The TV presenter, who has repeatedly dismissed the concerns of environmentalists, said he was convinced by the ”graphic demonstration of global warming” while filming in Cambodia.
As he tried to navigate the Mekong river system from Siem Reap to Vietnam he discovered that one lake had shrunk until it was barely knee-deep.
“It’s the first time that we’ve ever admitted to there being global warming,” Mr Clarkson told The Sunday Times. “It was alarming, genuinely alarming.”
He has previously been targeted by environmental campaigners because of his views on climate change.
“I do have a disregard for the environment,” Mr Clarkson said on one occasion. “I think the world can look after itself and we should enjoy it as best we can.”
In 2005 he admitted that “the world is warming up” but claimed it “isn’t even worthy of a shrug”.
A few months later he was hit in the face with a banana meringue after speaking to students at Oxford Brookes University.
Protesters also dumped manure on the front lawn of his home in Chipping Norton, Oxfordshire, in 2009 with a banner which read “this is what you’re landing us in”.
Just two months ago the car enthusiast continued his attacks on campaigners, describing 16 year-old Greta Thunberg as a “spoilt brat”.
“I agree with you that the world is heating up,” he told her in his newspaper column. “You may even be right that man has something to do with it... [but] science is what will solve the problem eventually.”
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