Climate change is causing the North Pole's location to drift, owing to subtle changes in Earth's rotation that result from the melting of glaciers and ice sheets. The finding suggests that monitoring the position of the pole could become a new tool for tracking global warming.

Computer simulations had suggested that the melting of ice sheets and the consequent rise in sea level could affect the distribution of mass on the Earth's surface. This would in turn cause the Earth's axis to shift, an effect that has been confirmed by measurements of the positions of the poles.

Now, Jianli Chen of the University of Texas at Austin and colleagues have shown that melting due to our greenhouse-gas emissions is making its own contribution to the shift.

The wobble in Earth's axis of rotation is a combination of two major components, each with its own cause. One is called the Chandler wobble and is thought to arise because the Earth is not rigid. Another is the annual wobble, related to Earth's orbit around the sun.

Additional wobble
Remove these wobbles, and you are left with an additional signal. Since observations began in 1899, the North Pole has been drifting southwards 10 centimetres per year along longitude 70° west – a line running through eastern Canada.

This drift is due to the changes in the distribution of Earth's mass as the crust slowly rebounds after the end of the last ice age. But Chen's team found something surprising. In 2005, this southward drift changed abruptly. The pole began moving eastwards and continues to do so, a shift that has amounted to about 1.2 metres since 2005.

To work out why the pole changed direction, Chen's team used data from NASA's GRACE satellite, which measures changes in Earth's gravity field over time. The data allowed them to calculate the redistribution of mass on Earth's surface due to the melting of the Greenland and Antarctic ice sheets and mountain glaciers, and the resulting rise in sea level. It correlated perfectly with the observed changes in the mean pole position (MPP).

"Ice melting and sea level change can explain 90 per cent of the [eastward shift]," says Chen. "The driving force for the sudden change is climate change."

Greenland thaw
Chen's team calculated that the biggest contribution is coming from the melting of the Greenland ice sheet, which is losing about 250 gigatonnes of ice each year. Another big factor is the melting of mountain glaciers, which contributes about 194 gigatonnes per year. The contribution from Antarctica adds up to 180 gigatonnes per year, but there is considerable uncertainty here because changes in the gravity field due to Earth's crust rebounding are less well understood over Antarctica than elsewhere.

Since the MPP can be accurately measured using multiple independent techniques, its position and drift can be used to gauge the extent of ice sheet melting, especially in between the end of the ageing GRACE mission and the launch of the next generation of gravity-field-measuring satellites, says Chen.

Jean Dickey of NASA's Jet Propulsion Laboratory in Pasadena, California, who was not associated with the study, agrees. "It's a way to monitor climate change by continuing to measure the deviation [of the MPP] from what we have seen in the past," she says.

Chen presented his findings this week at the annual meeting of the American Geophysical Union in San Francisco.

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Warming oceans could cause Earth's axis to tilt in the coming century, a new study suggests. The effect was previously thought to be negligible, but researchers now say the shift will be large enough that it should be taken into account when interpreting how the Earth wobbles.

The Earth spins on an axis that is tilted some 23.5° from the vertical. But this position is far from constant – the planet's axis is constantly shifting in response to changes in the distribution of mass around the Earth. "The Earth is like a spinning top, and if you put more mass on one side or other, the axis of rotation is going to shift slightly," says Felix Landerer of NASA's Jet Propulsion Laboratory in Pasadena, California.

The changing climate has long been known to move Earth's axis. The planet's north pole, for example, is migrating along 79 °W – a line of longitude that runs through Toronto and Panama City – at a rate of about 10 centimetres each year as the Earth rebounds from ice sheets that once weighed down large swaths of North America, Europe, and Asia.

The influx of fresh water from shrinking ice sheets also causes the planet to pitch over. Landerer and colleagues estimate that the melting of Greenland's ice is already causing Earth's axis to tilt at an annual rate of about 2.6 centimetres – and that rate may increase significantly in the coming years.

Now, they calculate that oceans warmed by the rise in greenhouse gases can also cause the Earth to tilt – a conclusion that runs counter to older models, which suggested that ocean expansion would not create a large shift in the distribution of the Earth's mass.

Tracking sea levels
The researchers modelled the changes that would occur if moderate projections made by the Intergovernmental Panel on Climate Change – a doubling of carbon dioxide levels between 2000 and 2100 – were to become reality.

The team found that as the oceans warm and expand, more water will be pushed up and onto the Earth's shallower ocean shelves. Over the next century, the subtle effect is expected to cause the northern pole of Earth's spin axis to shift by roughly 1.5 centimetres per year in the direction of Alaska and Hawaii.

The effect is relatively small. "The pole's not going to drift away in a crazy manner," Landerer notes, adding that it shouldn't induce any unfortunate feedback in Earth's climate.

But he says the motion is strong enough that it needs to be taken into account when interpreting shifts in Earth's axis. Tracking the motion of the poles could help place limits on the total amount of sea level rise over decades.

"The oceans take up at least 80 per cent of the heat that is added from greenhouse gases," Landerer told New Scientist. "They have a huge heat capacity, so this effect is going to be there for quite a bit."

Faster spin
Maik Thomas of the German Research Centre for Geosciences in Potsdam, who was not affiliated with the study, says the new work overturns previous ideas. "Up to now, people had believed that height variations [from ocean temperature changes] gave no contribution to polar motion," he told New Scientist. "This is an effect that now has to be considered."

But Thomas notes that polar motion is unlikely to yield a good measurement of sea level rise, whose signal may be difficult to disentangle from a host of other factors that contribute to changes in Earth's tilt, from movements in Earth's crust and mantle to the periodic effects of El Niño, an oscillation of the ocean-atmosphere system in the Pacific.

And climate change can also affect the Earth's spin. Previously, Landerer and colleagues showed that global warming would cause Earth's mass to be redistributed towards higher latitudes. Since that pulls mass closer to the planet's spin axis, it causes the planet to rotate faster – just as an ice skater spins faster when she pulls her arms towards her body.

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