The Rockefellers, the first family of oil, said Monday they would divest from fossil fuels in a fresh boost to the fight against climate change ahead of a UN summit.

A day after tens of thousands marched around the world urging world leaders to do more to stem climate change, private institutions, individuals and local governments with a total of more than $50 billion of assets announced in New York that they were divesting from fossil fuels.

The Rockefeller Brothers Fund, an $840 million endowment run by John D. Rockefeller's descendants, said it would reduce exposure to fossil fuels as much as possible, including ending investments in coal and tar sands -- two of the dirtiest forms of energy -- by the end of the year.

The fund said it was putting its investments in line with its values.

John D. Rockefeller was once the world's wealthiest person as the founder of Standard Oil -- whose descendant, ExxonMobil, is a frequent foe of climate initiatives.

Nobel laureate Desmond Tutu, the archbishop who fought South Africa's apartheid regime, took part in the announcement by video and urged a halt to new fossil fuel exploration.

"We can no longer continue feeding our addiction to fossil fuels as if there is no tomorrow, for there will be no tomorrow," Tutu said.

While the divestment is small compared with the vast size of the fossil fuel industry, campaigners hailed the announcement, which came a day before world leaders including US President Barack Obama hold a summit on climate change at the United Nations.

"Climate change is the defining issue of our time. Now is the time for action," said UN Secretary-General Ban Ki-moon, who called the summit and said that Sunday's march was his first rally since he protested South Korea's former military dictatorship.

- Time running out -

The summit aims to pave the way for a global agreement on fighting climate change -- a successor to the Kyoto Protocol -- during a conference in late 2015 in Paris.

Despite leaders' statements, a new study found that greenhouse gas emissions blamed for climate change rose 2.3 percent in 2013 to a new record and that time was quickly running out to check climate change at the key threshold of two degrees Celsius (3.6 Fahrenheit) above pre-industrial levels.

US Secretary of State John Kerry warned that hundreds of millions of people could face displacement if sea levels rise one meter (3.3 feet).

"You can make a powerful argument that (climate) may be the most serious challenge we face on the planet," Kerry told a conference in New York ahead of the summit. "I can show you parts of the world where people are killing each other today over drought and water."

"It is absolutely imperative that we decide to move and to act now," he said.

French Foreign Minister Laurent Fabius, who has been drumming up support for an agreement in Paris, said that he saw bright signs with both China and the United States -- the two largest carbon emitters -- doing more in recent years on climate change.

But he called for greater effort, including by helping the developing world to grow in a way that does not emulate the carbon-intense path of rich nations.

"Without alternative paths to development, which require finance and technology, the effects of climate disruption and pollution will become unmanageable, and they will ruin any hope of prosperity in most parts of the planet," Fabius said.