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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 say. They say oceans are getting deeper and sea levels have fallen by about 170 metres since the Cretaceous period 80 million years ago when dinosaurs lived. Previously, the little-understood fall had been estimated at 40-250 metres.

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

The scientists used a computer model to better understand shifts of continent-sized tectonic plates in the earth's crust. These models project 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, the UK 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.

"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," says Australian lead author Associate Professor Dietmar Müller of the University of Sydney.

Over time, Muller says there will be fewer mid-ocean ridges and a shift to more deep plains in the oceans as continents shift.

The Atlantic will widen and the Pacific shrink.

What about climate change?

Still, the projected rate of fall works out at 0.015 centimetres a century - irrelevant when the UN climate panel estimates that seas will rise by 18-59 centimetres 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," says 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 says.

Copyright: ABC