Australia seesaws back and forth with the seasons


Imperceptible continental tilts are driven by tiny shifts in Earth's centre of mass – and it means your GPS is out a little too. Kate Ravilious reports.


When the northern hemisphere packs on ice, the planet's centre of mass shifts a tad – and the continents on top rock along too (but only very slightly).
NASA Goddard's Scientific Visualization Studio / C. Starr

Right now, Sydneysiders are riding upwards while Darwin folk are descending. But come winter, Sydneysiders will drop and the north-west of the country will rise again.

This continent-wide seesawing was reported by Shin-Chan Han from the University of Newcastle in New South Wales. He took GPS measurements across Australia and combined them with changes in the Earth's gravitational pull and found the entire continent is imperceptibly tilting back and forth in response to seasonal shifts in the Earth's centre of mass.

The work was reported in the Journal of Geophysical Research: Solid Earth.

Our planet's centre of mass lies roughly at the middle of Earth's core, some 6,000 kilometres beneath your feet, but seasonal changes on the surface alter the exact position.

During northern hemisphere winter, snow and ice build-up on northern continents is enough to shift the centre of mass a few millimetres towards Europe.

Conversely, during the northern hemisphere summer when much of the snow and ice has evaporated into the atmosphere, the centre of mass shifts the opposite way – towards the South Pacific Ocean.

Han was curious as to whether these seasonal changes in Earth's centre of mass would be strong enough to exert a pull on Earth's smallest continent: Australia.

Some 14 land-based GPS stations across Australia provided the minuscule movements the continent makes.

Data from the Gravity Recovery and Climate Experiment (GRACE) – which uses two satellites to measure the change in the strength of the Earth's gravitational pull – determined what fraction of Australia's fidgeting was actually caused by movement of Earth's centre of mass.

Sure enough, he found that Australia does respond to this seasonal change, travelling around a millimetre north-west during the southern hemisphere summer – and back again in winter.

The entire country also tilts, so when the north-west drops by two or three millimetres, the south-east lifts the same amount. During southern hemisphere winter, the process reverses.

All continents feel the seasonal movement of the Earth's centre of mass, but Han believes Australia feels it more because it sits partway between Europe and the South Pacific Ocean.

“The continent happens to be located between the peaks and troughs of the global mean surface mass,” he writes. In other – very simplistic – words, if you can imagine a seesaw plank stretching between Europe and the South Pacific, it would pivot in Australia.

And his work also shows that if the rocking motion is not taken into account, GPS measurements across Australia are likely to be a millimetre or two out. “While this may not amount to much of a difference on a day-to-day basis, it could be a concern for precision measurements, like those taken to determine sea level.”

Meanwhile, Richard Gross from NASA’s Jet Propulsion Laboratory in Pasadena, California, who was not involved with the study, believes Han's work is likely to prompt other scientists to try to measure the movement of other continents. “This new way of determining the Earth's [centre of mass] … is a new and novel approach and will be taken up by others,” he told the American Geophysical Union.

  1. http://onlinelibrary.wiley.com/doi/10.1002/2016JB013388/abstract
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