professional bodybuilder, actor, businessman, investor, and politician. Schwarzenegger served as the 38th Governor of California from 2003 until 2011....

 issued an Executive Order in January 2007 for California adopt a Low-Carbon Fuel Standard

Low-carbon fuel standard

A low-carbon fuel standard is a rule enacted to reduce carbon intensity in transportation fuels as compared to conventional petroleum fuels, such as gasoline and diesel. The most common low-carbon fuels are alternative fuels and cleaner fossil fuels, such as natural gas...

 (LCFS). The LCFS requires oil refineries

Oil refinery

An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...

 to ensure that the mix of fuel they sell in the California Market meets, on average, a declining target for greenhouse gas

Greenhouse gas

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 emissions measured in CO₂-equivalent

Carbon dioxide equivalent

Carbon dioxide equivalent and Equivalent carbon dioxide are two related but distinct measures for describing how much global warming a given type and amount of greenhouse gas may cause, using the functionally equivalent amount or concentration of carbon dioxide as the reference.- Global warming...

 grams per unit of fuel energy sold. By 2020, the LCFS will produce a 10 % reduction in carbon intensity from production and use of transport fuels in California.

See also

• Emission standard
  Emission standard
  Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles and other powered vehicles but they can also regulate emissions from industry, power...
  
  
• Low-carbon economy
  Low-carbon economy
  A Low-Carbon Economy or Low-Fossil-Fuel Economy is an economy that has a minimal output of greenhouse gas emissions into the environment biosphere, but specifically refers to the greenhouse gas carbon dioxide...
  
  
• Sustainable mobility technology