One of America's top scientists has said that the world has already entered a state of dangerous climate change.
In his first broadcast interview as president of the American Association for the Advancement of Science, John Holdren told the BBC that the climate was changing much faster than predicted.
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For more than a year, the BBC has invited the US government to give its view on safe levels of CO2. Our request is repeatedly passed between the White House office of the Council on Environmental Quality and the office of the US chief scientist.
To date, we have received no response to questions on this issue that Tony Blair calls the most important in the world. Professor Holdren called on the US Government to back the UK position.
