Climate change can be reversed, DiCaprio's 'Ice on Fire' claims

"We don't need advanced technology, Mother Nature has seaweeds and shellfish which sequester five times more carbon than land-based plants," said Bren Smith, winching a glistening haul of glossy brown kelp out of the sea.

A fisherman turned "steward of the sea" who runs a restorative ocean farm growing seaweed and farming shellfish, Smith is one of dozens of characters who appear in "Ice on Fire", an eye-opening environmental documentary at Cannes film festival.

Produced by Hollywood star Leonardo DiCaprio and directed by Leila Conners, who worked together on 2007 climate documentary "The 11th Hour", the film offers a fascinating look at the innovations and people working to significantly reduce carbon in the atmosphere.

Take kelp. Not only does it soak up five times more carbon than plants on land, it is one of the fastest growing plants on earth and can be widely used for everything from food to fertiliser and even animal feed.

"If you provide a seaweed diet to cows, you get a 90 per cent reduction in methane output," said Smith, in what could play a key role in reducing the huge quantities of methane produced by livestock, a greenhouse gas that is 25 times more potent than CO2.

For Conners, it was learning about the properties of kelp and other innovations that provided the biggest surprise in putting together the film.

"If you plant kelp in nine per cent of global waters, you can reduce global emissions by 50 per cent," she told reporters in Cannes, flagging other innovations like the development of biochar (charcoal produced from plant matter) and a Swiss-made direct air capture machine.

"And that's completely doable."