The planet could face a freeze worse than an Ice Age starting in as little as 10,000 years, giving future societies a headache the opposite of coping with global warming, scientists said.

The researchers, based in Britain and Canada, said that now-vilified greenhouse gases might help in future to avert a chill that could smother much of Canada and the US, Europe and Russia in permanent ice.

They said the study, based on records of tiny marine fossils and the earth's shifting orbit, did not mean the world should stop fighting warming, stoked by human emissions of heat-trapping gases from burning fossil fuels.

"We're saying: 'don't push the panic button'," said Thomas Crowley, an American scientist at Edinburgh University who shared authorship of the study in the journal Nature with a colleague at Toronto University.

"There's no excuse for saying 'we've got to keep pumping carbon dioxide (CO2) into the atmosphere'," he said, adding that the cooling was projected to start in 10,000 to 100,000 years.

"Geologically it's tomorrow," he said. "But we have a lot of time to argue about the appropriate level of greenhouse gases."

The projected build-up of vast ice sheets across the Northern Hemisphere and overseas around Antarctica would also lower sea levels by perhaps 300 metres - connecting Russia to Alaska by land.

In the last Ice Age, sea levels fell about 130 metres and much of Russia escaped a big ice sheet. Scientists can build sea level records from fossils because ocean chemistry varies; salt, for instance, is more concentrated when there is less sea water.