The Earth's spin isn't as consistent as it may seem. In fact, it naturally drifts and wobbles on its axis over time, and that's generally chalked up to the way mass is distributed and redistributed across the planet's surface. Now, NASA scientists studying data gathered across the entire 20th century have identified three broad processes that play a part.
According to previous research, between 1900 and 2000 the Earth's spin axis drifted about 4 in (10 cm) every year, adding up to a total shift of more than 33 ft (10 m) by the end of the century. Traditionally, the main culprit for this kind of wobble is thought to be glacial rebound. During ice ages, glaciers form and their weight presses down on the Earth's crust. Later, as those glaciers melt – as is happening today at pretty alarming speeds – the rock below begins to rise up again, redistributing the Earth's mass and affecting its rotation.
But when the NASA researchers conducted a statistical analysis of that 20th century data, they found that glacial rebound only accounts for around a third of the observed axial wobble.
"The traditional explanation is that one process, glacial rebound, is responsible for this motion of Earth's spin axis," says Surendra Adhikari, first author of the study. "But recently, many researchers have speculated that other processes could have potentially large effects on it as well. We assembled models for a suite of processes that are thought to be important for driving the motion of the spin axis. We identified not one but three sets of processes that are crucial – and melting of the global cryosphere (especially Greenland) over the course of the 20th century is one of them."
The drastic climate shift we've seen over the last century has reduced glacial ice pretty much everywhere, but Greenland has been particularly hard-hit, losing about 7,500 gigatons of ice. That affects the Earth's spin not only by redistributing mass into the oceans, but because of its location on the globe.
"There is a geometrical effect that if you have a mass that is 45 degrees from the North Pole – which Greenland is – or from the South Pole (like Patagonian glaciers), it will have a bigger impact on shifting Earth's spin axis than a mass that is right near the Pole," says Eric Ivins, co-author of the study.
According to the study, the third major contributor to the wobble is mantle convection, referring to the movements of the molten rock deep inside the Earth. Not only is this kind of motion responsible for the movements of the tectonic plates, but the rise and fall of material in the mantle can redistribute mass and, as a result, affect the planet's spin.
Having identified these three key factors, the researchers say future work can do a better job of separating effects caused by longer-term Earth processes from effects caused by climate change. Of course, they are still linked too, and the team says that continued warming could accelerate the loss of ice in Greenland, which in turn could accelerate the rate of the planet's spin axis shift.
The research was published in the journal Earth and Planetary Science Letters.
Source: NASA/JPL
