Alister Doyle, Reuters 6 Mar 08;
OSLO (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 meters (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 meters.
"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 meters 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 centimeters 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 meters 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 meters rather than the projected 120.
The study challenges past belief that sea levels might have been only 40 meters 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 meters in the period, skewing the readings.
(Editing by Andrew Roche)
Sea Levels to Plunge Long Term, Study of Dino Era Says
John Roach, National Geographic News 6 Mar 08;
About 80 million years ago—a time when dinosaurs ruled the Earth—global sea levels were roughly 560 feet (170 meters) higher than they are today, according to a new study.
If sea levels were that high now, vast regions would be flooded: most of northern Europe, large sections of South America, the East Coast of North America, and parts of Australia.
In Washington, D.C., the tip of the Washington Monument would poke just above the water. The base of the 555-foot-tall (169-meter-tall) obelisk is currently 30 feet (9 meters) above sea level.
The finding stems from more than a decade of effort to virtually reconstruct ancient ocean basins to understand how their size and depth have changed since the Cretaceous, which lasted from 145.5 to 65.5 million years ago.
The result is a dramatic image of historic sea level change that goes beyond what is expected in the coming decades due to rapid global warming-induced ice cap melting.
"There're natural processes that also contribute to sea level change and are in fact independent of ice cap melting," said Dietmar Müller, a geologist at the University of Sydney in Australia.
In fact, the data reveal that the long-term trend in sea levels since the Cretaceous has been downward, said Müller, who led the study appearing in tomorrow's issue of the journal Science.
When this trend is extrapolated out 80 million years from now, it suggests that even if all of today's ice caps were to melt, sea levels would be 230 feet (70 meters) lower than they are today.
Rising and Sinking
Pictured on today's globe, that much of a sea level drop would mean that Indonesia would be largely connected to mainland Southeast Asia.
Furthermore, all the continents would be larger, so today's coastal cities would be stranded inland.
But it's hard to know exactly what Earth's landmasses might look like in 80 million years, because in addition to sea level changes, plate tectonics should significantly shift the continents.
Some plates will bump and grind, others will drift apart, and still others will dive under landmasses and melt within Earth's hot interior.
According to the new study, a key factor in sea level change is the creation and spreading of new ocean crust along underwater mountain chains called mid-ocean ridges, Müller said.
"As the ocean floor moves away from the hot and shallow mid-ocean ridges into parts of the abyssal plains, it cools and sinks," he explained.
Currently the mid-ocean ridges lie, on average, 1.6 miles (2.5 kilometers) beneath sea level, while the abyssal plains sit 3.7 miles (6 kilometers) deep.
"That's a huge difference, and if you change the relative proportion of mid-ocean ridges and abyssal plains in the ocean basins, you change [the ocean's] volume, and this is what we have tried to reconstruct," he said.
The team found that, during the late Cretaceous, huge mid-ocean ridges wrapped around the planet, making the global ocean much shallower on average than it is today.
In particular, a mid-ocean ridge system in an ancient ocean called Panthalassa—the precursor to the Pacific—was a crucial force driving sea level change through time, Müller said.
Much of that system no longer exists, which largely explains why sea levels have fallen over the last 80 million years.
Going forward, the researchers' model suggests the Atlantic Ocean will continue to grow and the Pacific Ocean will shrink as more mid-ocean ridges disappear.
"As time goes by, we will therefore increase the volume of the ocean basins because it will become deeper than average, so this will result in a long-term sea level drop," Müller said.
Commendable Job
Kenneth Miller is a geologist at Rutgers University in New Jersey.
He has also worked on late Cretaceous sea level estimates, and he commended Müller and colleagues' work.
"In ten years this is still going to be the most widely cited paper on this topic," he said, adding that the latest estimate for Cretaceous-era sea level is more reasonable than any other in the published literature, including his own.
According to Miller, studies like this are important to gain an understanding and appreciation of how plate tectonics drive sea level changes over million-year time scales.
But Miller cautioned against comparing the long-term, plate tectonic-driven changes in sea level to the short-term dangers from melting ice caps.
"The main effect for people who care about what's happening into the future is to understand how fast the ice caps are melting," he said.
And many studies have shown that the ice caps are melting rapidly, pushing sea levels higher.
