Every clean-energy entrepreneur and venture capitalist should now be applying for a United States visa. Barack Obama's speech yesterday was not only a reversal of the White House position on climate change, accepting the science and warning of the risk of “irreversible catastrophe” if it goes unchecked.

It was also a clear statement of the economic opportunities that will come from building a low-carbon economy. That includes the jobs involved in insulating homes and making fuel-efficient cars, plus the huge competitive advantage in renewable technologies, which America will be able to export.

In stating that America would not be “hostage to dwindling resources, hostile regimes and a warming planet”, President Obama deftly allied national security, prosperity and environmentalism. He pledged that America would lead the global fight against climate change and would begin at home.

The measures on cars are important, especially when the falling oil price has made drivers less conscious of the need to use less energy. Praising the Republican Governor of California, Arnold Schwarzenegger, and promising to review the block on Californian emission limits, was brave and right. Mr Obama's next and more fundamental step will be to set a price for carbon, by implementing one of the cap-and-trade schemes that his team have been debating.

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