Russian President Vladimir Putin has declared the fight against climate change "one of the bravest challenges humanity is facing".
Speaking on the opening day of UN conference talks in Paris, Mr Putin said Russia was "contributing actively" to addressing global warming and urged other nations to do the same.
He said it was important that there was a "joint effort" to reduce global warming, saying: "Our ability to successfully address climate change will determine the quality of life for all people on the planet."
US President Barack Obama has cited a warning from Martin Luther King in his appeal to his fellow world leaders to "act now" on climate change.
"I believe in the words of Dr Martin Luther King Jr that there is such a thing as being too late," he said. "And when it comes to climate change that hour is almost upon us."
Mr Obama said the scientific community have "broken the old arguments for inaction" on climate change, saying financial barriers no longer stood in the way of environmental progress.
"That should give it hope," he said as he joined other world leaders in making an introductory address at the UN conference.
But Mr Obama warned: "One of the enemies we face is cynicism."
He said the US would pledge new funds to help vulnerable countries who contribute the least pollution but are affected most by climate change.
French President Francois Hollande has said the battle against global warming is of equal importance to the world as the ongoing fight against Islamic State and other terror groups.
"I can't separate the fight with terrorism from the fight against global warming," Mr Hollande said in his address at the opening of the UN conference talks in Paris.
"These are two big global challenges we have to face up to," he said. "Because we have to leave our children more than a world freed of terror, we also owe them a planet protected from catastrophes."
Prior to his address Mr Hollande led world leaders in a minute's silence in honour of the victims of the recent jihadist attacks in the city, which claimed the lives of 130 people.
Prince Charles has urged world leaders to "think of your grandchildren, as I think of mine" as he demanded greater action to tackle climate change.
"On an increasingly crowded planet humanity faces many threats but none is greater than climate change," the royal told the UN conference in Paris.
Charles called on leaders gathered to take immediate action to tackle rising temperatures, which he said magnify "every hazard and tension of our existence".
"It threatens our ability to feed ourselves, to remain healthy and safe from extreme weather, to manage the natural resources that support our economies, and (impacts the) humanitarian disaster of mass migration and increasing conflict," he said.
Charles, who began his keynote speech with a tribute in French to the victims of the Paris attacks, added: "I can only urge you to think of your grandchildren, as I think of mine, and of those billions of people without a voice."
Prince Charles and David Cameron have both arrived at the climate change talks in Paris as world leaders and dignitaries descend on the French capital.
The Prime Minister will call for a robust deal that shows governments around the globe are serious about cutting carbon, though environmental commentators remain sceptical of agreements being forged.
The prospects of genuine progress being made in Paris have been played down by veteran naturalist and broadcaster Sir David Attenborough.
Of course I'm not confident. This is a hideous problem. Never in the history of humanity have all the people of the world got together to deal with one particular problem and agree on what the solution should be.
As Chinese President Xi Jinping joins other world leaders in Paris to tackle climate change, Beijing has been placed on special alert after dangerous smog reached its highest level for more than a year.
Pollution levels in the Chinese capital have soared to 17 times higher than levels deemed safe by the World Health Organisation.
From the canals of Venice to a remote island in the Pacific Ocean, the effects of climate change are worryingly visible across the globe.Read the full story ›
After decades of struggling negotiations both activists and delegates are hoping for some form of landmark agreement.Read the full story ›