Re: How could the increase in population affect earth's speed?

The long term change in Earth's rotation is caused mainly by 
the "breaking" of the Earth due to tidal friction. As our planet deforms 
continuosly by the effect of the attraction of the Sun and the Moon (and 
all the other bodies, in fact), the water in the ocean (and all other 
water masses) flows in the bottom, and the dissipated energy changes the 
rotation rate of the planet, by the conservation of angular momentum.

By this process, we have an increase in the length of the day by 2.3 
milliseconds per century nowadays. The fossil register indicates that some 
370 million years ago the day had 22 hours.

Only with the extended capabilities of measuring the daily variations in 
the rotation rate in recent years we could account for minor (or rapid) 
effects, such as the diurnal and semidiurnal variations. There are two 
main causes for these variations, the movement of water around the globe 
by the tides, that changes the moment of inertia of the earth, and the 
tidal velocity variations, that exchange angular momentum with the solid 
earth.

So, the increase in population and the damming of rivers can be classified 
in the diurnal and semidiurnal variations, although it should be stressed 
that the involved masses in this case are very small and the effect should 
be small if compared with the other effects.

Information about tides can be found in the web pages of Special Bureau 
for Tides (http://bowie.gsfc.nasa.gov/ggfc/tides/tides.html).

Some good references:

Munk, W. H. 'Once again: once again--tidal friction', Progress in 
Oceanography, vol. 40, pp. 7-36, 1997.

Munk, W. H. and G. J. F. MacDonald, The Rotation of the Earth: A 
Geophysical Discussion, Cambridge Univ. Press, 1960. 

Lambeck, K., The Earth's Variable Rotation: Geophysical Causes and 
Consequences, Cambridge Univ. Press, 1980.

Best regards

Eder C. Molina (eder@iag.usp.br)
Geophysicist
Dept. of Geophysics
Institute of Astronomy, Geophysics and Atmospheric Sciences
University of Sao Paulo
BRAZIL