The Prince of Wales - who has Prince George, 12, Princess Charlotte, 10, and Prince Louis, seven, with wife Catherine, Princess of Wales - has unveiled the finalists for this year's Earthshot Prize.
As he did, William reflected on how quickly the five years have passed since he first announced the initiative, which rewards those offering innovative solutions to the world's environmental challenges.
He said in a short film shared on his social media channels: "now, more than ever, is the time to fix the planet.
"I sat under this oak tree five years ago, soon after we launched The Earthshot Prize. The planet, the only home we have, needed our help, as scientists made it clear that we had to make significant changes by 2030.
"Back then, a decade felt a long time. George was seven, Charlotte five and Louis two; the thought of them in 2030 felt a lifetime away.
"But today, as we stand halfway through this critical decade, 2030 feels very real.
"The Earthshot Prize was founded because this decade matters. 2030 is a threshold by which future generations will judge us; it is the point at which our actions, or lack of them, will have shaped forever the trajectory of our planet."
William praised the "urgent optimism" of the finalists.
"The people behind these projects are heroes of our time, so let us back them," he said.
"Because, if we do, we can make the world cleaner, safer and full of opportunity — not only for future generations, but for the lives we want to lead now."
Among this year's finalists, Barbados has been nominated for its global leadership on climate with the island on track to become fossil-free by 2030.
The Chinese city of Guangzhou is shortlisted in the "Clean our Air" category for electrification of its public transport system.
And Sydney's Quay Quarter Tower is in the running after being billed as the world's first fully "upcycled skyscraper".
The building was one of thousands of 20th century towers now reaching the end of their lifespans.
Instead of demolition, which releases vast amounts of carbon and waste, a coalition of architects, engineers, building contractors and developers effectively "upcycled" the original structure.
This year's awards ceremony will take place in Rio de Janeiro next month.
11°C