Prince Harry called on "leaders to lead" in an impassioned plea for action over climate change.
The Duke of Sussex issued the appeal as he warned that "the world is on fire, again" amid soaring temperatures and wildfires in Europe.
The royal said the world is at a "pivotal moment" in the fight for the planet's future.
He spoke out about climate change as he delivered the Nelson Mandela International Day keynote speech at the UN headquarters in New York.
Harry said: "The Horn of Africa is enduring the longest drought it's faced in close to half a century.
"And what's happening in Africa is not an isolated event. The drought there is a reflection of extreme weather we are seeing across the world.
"As we sit here today our world is on fire, again. These historic weather events are no longer historic.
"This crisis will only grow worse unless our leaders lead."
He added: "This is a pivotal moment. In this moment we have a choice to make.
"We can grow apathetic, succumb to anger or yield to despair, surrendering to the gravity of what we're up against.
"Or we can do what Mandela did, every single day inside that 7ft x 9ft prison cell in Robben Island - and every day outside of it too."
Harry said those fighting climate change could follow Mandela's advice to "never give up the battle even in the darkest hour".
During his speech at the UN, the Duke of Sussex revealed he has a picture of his mum meeting Mandela on his wall.
The Duke of Sussex said he treasures the image of his mother meeting the former South African President.
Princess Diana met the anti-apartheid hero in Cape Town, South Africa, just three months before her death in August 1997.
Prince Harry observed his mother's "cheekiness" in the image as he paid tribute to Mandela during a speech at the UN headquarters in New York.
The royal said: "On my wall and in my heart every day is an image of my mother and Mandela meeting in Cape Town in 1997.
"The photo was presented to me by the late Archbishop Desmond Tutu. When I first looked at the photo straight away what jumped out was the joy on my mother's face, the playfulness, cheekiness even.
"The pure delight to be in communion with another soul so committed to serving humanity.
"And then I looked at Mandela, here was a man with the weight of the world on his shoulders.
"Yet in that photo and so many others he's still beaming, still able to see the goodness in humanity, still buoyant with a beautiful spirit that lifted everyone around him."
Princess Diana was pictured shaking Mandela's hand outside his presidential residence in May 1997.
Harry's mum spoke to the President about the battle against Aids during her visit.
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Following her death, Mandela paid tribute to Diana, describing her as "one of the best ambassadors of Great Britain".
He said: "I had the honour of hosting her a few months ago and was tremendously impressed by her.
"She was undoubtedly one of the best ambassadors of Great Britain. I found her very graceful, highly intelligent and committed to worthy causes."
