As a man whose enthusiasm for Hummers helped popularise General Motors' civilian version of the gas-guzzling military vehicle in the 1990s, Arnold Schwarzenegger makes for an unlikely environmental champion. Yet the actor turned Republican governor of California - one of whose tank-like runabouts is now converted to run on hydrogen - is taking a lead role in America's fight against climate change.

The signing of Assembly Bill 32 last August committed the most populous state in the US to a 25 per cent reduction in greenhouse gas emissions by 2020. Then, last week, Mr Schwarzenegger took the first step towards meeting that goal when he signed an executive order to establish a low-carbon standard for transport fuels sold in California.