The significance of the study of Earth rotation variations in modern geophysics is four-fold:
Modern space-geodetic Earth rotation observations provide access to globally integrated properties of the Earth system (total absolute angular momentum of air and water) over a range of periods. These observations provide not only a unique measure of long period fluctuations, normally thought of as global change, but also of the temporal continuum of such variations to periods as short as a few hours.
The nature of Earth rotation changes as integral measures of variability is unusual. Most surface and many satellite remote sensing observations of the Earth system record variations at isolated locations and times. Only a few measurements, including variations in the Stokes coefficients and Earth rotation changes, offer genuinely global measures.
Earth rotation studies are interdisciplinary, requiring the application and development of technology, theory, and observational programs in a variety of fields. Excellent space geodetic data are the product of many independent efforts, including: international cooperation; military-driven programs on positioning and laser tracking; fundamental radio astronomy of extra-gallactic objects; and the civilian space program. The task of interpretting these data belongs to a multitude of scientists: theorists interested in the physical properties of the Earth; oceanographers interested in fundamental and military aspects of the seas; atmospheric scientists interested in basic and applied (forecasting) problems, and hydrologists and glaciologists, addressing both fundamental and applied problems. Earth rotation studies are intimately connected to those of the gravity field, with again, both fundamental and applied (orbit prediction) aspects. Beyond these interconnections, Earth rotation research will also contribute to understanding global climate change, which connects all fundamental and applied earth and life sciences.
Earth rotation studies, by their global nature and significance in national security, time keeping, reference frames and positioning, require the convergence and international cooperation of scientists, societies, and cultures. The founding of the International Latitude Service at the end of the last century, and the continuing tradition of international cooperation and organization in the International Earth Rotation Service (and related activities) demonstrate that this science is able to transcend national and cultural boundaries in times of overwhelming political global change.
Acknowledgments. This review was prepared with support from NASA Grant NAGW-3131, and with additional support from the Geology Foundation of the University of Texas.