The planet will be in "huge trouble" unless Barack Obama makes strides in tackling climate change, says a leading scientist.

Prof James McCarthy spoke on the eve of the annual meeting of the American Association for the Advancement of Science (AAAS), which he heads.

The US president has just four years to save the planet, said Prof McCarthy. If major policy changes do not happen within Mr Obama's term of office, they will not happen at all, he warned.

"We have a moment right now of extraordinary opportunity, with a new president, positioned with scientific leadership that has known no equal in recent times," the AAAS president told BBC News. "The calibre of scientific advice that is close to this man is truly exceptional.

"If in his first term, in the next four years, we don't make significant progress in these areas, then I think the planet is in huge trouble. "Without US leadership, which has been sorely lacking, we will not get to where we need to be."