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Rodriguo is a colleague of mine (same floor at work, almost next office) and I immediately went to see him after I read this. I did not know he had done such research before in CNES (French NASA). I asked Rodrigo to give me a copy of a new paper he wrote, which is not yet pubished. So I cannot quote it directly. What he concludes is that over the studied time scale (~50 years only, which is still to few, but accurate measurements are only recent he says) the atmospheric angular momentum fluctuations (due to Global Warning he says) can explain 15% of the length of the day (LOD) variability, on a quasi biennal and triennal-quadrennial time scale. Which I find few. And that there is a six year period in LOD variability which cannot be explained this way and which is "probably" due to mantle-core exchanges of angular momentum.

Offered by Veronique.

Days Just Drag
New Scientist, July 3, 1999

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's 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 Niño 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's rotation, making it spin more slowly. Satellite observations showed that last year's El Niño 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's surface. These would also boost the angular momentum of the atmosphere, triggering a compensating slowing down in the Earth.

Now Rodrigo Abarca del Rio of the French space agency's Toulouse Space Centre has looked for direct evidence of this. Over the past 50 years, the Earth's average temperature has been increasing by around 0.79 °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). "There has also been a net loss in angular momentum by the solid Earth," he says. "The data suggest that global warming has caused a slowdown of the Earth at a rate of 0.56 milliseconds a century."

This implies that global warming is responsible for almost one-third of the slowing down in the Earth's 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 °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's 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. "Records of surface air temperature have been the main measure of global warming," says Abarca del Rio. "Measurements of atmospheric angular momentum may be an easier and more reliable way."

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. "Atmospheric angular momentum is increased by the El Niño event, so it is reasonable that it should also increase with global warming."