Sir David Attenborough said the Covid-19 pandemic was a threat to the environment as politicians deal with the crisis instead of climate change.
The naturalist and broadcaster expressed his fears to environmental activist Greta Thunberg at a virtual wildlife film festival in Bristol.
The 94-year-old said Thunberg had given the world hope by energising young people to fight for the environment.
The teenager also praised Sir David's new film A Life On Our Planet.
Sir David said several international climate conferences had been cancelled because of Covid-19.
"I am worried that people will take their eyes off the environmental issue because of the immediate problems they have on Covid-19," he said.
Speaking from his home in London as part of Wildscreen's "Planetary Crisis" conversation, he said Thunberg's campaigning had made other young people demand action to stop climate change.
"If there is any sign of hope, and there is to be truthful compared to what there was 25 years ago, it's because of what you've done and what you've done for young people," he said.
"The world owes you a lot and I hope you are not paying too high a price for it, and it looks from what you are saying that you are managing to survive alright."
'Create public opinion'
Speaking from Sweden, Thunberg, 17, said since becoming known around the world for her activism, people were talking about her instead of climate change.
"It takes away focus from the climate process. All we are trying to do is raise public awareness and create public opinion," she said.
"If enough people become aware and if enough people put enough pressure on people in power and the elected officials then they will have to do something because the politicians job is to get elected and to do as the voters ask."
She praised Sir David's A Life On Our Planet for the way it "connected all these issues, like the climate crisis, loss of biodiversity, loss of soil and over-fishing".