Partly because of high gas prices here, European cars are already far more efficient than those sold in the United States. The average new European car gets 40 miles to the gallon, double the average for new cars sold in the United States. Even so, total carbon emissions from all forms of transportation in Europe are rising. They are about a quarter higher today than they were 20 years ago, while emissions from industry have declined in the same period.

The European Union has committed to reducing its overall carbon dioxide emissions by at least 20 percent by 2020 and by half by 2050. To achieve those goals the region will have to curb auto emissions, most likely through changes in car design, fuel types, traffic control and driving habits.

"We believe that CO2 from personal transport has to be decreased in a holistic way, with contributions from all sectors — no one thing will bring levels down," said Ivan Hodac, secretary-general of the European Automobile Manufacturers' Association in Brussels. He said that if all sides cooperated, levels could come down 25 percent in the next five to seven years.

The industry is investing heavily in improving the fuel efficiency of conventional cars as well as in designing hybrids and vehicles powered by alternative fuels like electricity and hydrogen, Hodac said. But these design changes will take at least five years to come to market, and will not solve emissions problems on their own, he added.

"Everyone puts all the pressure on the manufacturers, and we're doing our part," he said. "But others should do their part as well; nothing much has been done with driving, improving infrastructure."

For individuals, lowering transport emissions can involve choosing more efficient vehicles, using public transportation or bicycles, driving at the most fuel-efficient speeds, and avoiding trips in congested areas. Several years ago, the Netherlands instituted a program to promote such behaviors, achieving a modest emissions reduction.

Recent studies have shown that rising fuel prices have to be combined with government intervention to force lasting changes in the transportation choices that people make.

Phil Goodwin, a professor of transportation policy at the University of West England, said research showed that a 10 percent increase in gas prices produced a 6 to 7 percent drop in fuel consumption over five years, because people drive less and switch to cars that are more fuel efficient.

"Price appears to be a highly effective way of influencing behavior," Goodwin said. "However," he added, "price alone doesn't win popular support or acquiescence. There has to be a package of a variety of different policies, including improvements to public transport, walking and cycling."