The largest gathering of world leaders on climate change opens at the United Nations on Tuesday amid calls for action to put the planet on course toward reversing global warming.

UN Secretary General Ban Ki-moon is hosting the summit of 120 leaders, the first high-level gathering since the Copenhagen conference on climate change ended in disarray in 2009.

Diplomats and climate activists see the event as crucial to building momentum ahead of the Paris conference in late 2015 that is to yield a deal on reducing greenhouse gas emissions after 2020.

But no-shows from the leaders of China, the world's biggest polluter, and India, the number three carbon emitter, are casting a cloud over the event.

"Climate change is the defining issue of our time. Now is the time for action," said Ban on the eve of the meeting opening at UN headquarters.

Ban is to kick off the summit alongside former US vice president and climate crusader Al Gore, Hollywood celebrity Leonardo DiCaprio, Chinese actress Li Bingbing and Nobel Peace Prize winner Rajendra Pachauri, head of the UN climate panel.

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

China is sending Vice Premier Zhang Gaoli while India will be represented by Environment Minister Prakash Javadekar.

Real commitments?

Despite much enthusiasm from climate activists for the summit's potential to create impetus, some see the event as falling short of what is needed to get serious about the environment.

"Few governments will be in a position to make any real commitments," wrote the aid agency Oxfam 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 ground-breaking," it added.

The summit is being held after marches drew hundreds of thousands of demonstrators on the streets in cities worldwide on Sunday in a show of "people power" directed at leaders reluctant to tackle global warming.

Key players from the private sector are also stepping into the fray to trumpet their commitment to greening, with Apple CEO Tim Cook announcing on Monday that the tech giant would prioritize low-carbon growth.

"Excuses for inaction have run out. The summit can be a major milestone, but only if it delivers the real world changes that we need," said Andrew Steer, of the World Resources Institute.

The summit talks are separate from the negotiations held under the UN Framework Convention on Climate Change (UNFCCC), which will culminate with the Paris conference in December 2014.

The United Nations is seeking to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, but scientists say current emission trends could hike temperatures to more than twice that level by century's end.

One recent report warned that a surge in carbon dioxide levels had pushed greenhouse gases to record highs in the atmosphere, increasing at their fastest rate in 30 years in 2013.

The 1997 Kyoto Protocol, the first international agreement to reduce emissions, expired two years ago and was never ratified by the United States.

Attempts to negotiate a new treaty ended in fiasco at the Copenhagen conference in 2009 and the pressure is on to avoid a repeat of that failure at the UN talks in Paris next year.

"The message from the climate summit and the message going forward to Paris is that it's not business as usual with a little bit of green attached," UN climate envoy Mary Robinson told AFP in an interview.

"It's changing course."