The Prince of Wales has warned against "doom and gloom" in discussions about tackling climate change.
He was speaking in New York as the finalists were announced for his flagship environmental project, the Earthshot Prize.
The prince said a dose of realism was important, but it was also necessary to give people a sense of hope.
Bill Gates, UN climate envoy Mike Bloomberg and former New Zealand PM Jacinda Ardern were among the guests.
"I think if we remark on how pessimistic and doom and gloom everything is, even though there is a healthy dose of that needed... it doesn't provoke the reaction from us humans that we would like," Prince William told the event in New York.
"An important part of the prize's design and development is not just to provide the solutions, but it's to make people believe there is hope."
He added that he was "impatient" to see a more rapid scaling up of new approaches to reducing environmental harm.
Prince William's New York visit this week has seen him stepping up on a global stage, including a meeting with UN secretary general Antonio Guterres.
But there have been comments on social media expressing cynicism about VIPs and celebrities flying so many miles to talk about decarbonisation and tackling climate change.
The Earthshot event has been held alongside New York's Climate Week and the United Nations has been staging its annual general assembly, including a keynote speech from US President Joe Biden.
The prince has used the week as a platform for his annual competition, which is dedicated to finding innovative ways to promote sustainability and tackle climate change.
The heir to the throne also held meetings with leaders of countries vulnerable to climate change, including the president of Ecuador.
A shortlist of 15 Earthshot finalists was announced, with nominees coming from countries including Peru, India, Sierra Leone and Poland.
One project, from the UK, aims to produce tyres for electric cars which will reduce harmful tyre pollution. And a finalist from the US aims to improve the treatment of industrial wastewater.
Five winning entries will receive £1m each at a ceremony in Singapore in November.
Former Microsoft boss and philanthropist Bill Gates spoke at the Earthshot event of his optimism about technological advances, saying that "innovation is delivering very well" in reducing environmental harm.
Others in attendance included Baroness Scotland, secretary general of the Commonwealth, and Maros Sefcovic of the European Commission.
The prize was inspired by US President John F Kennedy's "Moonshot" programme, which resulted in the US Apollo lunar launches and the first man setting foot on the Moon in 1969.
The late president's daughter Caroline Kennedy, US ambassador to Australia, was also among the guests in New York.
When he arrived in the US earlier this week, Prince William said of tackling the environmental crisis: "The challenge may feel huge, but as John F Kennedy taught us, we rise to the challenge not because it is easy, but because it is hard. And vital."