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What relation can there possibly be between the tiny, ten year timescale, variations of Earth's rotational period, the flows of liquid iron in the outer core and climatic variations? Apparently none, and yet a recent study by Jean Dickey and Steven Marcus (NASA's Jet Propulsion Laboratory, Pasadena, California) and Olivier de Viron (Université Paris Diderot and Institut de Physique du Globe de Paris) published in the Journal of Climate, shows otherwise. They show not only that the three phenomena are correlated, but that using a novel technique it is also possible to isolate the anthropic component of global warming, something that many (but by no means all) researchers attribute to human activity.
The fact that the period of rotation of the Earth shows variations of about 1 millisecond during the course of a year is well known and understood. The cause is the seasonal variations of temperature which cause exchanges of energy between the lithosphere, hydrosphere and atmosphere and the resulting motions in the gaseous and liquid masses.
However, there are also variations in the rotation period, of about 4 milliseconds, on longer timescales, especially with a period of 65-80 years.
These variations, too large to be attributed to variations on the surface, are thought to be caused by flows of liquid iron in the outer core, where the planet's magnetic field is generated. By studying the variations in the magnetic field it is possible to deduce the motions in the outer core using models, and these motions have been correlated with the variations in the rotation period.
Independent research has also found a relation between the long period variations of the length of the day and the variations in the mean surface air temperature (0.2°C).
Comparing the historical data from the last 160 years for both the length of the day and mean surface temperature, with the derived motions of the fluid motions of the outer core, Dickey's team have made a very interesting discovery, that highlights the global warming during the last 60 years.
The above graph summarises the results.
The black line is the mean surface temperature, the red line is this same temperature but corrected for human influences, the green line is the length of the day and the blue line is the angular momentum of the outer core. It is clear that between 1900 and 1950 all the variables correlate very well, but that after this the uncorrected surface temperature (black line) ceases to follow the trend. The fact that the human influence-corrected temperature (red) continues to correlate, is a very clear illustration of the human influences on the mean temperature.
It is very curious that movements of mass in the outer core can influence the Earth's mean surface temperature, and why this might be is not clear. One possibility is that variations in the intensity of the terrestrial magnetic field cause the flux of cosmic rays that enter the atmosphere to vary. According to some studies, this can influence cloud formation, and therefore indirectly affect the surface air temperature.
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