The Secretary-General of the United Nations, Kofi Annan, has told a climate change conference in the Kenyan capital, Nairobi, that there has been 'a frightening lack of leadership' in confronting one of the world's leading threats.
Representatives from 189 countries are attending the conference.
They want to move beyond the Kyoto Protocol limiting greenhouse emissions and find a way of bringing large carbon producing countries into the framework.
Mr Annan told delegates that sceptics had no valid arguments.
He told a news conference that his remarks were 'not directed at any individual or president' when asked if he was especially targeting the United States, the world's biggest source of emissions.
The US pulled out of Kyoto in 2001. President George W Bush said that caps on emissions would harm the US economy and that developing nations were wrongly omitted from goals for 2012.
Mr Bush favours big investments in new technologies, such as hydrogen or cleaner coal.