Warming oceans could cause Earth's axis to tilt,
according to a recently published study. The effect had
been thought 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 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.
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.