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.
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.
changing climate has long been known to move Earth's axis. The planet's
north pole, for example, is migrating towards 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.
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
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.
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.
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.
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.
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."
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."
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.
Journal reference: Geophysical Research Letters (in press)