Americans represent 5% of the world's population but drive almost a third of its cars, which in turn account for nearly half the carbon dioxide pumped out of exhaust pipes into the atmosphere each year, according to a report.

US cars play a disproportionate role in global warming because they are less fuel efficient than passenger vehicles used elsewhere in the world, emitting 15% more carbon dioxide, and because they are driven further across America's wide open spaces, said the report by the Environmental Defence watchdog group.

Americans drive 202m passenger vehicles out of 683m worldwide. The average US passenger vehicle, with a fuel economy of less than 20mpg, travels 11,000 miles a year, nearly a third more than cars elsewhere, according to Wednesday's report, Global Warming on the Road.

With suburban sprawl far outpacing the growth of public transport networks, Americans are commuting more each year, shopping more, and driving further to the shops each time. Between 1990 and 2001 the number of miles travelled on American shopping trips rose by 40%.

The boom in sports utility vehicles (SUVs) has peaked as a result of soaring fuel prices, but overall US fuel consumption will continue to rise in the next few years, the study found.

The huge Hummers, Chevrolet Suburbans and Ford Excursions bought in recent years will represent a bigger share of the cars on the road, as older, smaller cars end up on the scrapheap.

More SUVs are sold in the US than any other type of car, overtaking small cars in 2002. The report predicted that they "soon will be the main source of automotive CO2 emissions", emitting the equivalent of 55 large coal-fired power plants.

"The fuel economy of US vehicles has been declining since 1988, which means the CO2 emissions have been increasing, associated with a shift to large trucks," said John DeCicco, one of the report's authors.

While cars account for a tenth of greenhouse gas emissions around the world, American cars are responsible for 20% of US energy-related emissions. The Environmental Defence report found: "The amount of CO2 emitted from oil used for transportation in the United States is similar to the amount from coal used to generate electricity." General Motors, the biggest US car manufacturer, is responsible for nearly a third of those emissions, more than the biggest US power company, American Electric Power. GM and other car companies did not comment directly on the report, but noted that they were seeking to improve the energy efficiency and reduce the emissions of their fleets.

Mr DeCicco argues that far more stringent measures are needed, including a national cap on carbon emissions and a trading system that would allow companies that made more efficient cars to sell their carbon emissions rights to other firms. He also suggested that local authorities should be given financial credits for innovations that lead residents to drive less, providing better public transport or building shops closer to where people live.

The report also urges a rapid transition to fuels containing less carbon, like the ethanol President Bush has predicted will end his country's "addiction to oil".