Dr James Hansen. Photograph: AP Photos/The Daily Iowan/Melanie Patterson

Barack Obama has only four years to save the world. That is the stark assessment of Nasa scientist and leading climate expert Jim Hansen who last week warned only urgent action by the new president could halt the devastating climate change that now threatens Earth. Crucially, that action will have to be taken within Obama's first administration, he added.

Soaring carbon emissions are already causing ice-cap melting and threaten to trigger global flooding, widespread species loss and major disruptions of weather patterns in the near future. "We cannot afford to put off change any longer," said Hansen. "We have to get on a new path within this new administration. We have only four years left for Obama to set an example to the rest of the world. America must take the lead."

Hansen said current carbon levels in the atmosphere were already too high to prevent runaway greenhouse warming. Yet the levels are still rising despite all the efforts of politicians and scientists.

Only the US now had the political muscle to lead the world and halt the rise, Hansen said. Having refused to recognise that global warming posed any risk at all over the past eight years, the US now had to take a lead as the world's greatest carbon emitter and the planet's largest economy. Cap-and-trade schemes, in which emission permits are bought and sold, have failed, he said, and must now be replaced by a carbon tax that will imposed on all producers of fossil fuels. At the same time, there must be a moratorium on new power plants that burn coal - the world's worst carbon emitter.

Hansen - head of the Goddard Institute of Space Studies and winner of the WWF's top conservation award - first warned Earth was in danger from climate change in 1988 and has been the victim of several unsuccessful attempts by the White House administration of George Bush to silence his views.

Hansen's institute monitors temperature fluctuations at thousands of sites round the world, data that has led him to conclude that most estimates of sea level rises triggered by rising atmospheric temperatures are too low and too conservative. For example, the Intergovernmental Panel on Climate Change says a rise of between 20cm and 60cm can be expected by the end of the century.

However, Hansen said feedbacks in the climate system are already accelerating ice melt and are threatening to lead to the collapse of ice sheets. Sea-level rises will therefore be far greater - a claim backed last week by a group of British, Danish and Finnish scientists who said studies of past variations in climate indicate that a far more likely figure for sea-level rise will be about 1.4 metres, enough to cause devastating flooding of many of the world's major cities and of low-lying areas of Holland, Bangladesh and other nations.

As a result of his fears about sea-level rise, Hansen said he had pressed both Britain's Royal Society and the US National Academy of Sciences to carry out an urgent investigation of the state of the planet's ice-caps. However, nothing had come of his proposals. The first task of Obama's new climate office should therefore be to order such a probe "as a matter of urgency", Hansen added.