Automobile Emissions

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Automobile Emissions


Automobiles and trucks are major contributors to global climate change—17.8% of the world's energy-related greenhouse gas emissions come from road vehicles, including 10.2% from automobiles. There are over half a billion automobiles in the world today, almost all running on gasoline. Burning 1 gallon (3.8 liters) of gasoline releases 19.4 lb (8.8 kg) of carbon dioxide (CO2) into the atmosphere, along with a number of other pollutants. The average car emits its own weight in CO2 every year.

Emissions from automobiles are rising worldwide, as car ownership increases in the developing world and average vehicle size increases in the developed world. Increasing efficiency through decreased size, mileage increases, hybrid vehicles, and diesel engines is one way of decreasing greenhouse emissions from automobiles. Also, automobile usage may be reduced through public and alternative transport, zoning cities and other settlements so as to minimize commuting and traffic jams, and other measures.

Biofuels, which do not make a direct contribution to greenhouse gas emissions, are another option for reducing automobile emissions. Biofuels are controversial, however, because the agricultural sector (which would provide the feedstocks with which to make the biofuels) is itself fossil-fuel-intensive and causes other environmental damage as well, such as soil loss and oceanic dead zones.

Historical Background and Scientific Foundations

The first experimental automobiles were built in the mid-nineteenth century, and the first practical models were built in the 1880s. They remained a rare luxury item for several more decades, but in the early twentieth century a rapid shift to personal automobile ownership and reliance on motor-powered trucks for short- and mid-range transport of goods occurred. The lowering of automobile prices—accomplished through assembly-line techniques for mass production first applied on a large scale by American industrialist Henry Ford (1863– 1947)—was crucial to this transition. Ford's Model T automobile was first marketed in 1908. Eight years later Ford's company had sold half a million Model Ts and the price of the car was down to a mere $316 ($6,100 in 2006 dollars).

Although some early cars were powered by hydrogen, coal, or alcohol, gasoline soon came to completely dominate the automobile and truck world and continues to do so today with rare exceptions. Industrial society's dependence on road transport, and of road transport on petroleum, has resulted both in large-scale emissions of greenhouse gases and in strained international relations resulting occasionally in conflict in order to maintain access to petroleum resources. (Many of these petroleum resources are not located in the industrialized countries that consume the most oil.)

Analysts divide the world's economic activity into a number of sectors, such as buildings, industry, energy, forestry, agriculture, and transportation. Transportation, in turn, is broken down into rail, air, sea shipping, and ground transport, and ground transport is broken into five vehicle categories, namely light-duty vehicles (automobiles and small trucks and vans), two-wheelers (motorcycles, scooters, and the like), heavy freight trucks, medium freight trucks, and buses. In 2004, the transport sector as a whole accounted for 26% of world primary energy use and 23% of world energy-related greenhouse-gas emissions. (Other emissions came primarily from deforestation and agriculture.) All types of ground vehicles together accounted for 74% of transport emissions, that is, 17% of world energy-related greenhouse gas emissions. Automobiles accounted for 44.5% of transport emissions, that is, 10.2% of world energy-related greenhouse gas emissions.

Transport emissions consist mostly of CO2, but methane (CH4) and nitrous oxide (N2O) are also

emitted. CH4 accounts for 0.1–.3% of transport greenhouse gas emissions and N2O for 2–2.8%. In 2004, fluorine-containing compounds (F-gases), leaked from vehicular air conditioning systems, produced an amount of global warming equal to 5–10% of the warming produced by CO2 emitted by vehicles.

About 95% of transport energy comes from petroleum-based fuels, with most of the other 5% coming from biodiesel and ethanol. The three major petroleum-based transport fuels are diesel, which supplies 31% of total transport energy; gasoline, which supplies 47%; and aviation fuel, which supplies 9%.

The transport sector's contribution to global greenhouse gas emissions almost doubled from 1971 to 2000. Most of this increase was attributable to increased emissions from road transport. Two main factors led to this increase. One was the increase in vehicle numbers due to urbanization (increased numbers of people living in cities) and to decentralization (spreading-out) of cities. The sprawling patterns of urban transport that result from decentralization are not easily served by public transportation systems, which are best at transporting large numbers of people between a relatively small number of destinations. Two-wheelers and cars have, therefore, increased rapidly in many parts of the world. From 1950 to 1997, the world automobile fleet grew five times faster than population, from 50 million vehicles to 580 million. Some parts of the world, notably China, are poised for even more explosive growth. Chinese automobile sales grew from 2.4 million in 2001 to 7.2 million in 2006.

