The Duke of Sussex says there is "a race against time" to halt global warming, adding that he is "troubled" by climate change deniers.
"I don't believe that there's anybody in this world that can deny science," he said.
He called it "an emergency", adding "the world's children are striking" after teenage activist Greta Thunberg led a worldwide protest on Friday.
Prince Harry is visiting Botswana as part of a tour of southern Africa.
He says it was the place he went to "to get away from it all" after his mother's death.
The duke 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 and the most troubling part of that is that I don't believe that there's anybody in this world that can deny science."
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?"
Africa is close to Harry's heart. It is the continent where he appears most at ease.
He's been coming here since he was a teenager. It helped him to find solace after his mother's death and it was to Africa, specifically to Botswana, that he brought Meghan Markle shortly after their romance began.
Little surprise then that it is to southern Africa that they have come for their first official overseas trip as a family - Harry, Meghan and four-month-old Archie.
There has been ground to make up after what was regarded as recent ill-judged steps - involving celebrity friends, private jet flights and costly house renovations.
For several months the headlines have tended to be negative.
But this visit has marked a return to a more grounded approach with Meghan's fluent sincerity as she addressed the issue of violence against women in Cape Town's most violent township. She identified with them as a "woman of colour and as their sister."
Harry too has struck the right note in his comments about mental health and the need to mobilise more effectively against climate change.
Even young Archie did his bit with a happily headline-stealing appearance when the family went to visit Archbishop Desmond Tutu.
As Harry struck out on his own to Botswana, Angola and Malawi he is entitled to feel that everything, so far, has gone according to plan.
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".