Dams for Water Supply Are Altering Earth's Orbit, Expert Says

The insatiable thirst of the world's burgeoning billions has caused a spurt of dam building in temperate regions in the last 40 years, and a scientist with the space agency has found that the reservoirs are affecting Earth's orbital rotation.

Although Earth's rate of spin is gradually slowing because of the tidal drag of the moon, the slowing would have been measurably greater if it were not for the influence of 88 reservoirs built since the early 1950's, said the scientist, Dr. Benjamin Fong Chao, a geophysicist at the Goddard Space Flight Center, an arm of the National Aeronautics and Space Administration in Greenbelt, Md. Each of the reservoirs contains at least 2.4 cubic miles of water weighing 10 billion metric tons. The reservoirs contain the bulk of the world's impounded water.

The shift in the distribution of Earth's water caused by the reservoirs has tended to speed the planet's spin. Without lunar tidal drag, the reservoir effect would have reduced the length of a day by 0.2 millionths of a second a day for the last 40 years, Dr. Chao calculated.

The reason for this, he said, is that the shifting of water to mid-latitude reservoirs in the Northern and Southern Hemispheres has increased the amount of the world's water in those latitudes in relation to the Equator. In effect, more water is closer to Earth's axis. Moreover, Earth's axis is being slightly tilted by the weight of water that has collected in the 88 reservoirs, Dr. Chao found, and the shape of the planet's gravitational field has been altered.

These effects are several hundred times smaller than natural variations in Earth's motion, Dr. Chao said in an interview, and they pose no danger to people or the global environment. Still, he reported recently in Geophysical Research Papers, the effects of reservoir construction are significant enough that they will have to be taken into consideration in calculating long-term changes in global motion. His conclusions are based on geophysical measurements, international data bases and theoretical calculations.

Dam building in the former Soviet Union, Canada, Brazil and other mid-latitude countries has been rapid in the last four decades, and fresh water collected from rivers and other terrestrial sources has increased in this period by 10,000 cubic kilometers, or 10 trillion tons, an amount equivalent to all the moisture in Earth's atmosphere. This water mostly accumulates from rain, which in turn comes from clouds that draw their moisture largely from the evaporation of ocean water.

This enormous shift of water from the oceans has somewhat offset the continuing rise in global sea level, which would have been about 1.2 inches greater over the last 40 years if there had been no new reservoirs, Dr. Chao says.

Although scientists are uncertain about the causes of the observed rise in sea level, many say they believe that a major cause is global warming and the consequent expansion of liquid water and melting of mountain glaciers. In general, a rising sea level is regarded as a potential threat to many regions because it might eventually inundate low-lying countries like Bangladesh and coastal cities around the world. So the reservoir effect is desirable because it presumably slows the rise in sea level.

But another geophysicist, Dr. Dork L. Sahagian of the University of New Hampshire, has said he believes that the reservoir effect is too small to offset the rise sufficiently to rule out future coastal flooding.

Geophysicists have little doubt that dam building will continue rapidly until all sources of recoverable water have been exploited, a time some scientists calculate will come in the next century.