'If we say we want something more aggressive it could delay the process'
US Energy Secretary Steven Chu says the US will not be able to cut greenhouse emissions as much as it should due to domestic political opposition.
Prof Chu told BBC News he feared the world might be heading towards a tipping point on climate change.
This meant the US had to cut emissions urgently - even if compromises were needed to get new laws approved.
Environmentalists said Prof Chu, a Nobel physicist, should be guided by science not politics.
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.
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.
"With each successive year the news on climate change has not been good and there's a growing sensation that the world and the US in particular has to get moving," he said.
If you could convert (with photovoltaic cells) 20% of the Sun's energy into electricity you would need 5% of the world's deserts. This is not much land
Steven Chu, US Energy Secretary
"As someone very concerned about climate I want to be as aggressive as possible but I also want to get started. And if we say we want something much more aggressive on the early timescales that would draw considerable opposition and that would delay the process for several years.
The US energy secretary said that awareness of climate tipping points had increased greatly only in the past five years. He added: "But if I am going to say we need to do much, much better I am afraid the US won't get started."
To the anger of environmentalists, he said that one compromise would be approving new coal-fired power plants without obliging them to capture and store their carbon. The UK government has made this a stipulation for new coal plants but Prof Chu declined to explain why the US government would not follow suit.
The first step in America, he said, should be a massive programme of efficiency for commercial buildings. This could save 80% of their energy demand, he said. He said the government would provide the research and encourage states to adopt tough standards.
He envisaged a future in which the US was largely p
Professor Chu fears the world is heading towards a tipping point on climate change
owered by wind and solar but admitted there were technical difficulties.
On solar he explained: "The challenge is to make solar energy cost-effective. The amount of energy hitting the Earth - if you looked at it, if you could convert (with photovoltaic cells) 20% of the Sun's energy into electricity you would need 5% of the world's deserts. This is not much land. So the opportunity is enormous.
"The question is whether we can make it cost-effective. You have to transport this long distances because people don't live in deserts."
Similarly, on wind, Prof Chu told BBC News: "The good news is that many of the areas with good wind are where there aren't many people, so there are fewer objections to wind farms. The bad news is that there aren't many people. So we are planning to look at how you get an interconnecting (transmission) system, to allow us to develop these great resources."
An overriding challenge for both technologies was the need to develop storage for energy from renewables.
When informed about research from HSBC suggesting that China had invested twice as much in greening its economy with its fiscal stimulus as the US had done, he said he had not seen the figures but that he wished for more money for clean energy.
He said the challenge was pressing, and agreed that the world could face future spikes in energy prices because the recession had halted investment in many energy projects. This would be an issue at the upcoming G8 finance ministers meeting, he said.
Damon Moglen from Greenpeace USA was alarmed by Prof Chu's comments. "Obama has had something of a honeymoon with environmentalists," he said.
"But 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.
"It is out of the question that the US should agree new power stations burning coal - the dirtiest fuel. Our targets on emissions are too low anyway - and there is no way we will meet even those low targets if we allow more coal to be burned.
"Professor Chu's comments on coal are contradictory and illogical. This administration should give him the head to develop the sort of energy policy he knows we really need."
But Prof Chu said: "I am optimistic for the first time in my life that the US will start to move in this direction (of cutting emissions) and that's why I am heartened by these efforts. If you had asked me two or three years ago what's the possibility we could move in the direction of reducing carbon emissions in the US I would have said I don't know."
When asked whether he was frustrated, he said: "No, I am realistic about the politics and as in time we can make adjustments."
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