The Duke, who will be interviewed on Radio 4's Today programme on Thursday morning, said: "I felt very much that there's a lot of people wanting to do many good things for the environment and what they need is a bit of a catalyst, a bit of hope, a bit of positivity that we can actually fix what's being presented.
"And I think that urgency with optimism really creates action.
"And so The Earthshot Prize is really about harnessing that optimism and that urgency to find solutions to some of the world's greatest environmental problems.
"We believe that this decade is one of the most crucial decades for the environment and by 2030 we really hope to have made huge strides in fixing some of the biggest problems the Earth faces."
Jason Knauf, CEO of the Royal Foundation of the Duke and Duchess of Cambridge, added that the "sheer scale of the urgent situation", at a time when people are enduring enough immediate problems of their own, had run the risk of making challenges feel so insurmountable the public "feel little choice to look the other way".
But "the Duke wanted to turn the pessimism into the optimism and hope that will lead to real action," he said.
From now, a panel of nominators will seek out the best new ideas, technologies, policies or solutions across five categories: "Protect and restore nature", "Clean our air", "Revive our oceans", "Build a waste-free world", and "Fix our climate".
Each has $1.9m (£1m) in prize money per year which will support environmental and conservation projects agreed with the winners, who could be individuals, a group of scientists or activists, businesses, governments and even a city or country.
The Earthshot Prize takes its inspiration from the Apollo moon landings, nicknamed Moonshot, which helped advance mankind's technological achievements.
The project is expected to be seen as the duke's career-defining project, like his father's Prince's Trust or grandfather's Duke of Edinburgh Award scheme, and reflects his growing confidence and aim to play a global leadership role on the issue.
It will see him work with the Prince of Wales, a long-term environmental campaigner, and Sir David Attenborough, with organisations from Greenpeace to the UN forming a global alliance to share its message.
"The plan is to really galvanise and bring together the best minds, the best possible solutions, to fixing and tackling some of the world's greatest environmental challenges," said the Duke.
"We've got to harness our ingenuity and our ability to invent. The next 10 years are a critical decade for change.
"Time is of the essence, which is why we believe that this very ambitious global prize is the only way forward."
The prize fund will be provided by the project's global alliance founding partners, including the Paul G Allen Family Foundation, created by Microsoft co-founder Paul Allen, and the Jack Ma Foundation, the charitable body of the founder of the Chinese online retail giant Alibaba.
Key contributors also include the Aga Khan Development Network, Bloomberg Philanthropies, DP World in partnership with Dubai Expo 2020, and US internet entrepreneurs Marc and Lynne Benioff.
Every year an Ipsos Mori poll will be conducted to measure whether the public feel more optimistic about humanity's ability to solve the big issues.
Nominations for the prize open on November 1, with an annual global awards ceremony to be held in a different city each year, starting with London in autumn 2021.
