Michael McCarthy: At last, America seems serious about changing its car culture

Monstrous: the only world for the size of the US car fleet. At more than 250 million vehicles for a population of 300 million, that's more than one vehicle for every person eligible to drive when you factor in the old, the young, and the disabled. The US has a population that is five times the size of the UK, with a car fleet nearly 10 times bigger.

America is the spiritual home of the car; it sometimes feels as if its whole society runs on four squealing tyres. But that means it is also the spiritual home of vehicle pollution. Los Angeles is inseparable from the automobile, but also from the smog it produces. Those 250 million cars produce a gigantic amount of greenhouse gases; so if you bring in much tighter fuel efficiency standards, the emissions savings you make will be correspondingly huge.

President Obama's move yesterday is thus enormously significant on two levels: the symbolic and the real. America is at last trying to control one of its worst contributions to human society: tail-pipe pollution. No one can miss the significance of that. Furthermore, the actual savings of CO2 that these national emissions limits will make will go a long way to the US meeting the climate targets the President has at last introduced.

This is the third major initiative in climate change policy from Mr Obama, with his administration barely four months old. The first was the targets announcement itself (an 80 per cent saving of CO2 emissions by 2050); the second was the bill for a carbon trading system which the President has encouraged Congress to bring forward. Carbon trading by heavy industry will be America's principal weapon in forcing down emissions, but tighter fuel efficiency for cars will back it up strongly. The President has not waited for legislation, but is bringing in the new pollution limits by executive order. They may not be quite as tight as the limits being introduced in Europe but, by America's own standards, they are draconian. It all makes for a picture of a leader who promised he would act on climate change, and is keeping his word.