The American political system is in the throes of a fierce battle over climate policy. President Barack Obama says he wants cuts in greenhouse gases but has left it to Congress to make the political running.
Another triumph for democracy. Unsurprisingly environmentalists are alarmed. Damon Moglen from Greenpeace USA, echoing the views of Nasa scientist James Hansen, says:
The House of Representatives is debating a climate and energy bill but even if it passes it may be rejected by senators, many of whom are funded by the energy industry.
Prof Chu is a Nobel prize-winning physicist and a world expert on clean energy. But he said it was impossible to ignore political reality.
"we are getting very concerned. Professor Chu is a good man and a good scientist, but the science on global warming is clear and he should be guided by the science not the politics."