The Duke of Sussex has criticised climate change deniers, saying: “No-one can deny science.”
There was “a race against time” to halt global warming, he said, adding: “The world’s children are striking.”
A movement sparked by the teenager Greta Thunberg saw millions of people protest around the world on Friday.
Prince Harry is visiting Botswana as part of a tour of southern Africa – a place he says he went to “to get away from it all” after his mother’s death.
He said he had visited the country soon after Diana, Princess of Wales, died in August 1997 and had made “some of my closest friends” there.
“Now I feel deeply connected to this place and to Africa,” he said during a visit to the Chobe Tree Reserve.
The prince helped plant trees at the site, which has been affected by decades of deforestation.
“This last week, led by Greta, the world’s children are striking,” the prince said.
“It’s a race against time and one in which we are losing. Everyone knows it, there’s no excuse for not knowing that.”
He went on to say there had been scientific evidence of climate change for at least 30 years.
“And it’s only getting stronger and stronger,” he added.
He worked with a group to push a 10-metre tree tree upright and packed it with soil.
When he saw the large mound of earth that needed to be shovelled, he smiled and asked: “How long do we have?”
Prince Harry is on an official tour with the Duchess of Sussex and their baby son Archie, who was introduced to the renowned anti-apartheid campaigner Archbishop Desmond Tutu in South Africa on Wednesday.
The royal couple also met faith leaders at South Africa’s oldest mosque and visited a charity that provides mental health support to young people.
The duchess told girls in a deprived part of the country she was visiting South Africa not only as a member of the Royal Family, but also “as a woman of colour and as your sister”.