Scientists are facing an uphill battle to warn the public about pressing issues due to dissenters in their ranks who intentionally sow uncertainty, says a US historian.

These naysayers -- some of whom are paid by interest groups -- have helped undermine action on vital problems despite evidence of the need to respond, said Naomi Oreskes, a professor of history and science studies at the University of California at San Diego.

They sap convictions by endlessly questioning data, dismissing experimental innovation, stressing uncertainties and clamouring for more research, she said. Over the last half-century, they have helped weaken legislative action or brake political momentum on tobacco, acid rain, protection of the ozone layer and climate change.

"This strategy is so clever and effective," Oreskes said in an interview this week in Paris to promote a French translation of "Merchants of Doubt," a book she co-authored with California Institute of Technology historian Erik Conway.

"It takes something which is an essential part of science -- healthy skepticism, curiosity -- and turns it against itself and makes it corrosive."

Oreskes's book traces the starting point of professional science skeptics to when big tobacco companies were facing the first clear evidence that smoking caused cancer.

An internal memo, written by a Brown & Williamson Tobacco Corp. executive in 1969, spelt out the goal of weakening this link with expert help.

"Doubt is our product, since it is the best means of competing with the 'body of fact' that exists in the minds of the general public. It is also the means of establishing a controversy," according to the document, now placed in a US public archive.

Oreskes said a blatant example today was the sowing of doubt about global warming.

A "denial campaign" started to take root in the United States just before the Earth Summit of 1992 and amplified in the run-up to negotiations for the Kyoto Protocol in 1997, she said.

"They don't have to prove that they're right. They don't have to prove that there's no global warming," she said.

"They simply have to raise doubts and questions, because if they can raise doubts and questions, then they can say, 'Well, since the science is not settled,' they allege, 'therefore it would be premature to act on it.' And so they delay action and avoid the kind of actions they would like to avoid."

The tactic has been so successful that climate denialism is now firmly anchored in the higher reaches of US politics, said Oreskes.

"Major Republican (Party) leaders say in public that they believe it's a hoax. This is a very shocking state of affairs, and particularly from a party that once upon a time was considered to be more scientific and more environmental than the Democrats."

Oreskes was scathing about some US media which believed that story "balance" meant giving equal weight to opposing scientific views -- even if one opinion was backed only by a small minority in the face of massive evidence to the contrary.

According to Oreskes, scientists who push climate uncertainty are not necessarily hired guns, although "some of them get money, either directly through the fossil-fuel industry or indirectly through intermediaries."

"But I don't actually think money is the primary motivation. I think it's political, ideological, it's (the desire for) attention and sometimes it's narcissistic too."

For mainstream scientists, many of these full-time dissenters are time-wasters or intellectually valueless, she said.

"These people don't do work, they don't collect data. Instead, they just criticise other people's work. And then, when they make those criticisms, they don't take them to the scientific community for scrutiny. They publish it in The Wall Street Journal, which is not a scientific journal."