Variations in the rotation of the Earth include changes in the rate of rotation (altering the Length of the Day, LOD), in orientation of the rotation axis relative to a terrestrial frame (Polar Motion, PM) and in orientation relative to a celestial frame due to external torques (Nutation and Precession). Variations occur over a wide range of time scales, from hours to the age of the Earth. Scientific interest in and understanding of Earth rotation variations have proceeded rapidly over the last several decades due, in large part, to enormous improvements in observations by space geodetic means, including satellite laser ranging (SLR), very long baseline interferometry (VLBI), lunar laser ranging (LLR), and satellite positioning methods, especially the global positioning system (GPS). The study of the Earth's rotation is a mature interdisciplinary field, and extensive reviews of many aspects of the field are contained in the AGU monographs `Contributions of Space Geodesy to Geodynamics' (Smith and Turcotte, 1993). Articles by Eubanks (1993), Dickey (1993), Hide and Dickey (1991), and monographs by Lambeck (1988,1980) and Munk and MacDonald (1960) provide excellent background material on these problems, as well. The reader may also wish to review other IUGG Report articles in this series on related subjects, specifically those on the global gravity field, VLBI technology, satellite orbit dynamics, and GPS.
It is now virtually certain that air and water cause most observed PM and LOD variations at periods of a few years and less, excluding tidal variations in LOD due to the long period solid Earth tides (McCarthy and Luzum, 1993; Robertson et al, 1993). Thus, PM and LOD variations measure changes in global integrals of air and water mass distribution and momentum. The purpose of this article is to review the contributions that PM and LOD observations can make to understanding atmospheric, oceanic, and hydrologic system variations. The discussion is divided into three parts: a review of the relationships among air and water distribution and motion, Earth rotation changes and other geodetic problems; discussion of changes at periods of a few years and less; followed by discussion of changes at longer periods.