Headline from National Geographic: Sea Levels to Plunge Long Term, Study of Dino Era Says.
It paints a scary view of the Cretaceous: forget T. Rex. Did you know that sea levels were 560 feet higher than they are now? That would make the tip of the Washington Monument barely above sea level! (And, for that matter, the high points of Florida, Delaware, and Louisiana would be only accessible by scuba diving!) That's right... and it's all natural - it's due to the sea floor pulling apart!
Umm. Actually, I did know that. In fact, where I live, at 6500 feet above sea level, I can go see the shoreline of the Cretaceous Interior Seaway that flooded North America. The high sea levels of the Cretaceous are hardly big news to geologists, and the role of mid-ocean ridges in controlling global sea levels was accepted by the 1980's, at least.
The story is about an article coming out in Science tomorrow. Science is supposed to published ground-breaking, cutting-edge research. They wouldn't publish something that's been accepted for decades, would they?
Of course they wouldn't.
The paper discussed by National Geographic (Muller, R.D., Sdrolias, M., Gaina, C., Steinberger, B., Heine, C. (2008). Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics. Science, 319(5868), 1357-1362. DOI: 10.1126/science.1151540) deals with a far more subtle question. We know sea level was high during the Cretaceous, but how high was it, exactly? Different methods of estimation give different answers. Estimations of mid-ocean ridge volume gave numbers in the neighborhood of 250 meters (820 feet - above the high point of Rhode Island). Sedimentary rocks in New Jersey, on the other hand, imply that the highest Cretaceous sea level was only 40 meters (131 feet) - less than the sea level rise that would occur if we melted all the ice on the planet. Why the discrepancy?
Muller and colleagues started by assuming the sea level rise was mostly the result of tectonics. The young rocks at mid-ocean ridges ride higher than the old, cold rocks of the abyssal plains, so the first thing they had to do was figure out just how old the ocean floor was in the Cretaceous. That's not that easy, because several oceanic plates in the present-day Pacific have gone down subduction zones and disappeared. Then the authors had to account for hot spots (like Hawaii), and for sediments, and for changes in the area of oceans versus continents (such as that caused by the widening of Nevada during formation of the Basin and Range). And finally, yes, they calculated how much sea level would rise if our ice caps all melted.
They conclude that Cretaceous sea levels were 170 meters (~560 feet) above present-day sea level.
So why do the sedimentary rocks from New Jersey give such a different answer?
They blame the mantle. Actually, they blame the Farallon plate - the oceanic plate that subducted beneath North America during the Mesozoic (and whose remnants are still sliding under the Pacific Northwest, Central America, and South America). The old Farallon slab is still down there, sinking... and apparently it is dragging New Jersey (and presumably the rest of the East Coast of the US) down with it.
I'm not sure how to evaluate the explanation for New Jersey's low elevation. If it's correct, it implies that the mantle drives major changes in relative sea level. Now, heat in the mantle is blamed for the high elevations in rift zones (such as the East African Rift), but I hadn't heard of ancient subducting plates dragging their overriding continents down. (Is there a similar effect in eastern South America? Is northern Asia affected by subduction of the Indian plate?)
Whether their explanation for New Jersey is correct or not, however, the National Geographic article completely misses the point. Yes, I know that "Discrepancy Resolved Between Cretaceous Sea Level Estimates" doesn't have the same ring. But the Science article does not say anything about future drops in sea level. It certainly doesn't try to extrapolate the trend 80 million years into the future, as the National Geographic article does:
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.
This sounds like the advice of a bad stock analyst in the 90's: if it's going up now; it will always go up. And if it's going down now, it... will continuing decreasing forever.
My advice: if you are hoping for subduction of mid-ocean ridges to save your city from flooding due to global warming, don't hold your breath. Even if tectonics causes global sea levels to fall, the ocean floor isn't going to change very fast.
Edit: Reuters has an equally misleading headline. It sounds from the news release like the authors of the paper are claiming that the paper predicts the future. Hmmm. I guess it's possible to predict the future subduction of the East Pacific Rise and the Gorda/Juan de Fuca/Explorer Ridges. But is it possible to predict future hot spots/large igneous provinces, especially given that there's a debate about their origin? And do we know exactly why the Pacific plate appeared at the triple junction between three other oceanic plates? (Is it the result of some kind of triple-junction instability?)
Meanwhile, Ole has a good discussion of long-term versus short-term sea level changes. I can't comment on his site (it requires registration), so I'll say here: read it. It's a good discussion.