Nevertheless, 64% of transport emissions come not from developing countries, such as China, but from the world's 30 wealthiest states, all members of the Organ-isation for Economic Co-operation and Development (OECD). About 20% of the world's population resides in the OECD countries. The other 80% of the planet's people produced, as of 2007, only 36% of greenhouse emissions from transport. The non-OECD share of transport emissions was forecast by the United Nations Intergovernmental Panel on Climate Change (IPCC) to grow to 46% by 2030, if present trends continued.

The second main reason for the rise of transport emissions since 1971 has been growth in the size, weight, and power of automobiles, especially in the industrialized world. This growth has been largely driven by the surge in sport utility vehicle (SUV) sales since the 1970s. In 1975, SUVs accounted for only 2% of U.S. vehicle sales; by 2003, the share was 23%. SUVs are heavier than cars, carry no more passengers, and get much lower fuel economy. In 2007, the American SUV fleet average was 19.2 miles per gallon (8.2 km/L), compared to 26.9 miles per gallon (11.5 km/L) for family sedans.

The average U.S. new automobile of 2005 was 27% heavier than that of 1987; was able to accelerate 30% more quickly from 0–60 miles per hour (0–97 km/h); and got 5% poorer mileage. Note that mileage did not shrink in proportion to weight and performance, thanks to improvements in technological efficiency. However, weight and performance outpaced efficiency for a net loss. If the 2005 new-car fleet had remained at 1987

weight and performance levels, it would have had 24% better fuel economy due to efficiency improvements. In 2006, due to higher gas prices, sales of SUVs plummeted and several of the largest, most fuel-inefficient models were discontinued.

Impacts and Issues

According to the nonprofit Pew Center on Climate Change, the fuel economy of passenger road vehicles could be increased by one-third by 2030, without reducing size or performance, through adoption of already proven technologies. (Reducing size, performance, or both would result in even greater improvements.) These technologies include gasoline or diesel hybrid cars, advanced diesel engines, and electric vehicles powered by hydrogen fuel cells. However, such technological changes are likely to occur only gradually. The speed with which more efficient vehicles become common will depend on both market forces and government regulations.

In the United States, attempts by California and other states to further mandate better mileage for automobiles and trucks have met with resistance. The federal Clean Air Act (first passed 1963) gives California the unique ability to set automobile emissions standards that are stricter than federal standards, as long as the state receives a waiver (special permission) from the U.S. Environmental Protection Agency (EPA). In 2002, California passed a state law requiring automakers who wished to sell cars in the state to produce models emitting fewer greenhouse gases. Mandatory emissions reductions would start in 2009 and increase to 25% by 2030. Eleven other states (Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington) adopted versions of the California law that would only take effect if California's law did.


BIOFUEL: A fuel derived directly by human effort from living things, such as plants or bacteria. A biofuel can be burned or oxidized in a fuel cell to release useful energy.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosyn-thesis are no longer present and contributing to carbon storage.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

HYBRID VEHICLES: Vehicles that use electric motors for their sole or primary motive power but produce most or all of their own electricity using internal combustion engines.

HYDROGEN FUEL CELLS: Devices that combine hydrogen and oxygen, at a low or modest temperature, to produce electricity directly. In a fuel cell, hydrogen is not burned (combined rapidly with oxygen) to produce heat. Fuel cells are more efficient users of hydrogen than thermal systems such as hydrogen-burning internal combustion engines, but are also more expensive.

OCEANIC DEAD ZONES: Areas of the ocean where the deeper waters are devoid of life. Often occur where nutrients from agricultural fertilizers have been carried to the sea by rivers. Algae bloom (reproduce rapidly) in the nutrient-rich surface waters: when they die, they sink, and are digested by bacteria in deeper waters. These bacteria consume oxygen, resulting in low-oxygen deep waters in which all oxygen-dependent aquatic life, such as fish, dies. One of the largest hypoxic or dead zones is in the Gulf of Mexico: it is about 7,000 square mi (18,000 square km) in size, as large as the U.S. state of New Jersey, and is caused by pollution from U.S. agriculture draining into the Mississippi River.

