The Prince of Wales has praised the passion of people supporting the environmental prize he helped set up.
In a message to campaigners, the prince said protecting the planet had been a cause close to the Queen's heart.
"During this time of grief, I take great comfort in your continued enthusiasm, optimism and commitment", he told them.
His Earthshot project is the biggest environmental prize ever with £50m to be awarded over a decade.
The aim is to find 50 solutions to the world's gravest environmental problems by 2030.
Prince William had been due to attend a conference in New York with previous Earthshot prize winners but the Royal Family is continuing its period of mourning after Queen Elizabeth's death.
Instead, the prince sent a video message – recorded at Windsor the day after the Queen's funeral – to conference delegates in which he said: "During this time of grief, I take great comfort in your continued enthusiasm, optimism and commitment to the Earthshot Prize and what we are trying to achieve.
"Protecting the environment was a cause close to my grandmother's heart. And I know she would have been delighted to hear about this event."
BBC royal correspondent Sean Coughlan said the new Prince of Wales was already showing his commitment to campaigning over the environment, following in the footsteps of his father.
He added there are no signs of Prince William, now heir to the throne, stepping back from his role in warning about climate change.
Prince William also highlighted the inspiration of John F Kennedy, whose brief US presidency was associated with the optimism of a younger generation and a hopeful drive for social and technological progress.
He confirmed he would travel to Boston in December with the Princess of Wales for the second Earthshot award ceremony.
The royal couple last visited the US in 2014, when they travelled to New York for a three-day tour while Catherine was pregnant with Princess Charlotte.
In July this year, his environmental project became an independent charity, with the Prince as its president.
In his message on Wednesday, William acknowledged that the world was "an uncertain place right now", adding that many people "are facing unimaginable challenges – from conflict to energy crises and food shortages".
He warned that a changing global climate would "only serve to entrench these immediate challenges into fundamental global issues", adding: "That is why our collective mission is so urgent."
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