The duke called for society to “unite in repairing our planet” at the inaugural Earthshot Prize, as innovative green projects from across the globe were awarded £1m each.
Kate arrived at Alexandra Palace wearing an Alexander McQueen dress made for her in 2011, while her husband wore a dark green velvet blazer and a polo neck.
No stars flew to London and guests were asked by organisers to “consider the environment when choosing their outfit”.
Announcing Costa Rica as winner of the first ever Protect and Restore Nature prize, Kate said: “Nature is vital to us all.
“But for too long, we have neglected our wild spaces. And now we are facing a number of tipping points.
“If we don’t act now, we will permanently destabilise our planet. And we will rob our children of the future they deserve.”
William added to the Costa Rica winners: “Fantastic your celebrations were quite wild. You were quite moved when you won and taken aback.”
He called on younger members of society not to “give up hope” during the event and he announced the US would be the host nation for the next awards ceremony.
The move for the Earthshot Prize next year means William and Kate could travel to US to host the event, where the Duke and Duchess of Sussex have made their home.
At the end of the event, televised by the BBC, William, who was joined by Kate, took to the stage to tell the audience: “I want to say something to the young watching tonight.
“For too long, we haven’t done enough to protect the planet for your future, the Earthshot is for you.
“In the next 10 years, we are going to act.
“We are going to find the solutions to repair our planet.
“Please keep learning, keep demanding change, and don’t give up hope.
“We will rise to these challenges.”
Liverpool striker Mo Salah presented the first Revive our Oceans category to the Coral Vita project which grows coral on land to replant in oceans, giving new life to dying ecosystems.
Emma Watson awarded the Fix our Climate prize to AEM Electrolyser technology which turns renewable electricity into emission-free hydrogen gas.
During the show William and Kate clapped along and enjoyed music from the likes of Ed Sheeran.
New Delhi-based Takachar won the Clean our Air prize for its pioneering technology to help end the burning of agricultural waste, which causes severe air pollution.
The City of Milan Food Waste Hubs picked up the Build a Waste-free World for a programme recovering food from local supermarkets and restaurants to distribute to citizens in need.
Speaking on her way into the event, Dame Emma criticised throwaway culture and said her parents would have been horrified by how society dumps its waste on the streets.
She said: “If we had shown my parents how people live (today) how they will wander down the streets and coffee cup, immediately throw it away, eat, throw away, everything throwaway, they would’ve gone ‘what’s going on?’.”
Awards co-presenter Clara Amfo spoke about the duke’s acting skills.
She said: “He has been really great company and I am sure you saw our advert and he was quite the actor.
“We took a few takes and he took considerably less.”
William launched his prize to find solutions to the planet’s environmental problems and overcome the pessimism felt by many on its future.
Judges include broadcaster Sir David Attenborough, actress Cate Blanchett and singer Shakira.
During the ceremony, in a short film recorded in the London Eye, William will say: “We are alive in the most consequential time in human history.
“The actions we choose or choose not to take in the next 10 years will determine the fate of the planet for the next thousand.
“A decade doesn’t seem long, but humankind has an outstanding record of being able to solve the unsolvable.
“Many of the answers are already out there… but we need everyone – from all parts of society – to raise their ambition and unite in repairing our planet.
“The future is ours to determine. And if we set our minds to it, nothing is impossible.”
Ed Sheeran, Coldplay and KSI will perform during the ceremony, which will be shown on BBC One, and actors Watson, Dame Emma and David Oyelowo, and Liverpool FC striker Mo Salah, will hand out the awards.
— #EarthshotLondon2021 Awards | Sunday 8pm BST (@EarthshotPrize) October 17, 2021
The ceremony comes ahead of the Cop26 UN climate change conference in Glasgow in a few weeks, which the Queen, the Cambridges and the Prince of Wales and Duchess of Cornwall will attend.
Speaking ahead of the Earthshot ceremony, Charles said: “I am very proud of my son William for his growing commitment to the environment and the bold ambition of the Earthshot Prize.”
The prize takes its inspiration from the Apollo moon landings, nicknamed Moonshot, which helped advance mankind’s technological achievements.
It features five categories, or Earthshots – Protect and restore nature; Clean our air, Revive our oceans; Build a waste-free world; and Fix our climate – which, if achieved by 2030, would improve life for all, organisers say.
Every year from 2021 until the end of the decade, winners of the five Earthshots will each receive £1 million to develop their projects.
The inaugural 15 finalists include a 14-year-old girl from India who has designed a solar-powered ironing cart, the government of Costa Rica, which has pioneered a project paying local citizens to restore natural ecosystems, and a Chinese app that allows its citizens to hold polluters to account.