Tens of thousands of activists walked through Manhattan, New York, warning that climate change is destroying the Earth - in stride with demonstrators around the world who urged policymakers to take quick action.

Most came on foot, others with bicycles and walkers, and some even in wheelchairs. Many wore costumes and marched to drumbeats. One woman played the accordion. But their message was not entertaining.

"We're going to lose our planet in the next generation if things continue this way," said Bert Garskof, 81, as a family member pushed his chair.

He had first heard about global warming in 1967, "when no one was paying much attention", said Garskof, a native New Yorker and professor of psychology at Quinnipiac University, Connecticut.

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Organisers said more than 100,000 marched in New York, joined by United Nations Secretary-General Ban Ki-moon, former Vice-President Al Gore and New York City Mayor Bill de Blasio.

Tomorrow, more than 120 world leaders will convene for the United Nations Climate Summit in New York aimed at galvanising political will for a new global climate treaty by the end of next year.

The leaders, including US President Barack Obama, are expected to announce a major forest-restoration project as they seek to inject momentum into talks to agree a meaningful treaty to tackle climate change.

A contingent came from Moore, Oklahoma, where a huge tornado killed 24 people last year, as did hundreds of people affected by Superstorm Sandy, which the US National Oceanic and Atmospheric Administration and the British meteorological office said was made more likely by climate change.

"I am overwhelmed by such a strong power, energy and voice of people," said Ban. "I hope this voice will be truly reflected to the leaders ... Climate change is [a] defining issue of our time and there is no time to lose. If we do not take action now, we will have to pay much more."

The New York march was one of a series of events held globally to raise awareness about climate change.

In London, organisers said 40,000 marchers participated, while a small gathering in Cairo featured a huge art piece representing wind and solar energy. In Rio de Janeiro, marchers with green hearts painted on their faces rallied at Ipanema Beach.

Celebrities including actress Emma Thompson, musician Peter Gabriel and designer and activist Vivienne Westwood joined a march through Westminster calling on politicians to tackle warming.

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"This is important for every single person on the planet, which is why it has to be the greatest grassroots movement of all time," Thompson said.

"This is the battle of our lives. We're fighting for our children."

The backlash against fossil fuels is growing so fast nearly 700 financial institutions controlling 30 billion ($60 billion) of assets have pledged to exit investments that exacerbate climate change, it emerged yesterday.

Among those switching into cleaner power sources are the British Medical Association, the Joseph Rowntree Charitable Trust, and the Quakers, all of whom are part of the "dirty-energy divestment" campaign.

The three-year-old divestment campaign, which also includes Stanford University, the Rockefeller Brothers Fund and the city of Oakland in California, has doubled its membership this year and the organisers say new groups are signing up at an accelerating rate - a trend campaigners say is urgently needed.

Carbon emissions from burning fossil fuels are set to rise by 2.5 per cent to a record 40 billion tonnes this year, says new research from the University of East Anglia. At this rate, within 30 years the world will use up its "CO2 emission quota" - the amount of carbon dioxide it can pump into the atmosphere before catastrophic climate change becomes inevitable - the researchers say.

Scientists reckon the world's future carbon emissions cannot exceed 1200 billion tonnes if it is to have a reasonable chance of keeping global warming to 2C - the point beyond which the effects become increasingly devastating. Once in the atmosphere, CO2 can remain there for hundreds of years.

The divestment movement has 500 wealthy individuals and 168 institutions, mostly from the US but also from Europe, Australia, Africa and Britain.

- AP