Leonardo DiCaprio made an appearance at the Cop26 climate summit in Glasgow.
The Hollywood star, 46, was pictured at the conference surrounded by an entourage and wearing a blue suit featuring a colourful lapel pin.
He has worked on several documentaries about animal poaching and the environment, and was appointed a United Nations representative on climate change in 2014.
In 2019 Di Caprio met renowned climate activist Greta Thunberg, calling her a "leader of our time".
Kew Science, the research arm of the Royal Botanic Gardens in Kew, said the Titanic star had visited its Carbon Garden space, where scientists during Cop26 will be highlighting the role plants can play in providing solutions to climate change.
DiCaprio, who on Instagram describes himself as an actor and environmentalist, updated his biography before the event to include the hashtag #COP26.
He also added a link to the United Nations website page on climate change and tweeted a warning to world leaders that "the world is watching".
In 2016, he was an executive producer on The Ivory Game, which examined the ivory trade.
Oscar-winning director Fisher Stevens also filmed DiCaprio as he travelled around the world to see the effects of global warming for the 2016 feature Before The Flood.