GLOBAL warming is slowing the Earth down. A study of changes in wind patterns
linked to global warming over the past 50 years suggests they are slowing the
planet鈥檚 daily spin by around half a millisecond every century. These effects
open up a new way of tracking the progress of global warming without the
uncertainties in simple temperature measurements.
Ocean currents associated with El Ni帽o are already known to make
equatorial winds blow faster and boost the angular momentum of the atmosphere.
Momentum must be conserved, so the atmosphere steals momentum from the Earth鈥檚
rotation, making it spin more slowly. Satellite observations showed that last
year鈥檚 El Ni帽o made days drag on by an extra 0.4 milliseconds
(This Week, 4 April 1998, p 21).
Climate experts have suspected that steady global warming might have a
similar effect. Computer models of the effects of warming point to the
appearance of jets of fast-moving wind between the troposphere and stratosphere,
roughly 12 kilometres above the Earth鈥檚 surface. These would also boost the
angular momentum of the atmosphere, triggering a compensating slowing down in
the Earth.
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Now Rodrigo Abarca del Rio of the French space agency鈥檚 Toulouse Space Centre
has looked for direct evidence of this. Over the past 50 years, the Earth鈥檚
average temperature has been increasing by around 0.79 掳C per century.
Using wind data for the same period from the US National Centers for
Atmospheric Research and Environmental Prediction, Abarca del Rio calculated the
angular momentum of the atmosphere. He found it had increased in step with the
temperature rise (Annales Geophysicae, vol 17, p 806). 鈥淭here has also
been a net loss in angular momentum by the solid Earth,鈥� he says. 鈥淭he data
suggest that global warming has caused a slowdown of the Earth at a rate of 0.56
milliseconds a century.鈥�
This implies that global warming is responsible for almost one-third of the
slowing down in the Earth鈥檚 spin scientists have measured. It also suggests that
the length of the day could provide a means of monitoring global warming in
future. Every 0.1 掳C increase should produce a slowdown of 0.07
milliseconds, which is easy to measure.
However, Abarca del Rio says too many other complex effects influence the
Earth鈥檚 spin, such as movement of molten rock beneath its surface. Instead, he
suggests using records on atmospheric angular momentum, which have been kept
since the mid-1970s. 鈥淩ecords of surface air temperature have been the main
measure of global warming,鈥� says Abarca del Rio. 鈥淢easurements of atmospheric
angular momentum may be an easier and more reliable way.鈥�
Masaki Satoh, a climate modeller at Saitama Institute of Technology in Japan,
agrees that the results could provide a new and better way to monitor global
warming. 鈥淎tmospheric angular momentum is increased by the El Ni帽o event,
so it is reasonable that it should also increase with global warming.鈥�
