Science: Did the earth move for you?
Environmental abuse has been blamed for rising sea levels but new evidence suggests it may be a natural process.
Friday 19 June 1998
Ocean levels, for example, are known to rise and fall by immense amounts over geological time scales. During the heyday of the dinosaurs - about 100 million years ago - sea levels were between 100 and 200 metres (300 to 600ft) higher than today. Fifty million years later, when the first mammals appeared on the scene, sea levels were at least 100m lower than the present day.
For many years, scientists believed these long-term fluctuations in the earth's sea levels were the result of plate tectonics - the gradual motion of the continents across the earth's crust. More specifically, the culprit was thought to be "sea-floor spreading", when molten rock wells up from the earth's mantle (the layer below the crust), forcing the tectonic plates apart. Sea-floor spreading is most pronounced in the middle of the Atlantic Ocean, where the rising Mid-Atlantic Ridge is pushing Europe and Africa away from the Americas.
A new study, however, suggests that a very different process may be at work. According to Jerry Mitrovica and Jon Mound at the University of Toronto, the spinning of the earth on its axis may be the root cause. That spinning, it seems, causes the earth's crust to "wobble" around the rotation axis much like a jelly spinning on a plate.
There are different ways to picture this motion, depending on your perspective. As seen from space, the earth's axis remains fixed, while the crust moves with respect to the axis. To an observer on the ground, however, it looks like the earth's axis is moving; this motion, therefore, is often called "polar wander." The result, of course, is the same either way.
The earth's crust deforms, which in turn cause dramatic changes in sea levels. Mitrovica and Mound used a computer program to model the wobbling earth, and looked at the effect on the earth's crust and on ocean levels - what they call an "earth response calculation". What they found was a remarkable correspondence between the sea levels predicted by their model and the known changes in sea levels that are known to have occurred from studying the geological record.
"When Jon and I saw that, we were really shocked," Mitrovica says. "It looked so much like the records of sea level changes." At the very least, he says, the old textbook explanation may have to be re-written. "It certainly weakens, very significantly, the argument that all of these sea level fluctuations are due to the spreading rate [of tectonic plates]. They could very well have nothing to do with the spreading rate, and have everything to do with the earth's rotation."
Of course, there are many other processes that can affect sea levels. Ice ages, for example, can cause the oceans to rise and fall, but typically these changes occur over tens of thousands of years - much shorter than the periods studied by Mitrovica and Mound.
While the Canadian team was investigating these million-year fluctuations, an American geophysicist was studying much shorter time scales. Toshiro Tanimoto of the University of California at Santa Barbara was looking at tiny oscillations that seem to cause the entire planet to expand and shrink over a period of about five minutes. These vibrations - known as "fundamental mode oscillations" - are incredibly small, with the surface of the earth moving a mere two to three millionths of a centimetre (about one millionth of an inch).
In fact, the phenomenon is so subtle that there are only a few days each year when it can be measured; the rest of the time, the oscillations are drowned out by earthquakes and other seismic activity. But sensitive instruments installed at seismographic centres around the world clearly show this five-minute period of vibration.
Tanimoto believes this oscillation is caused by turbulence in the earth's atmosphere. He modelled the effect of such winds hitting the planet on a global scale, and found it could trigger oscillations of just the right size. Of course, other mechanisms have been put forward - everything from small earthquakes to the effects of rain or even the impact of billions of human beings walking on the earth's surface. But none of these have the right frequency and the right intensity to set the entire planet in motion. "Atmosphere is a very strong candidate," Tanimoto says.
A key question is whether these interactions are affecting the earth's climate over the kinds of time scales that human beings have to worry about. These studies, which seem to deal with periods too long or too short to be of concern, are - at the very least - suggestive. "I'm seeing the interaction between atmosphere and solid earth," Tanimoto says of his work. "We're starting to think about these interactions... it's going to be a fascinating field."
Mitrovica, who works with time periods in the millions of years, believes it's crucial to understand long-term interactions - especially in an age of near-panic over possible global warming. While sea levels are thought to be on the rise - by between two and three millimetres (about one-tenth of an inch) per year - the cause is not yet clear. "People are going around saying that the sea-level rise that we're seeing must be related to melting events over Antarctica and Greenland," Mitrovica says.
"That's over a human time scale. But they have no clue [as to the] kinds of processes that may be naturally leading to those kinds of changes in sea levels. They don't know whether it's really human intervention, or whether we're just going to see it reverse in 15 years. The only way you can unravel that is to get a better handle on all of the processes that are affecting sea level."
As an example of long-term geological activity affecting climate, Mitrovica cites the creation of the Tibetan Plateau, formed millions of years ago as tectonic plates ploughed into each other in central Asia. Those mountains, in turn, disrupted atmospheric circulation, which must have had an enormous impact on climate. While similar events may be shaping today's climate, the processes remain poorly understood. "In terms of the long-term history of the planet, connections between sea level changes, tectonic events, and climate are completely open," Mitrovica says.
Dan Falk is a science journalist based in Toronto
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