Image copyright Sundance Institute Image caption An Inconvenient Sequel: Truth to Power focuses on Al Gore's climate campaigning

Former US Vice President Al Gore says he believes climate change campaigners will "win" the debate, even with a new president in the White House who has previously called global warming "a hoax."

At the world premiere of his environmental documentary, An Inconvenient Sequel, at Utah's Sundance Film Festival, Gore called those demanding action on climate change "a mass movement - one that cannot be ignored".

Like its predecessor, 2006's Oscar-winning An Inconvenient Truth, this film, An Inconvenient Sequel: Truth to Power focuses on Mr Gore's lifetime campaigning for global action on climate issues, and on the search for renewable energies.

The politician says he was initially "reluctant" to make another film, but believes the 2015 climate conference in Paris was a "benchmark".

However, the film ends with the election of Donald Trump as president in November 2016.

Image copyright Getty Images Image caption Al Gore met Donald Trump in December at Trump Tower to discuss climate policy

Image copyright AFP/Getty Image caption Robert Redford: "We don't occupy ourselves with politics"

Donald Trump has previously described himself as "not a big believer in global warming."

In the documentary, directed by Bonnie Cohen and John Shenk, Mr Trump is heard calling for Mr Gore's Nobel Prize to be rescinded, while the politician describes the election of Mr Trump as a "setback".

Image copyright Getty Images Image caption Donald Trump has made his views on climate change clear

But he's also seen going into Trump Towers in New York for a meeting with the then president elect, and it's been reported in the media that now Mr Gore is in "continuous" dialogue with the new administration.

He now says he's "hoping for the best" from Mr Trump's environmental team.

"There's a little more openness to the idea of climate change. That's the only thing I am going to say about it," he said.

'Support storytellers' says Redford

"We'll find out those policies soon enough. I hope that those with responsibilities on their shoulders will listen carefully to what the experts and often their own advisors are telling them."

Festival founder Film Festival, Robert Redford, disagrees that there's a higher level of "political" filmmaking this year - although Sundance opened with An Inconvenient Sequel, along with Whose Streets?, a Sabaah Folayan's documentary about the killing of Michael Brown in Ferguson, Missouri.

It also has a late addition to the programme - a documentary called Trumped: Inside the Greatest Political Upset of All Time.

"We don't occupy ourselves with politics, we stay focused on what the stories are being told by artists," he said.

Image caption Hydroelectricity is a major source of renewable power

"The idea of us being involved in politics is just not so. We think it's far more important to support storytellers, and if politics comes up with them, so be it."

Nevertheless, the festival has established a New Climate section, to bring fresh film perspectives on the environment.

An Inconvenient Truth was the first hugely successful documentary on the topic, making more than $50m at the box office.

Gore, who was defeated by George W Bush in 2000 for the US Presidency, went on to win the Nobel Peace Prize in 2007.

In the decade since the first film was made, the politician says the biggest difference is that "solutions on renewable energy are now readily available - and in some cases cheaper than fossil fuels.

Image copyright PA Image caption Many countries still rely on coal-fired power

"Also, one of the things that is most encouraging and exciting to me is that silly partisanship is fading, that there's co-operation between Democrat and Republicans," he said.

"And then there's the younger generation. For them, climate change is a given. They just want to roll up their sleeves and say 'Yes, what do we need to do? Let's just get on with it.' That's why I think we'll win in the end."

However, An Inconvenient Sequel shows Gore's admission of "it's hard not to take it personally" when his beliefs on global warming are rejected, and his belief that "democracy is being hacked by big business" - a reference to political lobbying by fossil fuel industries.

Civil Rights 'parallels'

He also described his terror over what he calls "the Book of Revelations being enacted on our TV screens every night".

"There are fires, floods, droughts, and it keeps happening, it's just become something regular; 2016 was officially described as the hottest year ever, and that's the third year running it's happened.

"So this is real, and we have to act."

The environmental struggle, according to Gore, has parallels with the Civil Rights Movement of the 1960s. It's inevitable, he claimed, that they will win, adding: "But it could take a while.

"It was won over time by people who spoke up in conversations. We have to win those conversations."

An Inconvenient Sequel: Truth to Power will be released later in 2017. The Sundance Film Festival runs until January 29, 2017