A 190-nation U.N. conference in Copenhagen in December is due to agree a new U.N. pact to curb greenhouse gas emissions to slow a rise in temperatures to prevent floods, droughts, wildfires or rising sea levels.

Scientists have known for a century that greenhouse gases, for instance from burning coal, can warm the planet. But most experts say the evidence was thin until about the past decade.

"I don't think it (the world reaction) was too slow -- I think we have a very solid foundation for action now," said Yvo de Boer, head of the U.N. Climate Change Secretariat. "The sceptics are no longer derailing the process."

He noted it was only in 1995 that the U.N's Intergovernmental Panel on Climate Change blamed mankind for global warming, saying cautiously that the "balance of evidence suggests a discernible human influence on global climate".

"In recent years we've managed to move light years beyond that" level of certainty, he told Reuters during talks on the new U.N. treaty in Barcelona. The panel says it is at least 90 percent certain that global warming is man-made.

Still, warnings were around long ago. President Johnson said in a message to the U.S. Congress in 1965 that "This generation has altered the composition of the atmosphere on a global scale through...a steady increase in carbon dioxide from the burning of fossil fuels."

Thatcher told the United Nations in 1988: "The problem of global climate change is one that affects us all and action will only be effective if it is taken at the international level."

Naomi Oreskes, Professor of History and Science Studies at the University of California, San Diego, said that it was inevitably hard to accept that humans were to blame.

"It's not surprising that people have been resistant to the scientific conclusion, given that it means we have to change the energy source that is the basis of essentially all our economic activity," she told Reuters.

Mike Hulme, founding director of the Tyndall Centre in England, wrote in a 2009 book that the history of climate change "is not a simple one of science progressing purposefully in a straight line from blissful ignorance to a state of confident knowledge."

And he said that arguing over the human element in climate change was in some ways a distraction from a longer-term need for a stronger fight against poverty. Rich nations have failed to keep repeated promises of more aid to help the poor.

More aid would have built trust and helped "strengthen the capacity of developing nations to minimise some of the risks posed by natural climate (not climate change): drought, flood, hurricane," he said.

"To this extent, arguments about whether or not human influence on global climate has been detected have been a distraction from what could and should be done," he said.