In 2005, California requested that the EPA give it a waiver so that it could impose its stricter emissions standards. In the spring of 2007, automobile manufacturers brought suit against Vermont, arguing that its pending emissions law, modeled on California's, was illegal. In September 2007, a federal judge in Vermont ruled against the automakers and in favor of the state, making it likely that all the other states' emissions laws would be upheld. As of October 2007, a suit against the new standards brought by the Association of International Automobile Manufacturers (Honda, Nissan, Toyota, and 11 other foreign car makers) was still pending in a California federal court.

In April 2007, after more than a year of silence from the EPA in response to California's request for a waiver, Governor Arnold Schwarzenegger announced that if the EPA had not issued the waiver in six more months the state would file suit against the EPA. In October, the deadline expired and the California attorney general announced plans to sue. The EPA rejected the waiver in December, prompting California and other states to sue in January 2008. The outcome of such legal struggles may have a significant effect on automobile emissions around the world, since manufacturers that develop low-emissions technologies for the large U.S. market would be likely to market them globally.

Primary Source Connection

This source from the California Environmental Protection Agency (EPA) explains the process by which the California Air Resources Board (ARB) adopts emissions standards for automobiles that are stricter than the national standard. The proposed California emissions standard would greatly reduce greenhouse gas emissions, but the United States Environmental Protection Agency has yet to grant approval to California's proposed standards.


What are California's Motor Vehicle Greenhouse Gas Emissions Standards?

In September 2004, the California Air Resources Board approved regulations to reduce greenhouse gas emissions from new motor vehicles, based on a law by former Assemblywoman Fran Pavley, signed in 2002. The law directed the Board to adopt regulations that achieve the maximum feasible and cost effective reduction in greenhouse gas emissions from motor vehicles. The regulations establish emission standards for new passenger vehicles and light duty trucks beginning with the 2009 model year.

What are the greenhouse gas benefits expected from the motor vehicle greenhouse gas emissions standards?

In California alone, officials estimate that the standard will reduce climate change emissions by approximately 30 million metric tons, in 2020 and over 50 million metric tons in 2030. This equates to an overall 18% reduction in climate ‘change’ emissions from passenger cars in 2020 and a 27% reduction in 2030. In addition, staff estimates that the regulation will reduce “upstream” smog-forming emissions of hydrocarbons and oxides of nitrogen by approximately 6 tons per day in 2020 and 10 tons per day in 2030.

Reductions from the standards for vehicles in all 12 states that have adopted California's standards will reduce greenhouse gas emissions by 74 million metric tons in 2020. Adoption by the six additional states that are considering the policy would increase the total emissions reduction to 100 million metric tons in 2020.

How many other states have adopted California[‘]s Motor Vehicle Greenhouse Gas Emissions Standards?

Eleven states: Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont and Washington. Six additional states are actively considering adopting the standards: Arizona, Colorado, Illinois, New Hampshire, New Mexico and North Carolina.

Why does California need a waiver from theU.S.EPA to enforceits ownvehicle standards?

California is the only state, under the Federal Clean Air Act, with the unique ability to set stricter-than-federal standards for vehicles, as long as it gets a waiver from the federal government. The Federal Clean Air Act preempts state and local governments from adopting or enforcing standards to control emissions from new motor vehicles or engines. However, once California receives a waiver of preemption from the federal government, then other states can adopt California's standards.

How long has California been waiting for the U.S. EPA to grant a waiver?

In December 2005, the California Air Resources Board requested a waiver from the U.S. EPA. In April 2006 and again in October 2006, Governor Arnold Schwarzenegger followed up the request with letters urging swift action. In April 2007, Governor Schwarzenegger met with U.S. EPA Administrator Stephen Johnson to personally request assistance granting California its waiver. In April 2007, the Supreme Court ruled that the U.S. EPA must take action regarding greenhouse gas emissions. In April 2007, the U.S. EPA finally announced two public hearings to consider California's request for a waiver, both in May, one scheduled in Washington D.C. and one in Sacramento.

How does the U.S. EPA waiver process work?

