THE EFFECT OF HUMAN ACTIVITIES ON GLOBAL SEA LEVEL

A significant fraction of the Earth's hydrologic cycle is artificially controlled by human activity to the point that it is not only affecting continental water resources, but global sea level as well. Some activities that transfer water away from continental storage reservoirs and into the ocean, thus contributing to sea level rise. A few of these include irrigation, deforestation, and wetland filling or drainage. However, new dam construction has sequestered water on the continents at a fairly constant rate throughout the second half of the 20th century. The rate of water impoundment behind dams may have been overwhelming the cumulative effect of all other, positive anthropogenic contributions to sea level rise ameliorating sea level rise in the 20th century. However, there is some reason for concern:

The amelioration of sea level rise by new dam construction will cease unless globally, we continue to build new dams and impound water at the rate that we did in the latter half of the 20th century. This is unlikely. In this case, we will observe an INCREASE in the rate of sea level rise in the 21st century.

Furthermore, present compilations of reservoirs impounded by dams include only the results of major engineering projects. Smaller impoundments have largely been ignored. The cumulative volume of the literally millions of small reservoirs such as farm ponds and rice paddies may approach or even exceed that of the larger documented reservoirs. In addition, the quantity of water stored in artificially raised water tables behind dams has not been addressed, but it may exceed that of surface water.

Estimates based solely on major dammed reservoirs suggest that if new dam construction is not maintained in the 21st century, the rate of sea level rise could increase by about 0.5 mm/yr. If the water stored in small impoundments and ground water is taken into account, this figure could be much greater. This would be in addition to any increases in the rate of sea level rise caused by other factors such as glacial retreat and ocean thermal expansion.