By Alister Doyle, Environment Correspondent

OSLO, March 6 (Reuters) - Sea levels are set to fall over millions of years, making the current rise blamed on climate change a brief interruption of an ancient geological trend, scientists said on Thursday.

They said oceans were getting deeper and sea levels had fallen by about 170 metres (560 ft) since the Cretaceous period 80 million years ago when dinosaurs lived. Previously, the little-understood fall had been estimated at 40 to 250 metres.

"The ocean floor has got on average older and gone down and so the sea level has also fallen," said Bernhard Steinberger at the Geological Survey of Norway, one of five authors of a report in the journal Science.

"The trend will continue," he told Reuters.

A computer model based on improved understanding of shifts of continent-sized tectonic plates in the earth's crust projects more deepening of the ocean floor and a further sea level decline of 120 metres in 80 million years' time.

If sea levels were to fall that much now, Russia would be connected to Alaska by land over what is now the Bering Strait, Britain would be part of mainland Europe and Australia and Papua island would be the same landmass.

The study aids understanding of sea levels by showing that geology has played a big role alongside ice ages, which can suck vast amounts of water from the oceans onto land.

DOWN NOT UP

"If we humans still exist in 10, 20 or 50 million years, irrespective of how ice caps are waxing and waning, the long term ... is that sea level will drop, not rise," said lead author Dietmar Muller of the University of Sydney.

Over time, Muller told Science in a podcast interview there would be fewer mid-ocean ridges and a shift to more deep plains in the oceans as continents shifted. The Atlantic would widen and the Pacific shrink.

Still, the projected rate of fall works out at 0.015 centimetres a century -- irrelevant when the U.N. Climate Panel estimates that seas will rise by 18-59 cms by 2100 because of global warming stoked by human use of fossil fuels.

"Compared to what is expected due to climate change, the fall is negligible," said Steinberger. Cities from Miami to Shanghai are threatened by rising seas that could also swamp low-lying island nations in the Pacific.

Rising temperatures raise sea levels because water in the oceans expands as it warms, and many glaciers are melting into the seas.

Antarctica and Greenland now contain enough ice to raise sea levels by 50 metres if they all melted, the article said. If all ice on land were gone in 80 million years' time, the net drop in ocean levels would be 70 metres rather than the projected 120.

The study challenges past belief that sea levels might have been only 40 metres higher than today in the Cretaceous period by arguing that measurements from New Jersey in the United States had underestimated the fall.

It said that the New Jersey region had itself subsided by 105 to 180 metres in the period, skewing the readings.

-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Andrew Roche)

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