As the price of oil balloons and trips to the gas pump cost more, U.S. fuel economy standards -- aimed at making vehicles more fuel-efficient -- are coming under renewed scrutiny for their impact on global warming.

But while some lawmakers and environmental groups are looking to increase fuel economy, other analysts say an increase will only make automobiles more costly and keep more higher-emitting vehicles on the road.

Transportation accounts for one-third of greenhouse gas emissions in the United States -- the main manmade greenhouse gas is carbon dioxide (CO2), a major byproduct of gasoline.

The Environmental Protection Agency regulates fuel quality and automobile emissions under the Clean Air Act.

However, it's the Department of Transportation's National Highway Traffic Safety Administration that regulates car efficiency, including CO2 emissions, through corporate average fuel economy (CAFE) standards under the Energy Policy and Conservation Act.

CAFE standards, covering passenger cars and light trucks but not heavier vehicles used primarily for farming, were established shortly after the Arab oil embargo in the 1970s to save consumers money and decrease dependence on foreign oil.

Under CAFE, the actual fuel economy of the passenger vehicle fleet rose from 19.9 miles per gallon (mpg) for the 1978 model year to 26.2 mpg for the 1987 model year, but declined for the next 14 years as fuel prices decreased and sport utility vehicles, classified as "light trucks" set to a less stringent standard, became popular.

Recently, light truck standards were revised to include vehicles weighing up to 10,000 pounds, which encompasses larger SUVs and vehicles such as the Hummer H2 and Ford Excursion. (Click here for a chart of CAFE standards)

Calls for fuel economy have amplified as oil prices have surpassed $70 per barrel and the average price of gasoline at the pump has risen about 70 cents since last summer to $3.00 per gallon, according to the Department of Energy. President Bush, for his part, acknowledged the country's "addiction to oil" in his Jan. 31 State of the Union speech.

In response, House Science Committee Chairman Sherwood Boehlert, R-N.Y., reintroduced his plan to increase CAFE standards to 33 mpg. "I think that raising fuel economy standards is the single most important step the Congress can take to reduce what the president has correctly identified as the U.S. 'addiction' to oil," he said in a press statement.

On the Senate side, Richard Durbin, D-Ill., and Dianne Feinstein, D-Calif., have proposed raising the fuel economy to 35 mpg by the 2017 model year.

However, neither bill has seen any action in Congress.

Environmental organizations contend that fuel economy could be raised to 35-40 mpg over the next 10 years using current technology, without creating a burden on the automobile industry that contributes 4 percent of the country's gross domestic product.

"You can significantly boost fuel economy/performance, and do it in a way that's not going to be harmful to automakers," said Deron Lovaas, vehicles campaign director at Natural Resources Defense Council.

But other analysts say not so, and that higher CAFE standards may not necessarily have the desired effect.

"There's one ironic aspect to CAFE, especially higher CAFE," said Sam Kazman, general counsel for the Competitive Enterprise Institute. "It's being sold as a cure for oil addiction, but what it really does is reduce the price of driving," offering less of an incentive for people to drive less.

"There will be a little push back in the marketplace," Lovaas countered, "but it will be trumped by the gains in reduced fuel use and therefore reduced carbon dioxide pollution."

Ben Lieberman, senior policy analyst at the Heritage Foundation, on the other hand, said increases in CAFE standards would "raise the cost of new vehicles and keep existing, higher emitting vehicles on the road longer."

Don MacKenzie, a vehicles engineer with the Union of Concerned Scientists, said technologies already are available that could decrease fuel intensity but they are not being offered to consumers. "When consumers go to the showroom and they say, 'I want a vehicle that gets 30 mpg' they get told, 'Hey, well, buy a compact.' Now if you want a vehicle for your family, you need an SUV or something with a lot of space, well, your choices are: 16, 17 or 18 mpg. And that's not a real choice," he said.

"I don't see any need for anything more than the high price of gasoline acting as a market signal," Lieberman contended. "There's fuel-efficient vehicles out there for those who want them. It doesn't take CAFE standards for that option to be out there -- including hybrids, which were introduced without any pressure from new CAFE standards."

Gas-electric hybrid vehicles and diesel-fueled vehicles accounted for about 2 percent of 2005's car fleet, but the choice of alternatively fueled vehicles is expected to increase. Hybrids such as Toyota's Prius and Honda's Insight get mileages in the 50s. (Click here to see how hybrid vehicles compare to those that run on conventional gas)

Hydrogen-powered fuel-cell vehicles, which produce negligible greenhouse gas and air pollutants at the tailpipe, are not yet widely available.

Diesel fuel costs less, gives better mileage, and produces less CO2 than similar conventional gasoline vehicles, and the recent implementation of diesel fuel standard improvements, including a large reduction in sulfur emissions, could make diesel cars more attractive for car buyers and carmakers.

But American carmakers are pursuing what they think is a more practical option: dual-fueled vehicles. An estimated 5 million cars now on the road in the United States can accept "flex-fuel," a blend of conventional gasoline and ethanol, a high-octane but lower energy fuel that is derived primarily from corn.

The use of "E85" fuel, an 85 percent ethanol-15 percent gas blend, shows only a small decrease in CO2 emissions at the tailpipe. The reduction in CO2 goes up to about 20 percent when fuel recovery, production and consumption are taken into account, according to data from the Center for Transportation Research of Argonne National Laboratories.

When ethanol is derived from cellulose instead of corn, carbon emissions decrease by 85 percent. And because the carbon in ethanol comes from atmosphere, the net change in CO2 is even more pronounced.

As for the production of ethanol, the United States produces almost all it uses, with only about 2.5 percent coming from Brazil.

Despite these advances, the United States still lags behind a number of other countries' fuel economy standards. The European Union and Japan, both large carbon emitters on the global scale, have standards almost double those of CAFE.

China, which recently overtook the EU as the world's second largest carbon dioxide emitter, has adopted the EU's stringent automobile emissions standards, but Chinese oil consumption, along with its economy, is growing rapidly.

California, meanwhile, has taken matters into its own hands by setting its own emissions standards -- as allowed under the Clean Air Act -- and introducing a plan for reducing greenhouse gas emissions, including CO2, from automobiles by 30 percent by the 2016 model year. The law says other states may voluntarily adhere to California's criteria, but some environmental groups would like the standards to be mandatory.

Several environmental groups, including the Natural Resources Defense Council, the governments of New York City and Baltimore, and 12 states representing one third of the vehicles in the United States -- California, Connecticut, Illinois, Maine, Massachusetts, New Jersey, New York, Oregon, Rhode Island, Vermont, Washington and the District of Columbia, along with American Samoa -- have decided to sue the EPA over its obligation to regulate pollutants that endanger public welfare through global warming.

The Supreme Court has agreed to hear arguments when it begins its next session in October on whether the federal government must consider CO2 an "air pollutant" and regulate vehicle's carbon emissions, implying the highest court's entry into the global warming debate.

-- By Adnaan Wasey, Online NewsHour