Arguing that global warming merits a national solution and not "a confusing patchwork of state rules", the US Environmental Protection Agency (EPA) last month rejected a request by California to impose state limits on greenhouse-gas emissions from motor vehicles.

The California regulations would have imposed a reduction of 30% in greenhouse-gas emissions in vehicle exhausts by 2016. Seventeen other states were expected to adopt the standards, which would then have covered around 45% of annual car sales, advocates of the proposal say.

The EPA decision followed federal legislation on 19 December requiring motor-vehicle manufacturers to increase fuel efficiency by 40%, to 35 miles per gallon, by 2020.

Calling the EPA decision "completely absurd", California attorney-general Edmund Brown promised to challenge it in the courts.