United States Energy Secretary Steven Chu is in Europe this week to begin talks that will be crucial in the global battle against climate change.

The 61-year-old physicist yesterday held key discussions with energy ministers from G8 nations in Rome before travelling to London to take part in a debate, with Nobel prize winners, on global warming.

The arrival of Chu, himself a physics Nobel prize winner, comes as the scientist-turned-politician finds himself under fire from environmentalists over decisions he has made about America's campaign to fight global warming.

Green groups have accused him of being "contradictory and illogical" and of failing to demonstrate sufficient dynamism in establishing a new, low-carbon approach to transport and power-generation in the US.

In recent weeks, Chu - appointed by President Barack Obama in December - has revealed he is no longer willing to block the construction of new coal-powered electricity plants in the US despite widespread opposition from green groups and despite having said, initially, that he would not permit their construction.

Environmental campaigners rail against coal plants - which atmosphere scientist James Hansen, director of Nasa's Goddard Institute for Space Studies in New York, recently labelled "factories of death" - because of their high carbon emissions.

Chu has also called for a slowdown in the development of hydrogen-powered vehicles in the US and slashed funding for new projects by 60 per cent.

"We asked ourselves, is it likely in the next 10 or 15, or even 20 years that we will convert to a hydrogen car economy?" Chu explained. "The answer, we felt, was no."

On top of these controversial calls, Chu has eliminated funding for a project to build a nuclear waste store at Yucca Mountain in Nevada.

Instead of storage, he has backed the building of fast neutron reactors that could burn long-lived waste. Such a move, which would require a major expansion of the US nuclear industry, has horrified ecology groups.

Yet most US eco-campaigners were overjoyed by Chu's appointment last year. They saw his arrival as the start of a new, enlightened approach.

Chu says he is merely being pragmatic and is still enthusiastically committed to cutting US carbon emissions. "[But] if we say we want something much more aggressive [early] that would draw considerable opposition and that would delay the process for several years."

The problem is that America isolated itself over the issue of climate change for eight years under George W. Bush. Opposition to the idea of reducing carbon emissions has become entrenched.

Obama has indicated he wants America to cut its greenhouse gas emissions significantly but has left it to Congress to pass the necessary laws. However, the energy industry continues to lobby politicians fiercely and the climate and energy bill faces considerable hurdles.