OSLO 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 on Wednesday.
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 United States, 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 told Reuters by telephone, 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 over seas around Antarctica would also lower sea levels by perhaps 300 meters (980 ft) -- connecting Russia to Alaska by land.
In the last Ice Age, sea levels fell about 130 meters 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.
"Presumably, future society could prevent this transition indefinitely with very modest adjustments to the atmospheric CO2 level," they wrote. Greenhouse gases are widely blamed as the main cause of current warming that may bring more heatwaves, droughts, food shortages and rising seas.
A shift to a bigger blanket of ice would mark the end of a period of warming that began 50 million years ago, when even Antarctica was almost ice-free.
The scientists said the recent swings between Ice Ages and warmer periods such as the present, over the past 900,000 years, were getting sharper. Models suggested that instability could herald a shift to a new, far colder and stable state.
A similar shift happened more than 34 million years ago when Antarctica was first covered by ice, the scientists said. A trigger could be a slight growth of polar ice sheets, with ice and snow reflecting more of the sun's heat back into space. That could accelerate a cooling.
"Historians of science hate to say 'this is a special time'," Crowley said.
"But when you go through the models, each step seems reasonable and you get to an astonishing conclusion that we are right at the end of a 50-million-year evolution."
Modern human societies might never have developed if such a freeze had happened slightly earlier. "Anatomically modern humans evolved only 150,000 years ago," he said.
Crowley said more tests of the projections were needed.
"It might not come for tens of thousands of years," he said. "I'm sure some headline writers will want to say 'CO2 good for the atmosphere', or 'CO2 is good for us'. That's not the case."
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(Editing by Tim Pearce)