In an article to be published in the Jan. 23 issue of the journal Science, U of T physicist Jerry Mitrovica and PhD student Jon Mound use numerical simulations to show how long-term changes in the orientation of the Earth's rotation axis, or "wobbling" of the planet, can produce sea-level variations which exceed 100 metres.

"It's quite possible that all of the records of long-term sea-level change that people have been interpreting for decades will have to be entirely reinterpreted," Mitrovica says. "The amplitude of sea-level trends associated with the wobbling are large enough that they can contribute significantly to the observed sea-level variations."

One particularly dramatic example of a large-scale sea-level change is the rise and fall of the oceans of 100 to 300 metres which has occurred over the last 140 million years. Geophysicists have long believed this and other similar variations were linked to changes in the elevation of the ocean floor caused by changes in the rate of sea-floor spreading, the process by which oceans grow and continents drift apart.

Mitrovica says the effect of a wobbly planet on long-term sea-level variations had never been considered. "We can essentially match the amplitude and timing of the sea-level change over the last 140 million years just by using what we know about the history of the planet's rotation," he says. "It doesn't necessarily have anything to do with changes in the elevation of the ocean bottom due to changes in the rate of ocean spreading."

According to PhD student Jon Mound, the effect of the wobbling, also known as polar wander, means rises and declines in sea-level are not constant around the globe. "It had been assumed that you could apply sea-level curves globally whereas our numerical predictions show they're in fact regional," he says. "At the same time sea-levels rose to flood parts of North America, it may have fallen in South America."

"Jon's calculations tell us that the classic interpretation of those sea-level variations will have to be revised," Mitrovica says. "In the past, sea-level variations have been linked to sea-floor spreading rates, and spreading rates are necessary for developing models for the chemical evolution of the Earth's oceans and atmosphere and, ultimately, the evolution of the climate. One of the links in this chain of logic has been significantly weakened."

Mitrovica's research is funded by the Natural Sciences and Engineering Research Council (NSERC) and the Canadian Institute for Advanced Research (Earth Systems Evolution).