One of the world's leading climate change scientists yesterday called for a major overhaul of US climate change policy, warning that there is just one or two years left for action to be taken before irreversible climate "tipping points" are reached.

Speaking exactly 20 years after he first told Congress of the threat posed by global warming, James Hansen told a congressional briefing that the Earth's climate had now reached "an emergency situation" and that urgent action is required.

Hansen, the director of NASA's Goddard Institute of Space Studies and one of the world's most widely respected climate scientists, said that a carbon tax would be the most efficient means of curbing carbon emissions.

"We have to level with the public that there has to be a price on carbon emissions," Hansen said. "That is the only way we are going to begin to move toward a carbon free economy."

He added that a such a tax could be applied on coal, gas and oil at the first point of sale or port of entry, and that the entire tax could then be returned to the public in the form of a monthly dividend that they would then be encouraged to spend on more energy and carbon efficient technologies.

Presidential candidates John McCain and Barack Obama have both outlined plans for a cap-and-trade scheme as a means of placing a price on carbon emissions, but Hansen argued that a tax would represent a simpler and more effective means of imposing the price signals required to drive demand for lower carbon products.

He also called for a moratorium on coal-fired power plants that do not include carbon capture systems, arguing that the size of coal reserves meant that it posed a greater threat than emissions arising from oil.

Moreover, he urged the next president to make the development of low loss electric grid based on emerging direct-current high-voltage cables an i mperative and called for an overhaul of energy regulations to ensure utilities are rewarded for promoting energy efficiency.

Hansen warned that without the urgent adoption of such measures the world's climate would be changed irreversibly, adding that the atmosphere has already passed the "dangerous level" for greenhouse gases in the atmosphere. He warned that tipping points, whereby the increase in temperature leads to an acceleration in future temperature rises, had already been reached, citing the example of the warming effect on sea temperatures resulting from the melting of the Arctic ice cap.

He added that recent research had shown that the "safe level" for carbon dioxide in the atmosphere stands not at 400 or 450 parts per million (ppm) of CO2, but rather at 350ppm, about 10 per cent lower than current levels.

Particular ire was reserved for those energy firms that have previously expressed doubt over climate change science. Writing ahead of his address, Hansen said that the chief executives of such firms should face criminal charges.

"Instead of moving heavily into renewable energies, fossil companies choose to spread doubt about global warming, as tobacco companies discredited the smoking-cancer link," he said. "Methods are sophisticated, including funding to help shape school textbook discussions of global warming.

"CEOs of fossil energy companies know what they are doing and are aware of long-term consequences of continued business as usual. In my opinion, these CEOs should be tried for high crimes against humanity and nature."