Warming oceans could cause Earth鈥檚 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掳 from the vertical. But this position is far from constant 鈥� the planet鈥檚 axis is constantly shifting in response to changes in the distribution of mass around the Earth. 鈥淭he 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,鈥� says of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California.
The changing climate has long been known to move Earth鈥檚 axis. The planet鈥檚 north pole, for example, is migrating along 79 掳W 鈥� a line of longitude that runs through Toronto and Panama City 鈥� 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.
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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鈥檚 ice is already causing Earth鈥檚 axis to tilt at an annual rate of about 2.6 centimetres 鈥� 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 鈥� 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鈥檚 mass.
Tracking sea levels
The researchers modelled the changes that would occur if moderate projections made by the 鈥� a doubling of carbon dioxide levels between 2000 and 2100 鈥� were to become reality.
The team found that as the oceans warm and expand, more water will be pushed up and onto the Earth鈥檚 shallower ocean shelves. Over the next century, the subtle effect is expected to cause the northern pole of Earth鈥檚 spin axis to shift by roughly 1.5 centimetres per year in the direction of Alaska and Hawaii.
The effect is relatively small. 鈥淭he pole鈥檚 not going to drift away in a crazy manner,鈥� Landerer notes, adding that it shouldn鈥檛 induce any unfortunate in Earth鈥檚 climate.
But he says the motion is strong enough that it needs to be taken into account when interpreting shifts in Earth鈥檚 axis. Tracking the motion of the poles could help place limits on the total amount of sea level rise over decades.
鈥淭he oceans take up at least 80 per cent of the heat that is added from greenhouse gases,鈥� Landerer told New 杏吧原创. 鈥淭hey have a huge heat capacity, so this effect is going to be there for quite a bit.鈥�
Faster spin
of the German Research Centre for Geosciences in Potsdam, who was not affiliated with the study, says the new work overturns previous ideas. 鈥淯p to now, people had believed that height variations [from ocean temperature changes] gave no contribution to polar motion,鈥� he told New 杏吧原创. 鈥淭his is an effect that now has to be considered.鈥�
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鈥檚 tilt, from movements in Earth鈥檚 crust and mantle to the periodic effects of , an oscillation of the ocean-atmosphere system in the Pacific.
And climate change can also affect the Earth鈥檚 spin. Previously, Landerer and colleagues showed that global warming would cause Earth鈥檚 mass to be redistributed towards higher latitudes. Since that pulls mass closer to the planet鈥檚 spin axis, it causes the planet to rotate faster 鈥� just as an ice skater when she pulls her arms towards her body.
Journal reference: (in press)
