Climate change is the biggest challenge facing the world, naturalist Sir David Attenborough has said.
The veteran broadcaster said scientific data clearly showed that human-induced climate change was now beyond doubt.
Sir David, 80, added that everyone had a responsibility to change their behaviour, including being less wasteful and more energy efficient.
It is the first time Sir David has voiced his concerns in public about the impacts of global warming.
His comments come ahead of a two-part BBC series in which he examines the impacts of global warming on the Earth.
Sir David has been criticised by environmentalists in the past for not speaking out on the matter.
"If you take one moment in time, you can't be sure what the trend is," he told the BBC.
"Now... when we look at the graphs of rising ocean temperatures, rising carbon dioxide in the atmosphere and so on, we know that they are climbing far more steeply than can be accounted for by the natural oscillation of the weather."
Sir David, whose distinguished broadcasting career spans more than half a century, says everyone has a responsibility to act: "What people (must) do is to change their behaviour and their attitudes.
"If we do care about our grandchildren then we have to do something, and we have to demand that our governments do something.
His comments came as a UK parliamentary body, the All-Party Environment Group, issued a report labelling the government a "climate laggard" for its record on reducing emissions.
Sir David, whose natural history programmes have been watched by millions of people around the world, is the latest high-profile figure to say the world is facing a climate crisis.
The Archbishop of Canterbury, Dr Rowan Williams; the government's chief scientist Sir David King, and former Royal Society president Sir Robert May have all expressed public concern on the issue.
Sir David's programmes have been watch by millions worldwide
This week, former US Vice President Al Gore has been at the Cannes Film Festival to promote a documentary on climate change.
Mr Gore told festival goers that the world was facing a "planetary emergency" due to global warming.
The man who beat him to the White House in the 2000 US presidential elections, George W Bush, remains sceptical about the influence of human activity on the state of the planet's atmosphere.
He says binding targets to reduce greenhouse emissions are inefficient and would harm the US economy.
Last year, he launched a partnership alongside five Asia-Pacific nations to promote technological solutions for reducing the world's dependency on fossil fuels.
Sir David will present a two-part television programme that will explore how climate change is altering the planet, from drought-hit rainforest to the decline of polar bears.
Sir David Attenborough presents Are We Changing Planet Earth? on BBC One, Wednesday, 24 May 2006, at 2100 BST