By Roger Harrabin
BBC environment analyst
One of America's top scientists has said that the world has already entered a state of dangerous climate change.
Sea levels could rise by 4m over the coming century, he warns
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.
"We are not talking anymore about what climate models say might happen in the future.
"We are experiencing dangerous human disruption of the global climate and we're going to experience more," Professor Holdren said.
He emphasised the seriousness of the melting Greenland ice cap, saying that without drastic action the world would experience more heatwaves, wild fires and floods.
He added that if the current pace of change continued, a catastrophic sea level rise of 4m (13ft) this century was within the realm of possibility; much higher than previous forecasts.
To put this in perspective, Professor Holdren pointed out that the melting of the Greenland ice cap, alone, could increase world-wide sea levels by 7m (23ft), swamping many cities.
He blamed President Bush not only for refusing to cut emissions, but also for failing to live up to his rhetoric on harnessing technology to tackle climate change.
"We are not starting to address climate change with the technology we have in hand, and we are not accelerating our investment in energy technology research and development," Professor Holdren observed.
He said research undertaken by Harvard University revealed that US government spending on energy research had not increased since 2001. In order to make any progress, funding for climate technology needed to multiply by three or four times, Professor Holdren warned.
Last year, the UK's Prime Minister, Tony Blair, held a science conference to determine the threshold of dangerous climate change. Delegates concluded that to be relatively certain of keeping the rise below 2C (3.6F), CO2 levels in the atmosphere should not exceed 400 parts per million (ppm) and the highest prudent limit should be 450 ppm.
In October, at an international conference in Mexico, UK environment and energy ministers will try to persuade colleagues from the top 20 most polluting nations to agree on a CO2 stabilisation level.
Professor Holdren expressed doubt that progress could be achieved because if the US administration agreed that there was a need to limit CO2, this would inevitably lead to mandatory caps. President Bush has already rejected that option.
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.
John Holdren, in addition to his presidency of the AAAS, is director of the Woods Hole Research Center, and the Teresa and John Heinz Professor of Environmental Policy at Harvard University.