LOS ANGELES-FRANCE: “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.

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

“If you provide a seaweed diet to cows, you get a 90% reduction in methane output,” said Smith, in what could play a key role in reducing the huge quantities of the greenhouse gas produced by livestock. Methane 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 9% of global waters, you can reduce global emissions by 50%,” she told reporters in Cannes. “And that’s completely doable.”

Even though warnings about climate catastrophe grow ever louder, the film argues, the world is still spending billions of dollars every year to subsidise the very fossil fuels that are causing the planet to overheat. “Excess carbon creates climatic disruption; it changes the weather patterns and life support systems that society relies on to survive,” said the voiceover by DiCaprio, long an environmental activist. “But climate change can be reversed if we act now,” he added.

There are two ways of doing that: either stopping greenhouse gases entering the atmosphere or bringing CO2 levels back down through photosynthesis, by which carbon is extracted from the air and put back into the earth. Ice on Fire focuses on the latter.

“How we get out of this is by very practical,” said Conners. “We went to people who were practically solving this rather than just talking about it.”

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