GLOBAL ANGULAR MOMENTUM

It follows from Newton's laws that the angular momentum of a spinning body, in the absence of outside forces, must be conserved. The mass orbiting a point multiplied by the distance to that point and the velocity of the motion measures angular momentum. This is summed over all the points of mass in an object. Therefore, when a spinning figure skater draws his arms inward, the velocity with which he spins increases because this preserves angular momentum.

Science News 149, p 108 (1996), says that B. F. Chao (December 15, 1995, Geophysical Research Letters) has found effects on the earth's rotation from manmade water reservoirs. The shift in mass caused by 10 trillion tons of water storage in the Northern Hemisphere over the past 40 years has changed the rotation rate of the earth by 8 millionths of a second per day. This storage has the effect of moving mass closer to the axis of spin because much of it is removed from the equatorial oceans. So the earth, like the figure skater, is rotating faster.

Moreover, since this rearrangement of mass is not uniform over the globe, it has also increased the rate of drift of the earth's axis of rotation. Since 1940, this effect has "pushed the axis of rotation about 60 centimeters away from the North Pole toward Western Canada'' and "equals 5% of the natural axis drift over the last 100 years.'' Will this become the next environmental crisis? We can imagine the headlines - "Earth wobbling uncontrollably as a result of the stock watering ponds of Western ranchers, Vice-President warns. Loss of trees from equatorial rain forests also a culprit.''