The slow rise and fall of sea levels over tens of millions of years may be due in part to meanderings of Earth's spin axis, according to a provocative study  in today's issue of Science. Geophysicists propose that "true polar wander"--literally, the drifting of Earth's axis of rotation--may have helped bulge some oceans by 100 meters or more during the time of the dinosaurs, then shrank them back to today's levels. If borne out by observations, the hypothesis would force scientists to revise their explanations of long-term sea-level change.
Geologic records show that Earth's axis is not rooted to the spots of the current North and South Poles. Rather, the most stable axis of rotation wanders as drifting continents and other tectonic processes redistribute the planet's mass. About 130 million years ago, for instance, the North Pole sat where northern Greenland is today. It then sauntered away from North America toward Asia at a rate of about 450 kilometers per 10 million years, finally reversing direction about 30 million years ago. These changes, some researchers reasoned, might influence sea levels in certain parts of the globe, because Earth's spin causes water to pile up near the equator and drains it from the poles.
Geophysicist Jerry Mitrovica and graduate student Jon Mound of the University of Toronto created a computer model to explore this spin-driven sloshing. Sea levels, they found, responded differently to polar wander in four quadrants of the globe: When the spin axis drifted away from a quadrant (moving that part of Earth closer to the new equator), oceans there rose markedly; when it approached a quadrant, they fell.
Mound says that true polar wander could adjust regional sea levels, while other mechanisms--such as fluctuations in ocean volume triggered by variations in sea-floor spreading rates--would be global. Actual records of sea levels in North America and Europe broadly reflect this pattern, the researchers claim, but it's less clear in other regions.
Other experts praised the work as creative and grounded in solid physical principles. However, most will remain wary until geologists find more widespread records of sea-level changes. Such skepticism is appropriate, says Mitrovica, because the degree to which the planet itself bulges would influence how the seas respond. "True polar wander could plausibly contribute to sea-level changes," he says, "but the effect ranges from 10 meters to 150 meters depending on our assumptions of what Earth is made of."