Evidence for global warming has mounted but public awareness of the threat has shrunk, due to a cold northern winter and finger-pointing at the UN's climate experts, a top scientist warned Wednesday.

James Hansen, a leading NASA scientist whose testimony to the US Congress in 1988 was a landmark in the history of climate change, said he was worried by "the large gap" in knowledge between specialists and the public, including politicians.

"That gap has increased substantially in the last year," Hansen told a press conference during a visit to Paris.

"While the science was becoming clearer, the public's perception became less clear, in part because of the unusually cold winter in both North America and Europe, and in part because of the inappropriate over-emphasis on small minor errors in IPCC documents and because of the so-called Climategate."

The IPCC -- the UN's Intergovernmental Panel on Climate Change -- is under fire for several errors that appeared in a key 2007 report.

Its authors have acknowledged the mistakes, but say the overall conclusions of the report, that man-made greenhouse gases are changing the climate, remain solid.

The "Climategate" affair relates to stolen emails exchanged among British scientists that, sceptics said, showed they had ignored evidence that natural, rather than man-made, causes were to blame for climate change.

The scientists have been cleared by a British parliamentary panel.

"The winter was not cold if you look over the whole world: December, January, February was the second warmest in 130 years," Hansen noted.

"It was cool at mid-latitudes in the northern hemisphere but unusually warm in the Arctic and that has a simple explanation: there is a chaotic variation in the pressure in the Arctic region. But it's just chaotic variation, there is no reason that it will be repeated."

He added: "We have to look at the frequency of events. Seven out of the last 10 winters in Europe have been warmer than the long-term average, and eight out of 10 in the United States.

"So the climate dice are being loaded at a rate which is in very close agreement with what was predicted ago a few decades ago based on the expected global warming."

Hansen is director of NASA's Goddard Institute for Space Studies, but stressed he was speaking in a private capacity.

He blasted governments for "ignoring... basic scientific facts" by continuing to depend on fossil fuels, build more coal-fired plants and drill for oil in the deep ocean and the Arctic.

And he said that the poor outcome of December's climate summit in Copenhagen was predictable.

"Frankly, there was a realisation that you can't have 180 countries making the initial agreement, it has to be the major players. So, Europe, the US and China, and probably India, need to agree that there needs to be a carbon price and then it's very easy to make that global."