Once California has determined that its state standards are as protective of the public health and safety as applicable federal standards, and has applied for a waiver, the Clean Air Act directs U.S. EPA to provide the opportunity for a hearing and then grant the waiver.

What are the reasons that the U.S. EPA would deny a waiver request?

The U.S. EPA must grant a waiver unless the federal agency makes one of three findings:

  1. That California's “protectiveness finding” is arbitrary and capricious;
  2. That California does not need its state standards to meet compelling and extraordinary conditions; and
  3. That the state standards are not consistent with section 202(a)-part of the Clean Air Act provisions on U.S. EPA's adoption of motor vehicle emission standards. Section 202(a) indicates that (i) the California Standards must provide manufacturers with adequate lead time, taking the cost of compliance into account, and (ii) manufacturers must be able to put a vehicle through one set of tests to determine compliance with both the state and federal standards.

How many waiver requests has U.S. EPA approved or denied?

Since 1968, EPA has granted about 50 new (or “full”) waivers and about 40 determinations that amendments were within the scope of prior waivers. Five waivers have been denied, the last in 1975. Since 1975, there have been instances where ARB made modifications to regulations where a waiver was pending or, in the case of the Zero Emission Vehicle regulations, where a waiver was granted only until model year 2011, meaning ARB will have to do a new waiver application for model years 2012 and beyond.

Does the technology exist to meet the vehicle emissions standards?

Yes. Technology is in use today to meet the motor vehicle greenhouse gas emissions standards. Even in 2002 when the law was signed, technology was available that could be used by auto manufacturers to reduce their fleet average emissions.

Does theU.S.EPA have theauthority to regulate greenhouse gases?

Yes. The U.S. Supreme Court recently ruled that the U.S. EPA had the authority to regulate greenhouse gases, and the obligation to review the mounting scientific data. The court decision countered the U.S. EPA's argument that it could not regulate greenhouse gas emissions because of the “substantial scientific uncertainty” about the harmful effects of greenhouse gas emissions.

In its ruling, the Supreme Court stated that, gases such as carbon dioxide “act like a ceiling of a greenhouse, trapping solar energy and retarding the escape of reflected heat.” The Supreme Court indicated that the effects of greenhouse gases on climate and weather are covered under the Clean Air Act as such effects threaten human welfare. Following the ruling, Governor Schwarzenegger expected the U.S. EPA to move quickly now in granting the ARB's request for a waiver.

Does the President's Executive Order from 5/14/07 affect California[‘]s request for a waiver of preemption?

No. California's standing under the Clean Air Act does not relieve the U.S. EPA from taking swift action to grant a waiver. Although it appears the Executive Order is an effort by the Federal government to interfere with California's clear right under the Clean Air Act to regulate greenhouse gas pollutants from vehicles, the ARB remains optimistic that the U.S. EPA will grant the waiver.

california epa. “climate change emissions standards for vehicles: actions to reduce greenhouse gases from cars and trucks.” may 2007. <> (accessed november 30, 2007).

See Also Aviation Emissions and Contrails; Biofuel Impacts; Petroleum: Economic Uses and Dependency; United States: State and Local Greenhouse Policies.



Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.


Egelko, Bob. “California to Sue Bush Administration Over Law to Limit Emissions.” San Francisco Chronicle, October 20, 2007.

Freeman, Sholnn. “States Adopt California's Greenhouse Gas Limits.” Washington Post, January 3, 2006.

Web Sites

“California AG to sue EPA to Force Emissions Decision.” MSNBC, October 23, 2007. <> (accessed November 5, 2007).

“Climate Change Emissions Standards for Vehicles.” California Environmental Protection Agency. May 2007. <> (accessed November 5, 2007).

“Emission Facts: Greenhouse Gas Emissions from a Typical Passenger Vehicle.” U.S. Environmental Protection Agency (EPA), February 2005. <> (accessed November 5, 2007).

Greene, David L., and Andreas Schafer. “Reducing Greenhouse Gas Emissions from U.S. Transportation.” Pew Center on Global Climate Change, May 2003. <> (accessed November 5, 2007).

Meltz, Robert. “The Supreme Court's Climate Change Decision:Massachusetts v. EPA.” Congressional Research Service Report for Congress, May 18, 2007. <> (accessed October 15, 2007).

Larry Gilman

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Automobile Emissions

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