UN chief urges the world to set a 'new course' on climate

The largest gathering of world leaders ever to deal with climate change opened at the United Nations on Tuesday with a call to action that advocates say would put the planet on course toward reversing global warming.

"Today, we must set the world on a new course," UN Secretary-General Ban Ki-moon told leaders from 120 countries. "I am asking you to lead."

The meeting is the first high-level gathering since the Copenhagen conference on climate change ended in disarray in 2009.

Climate activists see the event as crucial to building momentum ahead of the Paris conference in late 2015 that they hope will produce a deal on reducing greenhouse gas emissions after 2020.

Ban called for a reduction in greenhouse gas emissions and declared that by the end of the century the world must be carbon neutral.

"Climate change threatens hard-won peace, prosperity and opportunity for billions of people," Ban said. "We are not here to talk. We are here to make history."

Ban was joined at the opening by former US vice-president and climate crusader Al Gore, Hollywood celebrity Leonardo DiCaprio, Chinese actress Li Bingbing and Rajendra Pachauri, head of the UN climate panel, which won the Nobel Peace Prize in 2007.

Leaders are to take turns at the podium throughout the day, from President Barack Obama, representing the world's second biggest polluter, to Prime Minister Enele Sopoaga of Tuvalu, the Pacific island-nation that faces the prospect of being wiped out by rising sea levels.

Despite much enthusiasm from climate activists for the summit's potential to create momentum, some see the event as falling short of what is needed to seriously address environmental issues.

"Few governments will be in a position to make any real commitments," wrote Oxfam, an aid agency, in an assessment of the summit's likely outcome.

The initiatives to be unveiled by the private sector, foundations and green groups at the summit "are helpful, but few, if any, are really groundbreaking", Oxfam said.

The summit is being held after marches drew hundreds of thousands of demonstrators to the streets in cities worldwide on Sunday.