Unless we protect resources and the Earth's natural capital, we shall not be able to sustain economic growth

Kofi Annan

He said conserving energy and using it more efficiently was more than a matter of individual choice. And he insisted that the science of climate change, despite the criticisms of the sceptics, was well-founded.

Mr Annan was giving the commencement address at the Fletcher School of Law and Diplomacy at Tufts University in Massachusetts, US.

His intervention was pointed, particularly since it is only three days since the US administration unveiled its national energy plan.

US plan attacked

The plan has been severely criticised by many environmental groups for its continued reliance on polluting fuels.

Mr Annan: Global warming is a sober prediction for tomorrow

Mr Annan had earlier described as "unfortunate" President Bush's announcement that the US would not implement the international climate treaty, the Kyoto Protocol.

Under Kyoto, developed countries agree to reduce their emissions of greenhouse gases by an average of 5.2% on their 1990 levels over the next decade.

Returning to the issue, Mr Annan told his audience at Tufts that the US had a peculiar responsibility in the fight to limit global warming.

He said: "The United States, as you probably know, is the world's leading emitter of greenhouse gases, largely because it is the world's most successful economy.

"That makes it especially important for it to join in reducing emissions, and in the broader quest for energy efficiency and conservation. There is concern throughout the world about the decision of the new administration to oppose the protocol."

Economic argument

Mr Annan also dismissed the idea that implementing Kyoto would be economically damaging, although this was one of the reasons Mr Bush gave for rejecting it.

"The opposite is true," Mr Annan said.

Mr Bush has drawn environmentalists' ire

"Unless we protect resources and the Earth's natural capital, we shall not be able to sustain economic growth."

The US vice-president, Dick Cheney, said recently that energy conservation could be a personal virtue but was no basis for a sound energy policy.

Mr Annan's message for him was clear: "It is said that conservation, while admirable, has only limited potential. But economists now broadly agree that improved energy efficiency and other 'no regrets' strategies could bring great benefits at little or no costs."

Dispute over science

Another of Mr Bush's objections to Kyoto is his belief that it is not based on sound science.

Mr Annan made it clear he does not share the president's reservations.

Developing countries will have to do their part in due course

Kofi Annan

"Imagine melting polar icecaps and rising sea levels, threatening beloved and highly developed coastal areas such as Cape Cod with erosion and storm surges", he said.

"Imagine a warmer and wetter world in which infectious diseases such as malaria and yellow fever spread more easily.

"This is not some distant worst-case scenario. It is tomorrow's forecast. Nor is this science fiction. It is sober prediction, based on the best science available."

Developed v developing countries

Mr Annan said developed countries had to take the lead on climate change because they emitted most of the pollutants scientists believe are contributing to it.

But he added: "Developing countries will have to do their part in due course. Their exclusion from emissions commitments is only for the protocol's first phase."

Mr Annan's challenge to President Bush will encourage many environmental groups disheartened by his repudiation of Kyoto.

But it will also dismay those scientists who remain sceptical about the science, and about what they say is an attempt to marginalise any doubters.