California has led the world and irritated the car industry for more than 60 years in its fight against the atmospheric pollution that fouls its cities and is blamed for killing trees hundreds of miles away.

As long ago as 1943, Los Angeles scientists recorded the first episode of smog formation in the city's basin and identified vehicle emission as the cause, leading to its first air pollution law in 1947. Last year, the state incensed manufacturers by imposing a 30 per cent reduction in exhaust pipe emissions of greenhouse gases - mainly carbon dioxide - from all new cars sold in the state. So when in 10 days' time a group of 60 city mayors from around the globe arrive in San Francisco for a five-day meeting to mark the UN's World Environment Day by discussing measures they can take to combat climate change, they will find plenty of inspiration from local lawmakers.