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Sea Levels to Plunge Long Term, Study of Dino Era Says

John Roach
for National Geographic News
March 6, 2008
 
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.

(Related news: "Global Warming Is Rapidly Raising Sea Levels, Studies Warn" [March 23, 2006].)

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.

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