Published online: 1 April 2007; | doi:10.1038/news070326-16 /
http://www.nature.com/news/2007/070326/full/070326-16.html
Warmer waters could spin the Earth faster
The oceans' heating will shave instants off the day.John Whitfield
| Stranger than fiction: on 1 April 2004, news@nature.com reported that climate change was making the day longer - as an April Fools' day joke. "I'm astonished we got anywhere close to the truth," says Nicola Jones, author of the original piece. "I just made that up." NASA |
|
The
warming of the world's oceans is going to shorten the day, say German
researchers. But there's no need to adjust your watch: the shortening
will be by only 0.12 milliseconds over the next 200 years, they
estimate.
As
water warms, it expands, causing sea level rise. Felix Landerer of the
Max Planck Institute for Meteorology in Hamburg, Germany, and his
colleagues used a computer model to find out what effect this expansion
will have on the distribution of water around the globe.
The
researchers looked only at heating, and not at the melting of the ice
caps. So in their model, the change was not in the total mass of water
in the ocean, but in the water's density and distribution. As the ocean
expands and water creeps up the shorelines, the net effect is to
transfer mass away from the central ocean and towards the shore.
Two
factors mean that this tends to move water's mass away from the equator
and towards the poles. First, the depths of the North Atlantic should
warm more quickly than depths elsewhere, thanks to a current there that
carries water down from the surface. So the expansion and movement of
water is strongest there. Second, a quirk of our planet's geography
means that the continental shelf's surface area happens to be larger at
high latitudes than around the equator.
Having
more mass at the poles, which is closer to Earth's axis of rotation,
will make the planet spin faster. "It's like a figure skater doing a
pirouette," Landerer explains. "When the arms are close to the body,
you turn quicker than when they are stretched out." He reports the
results of his model in
Geophysical Research Letters1.
"It
highlights how massive a change [in climate] is going on," says Richard
Gross, an expert on the Earth's rotation working at the Jet Propulsion
Laboratory in Pasadena, California. "The Earth is such a large body
that it takes a huge change in mass distribution to show up in
rotation."
What time is it?"If
you warm the ocean at different depths and places, water is going to
spill over," agrees Rui Ponte, a climate researcher at Atmospheric and
Environmental Research in Lexington, Massachusetts.
But,
he says, whether and how this will affect the length of the day is less
certain because we don't know much about how the oceans' depths respond
to warming. "The models have various approximations and problems,"
notes Ponte. And melting ice caps, he points out, might take mass away
from the poles and distribute it more evenly around the planet,
counteracting the effect that Landerer highlights.
Several
other factors also affect the length of an Earth day. The Moon's
gravitational pull, for example, lengthens the day by 2.3 milliseconds
a century. And interactions between Earth's liquid core and its solid
mantle layer are also thought to subtly change day length.
In
2002, a group of researchers argued that climate change could make the
day longer, by 0.1 millisecond a century, because of an increase in
winds blowing from west to east - the opposite direction to the
planet's rotation
2.
If all these predictions are accurate and complete (which
is almost certainly not true), then that should add up to a day being
2.34 milliseconds longer a hundred years from now. That's a measurable
difference, although it is important to keep in mind that day length
varies by about one millisecond from day to day, says Gross, mainly
because of changes in the winds.
Landerer's
team are now trying to work out the other effects of redistributing the
ocean. For example, he says, the Earth's axis could move slightly,
changing the position of the geographic poles.
References- Landerer F. W., Jungclaus J. H. & Marotzke J. J. Geophys. Res. Lett.,
34
. L06307 (2007).
- de Viron O., Dehant V., Goosse H. & Crucifix M. Geophys. Res. Lett.,
29
. (2007).
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