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Summary: Earth orientation parameters comprising variations of both the position of the rotational pole and the spin rate are precisely observed by modern space geodetic techniques for several decades already. Moreover, optical astrometric observations extending back in time over more than 100 years provide even carry information about the mass transport and mass distribution processes acting on Earth at historical times that might be explored to quantify slow and subtle variations in the Earth's climate. This working group will study the various contributors of the global and interactively coupled climate system to the observed changes of the Earth's orientation on time-scales from days to centennials. It will explore possibilities to validate numerical climate models and its individual components by means of assessing the angular momentum budget and the associated torques. The working group will further investigate predictive limits of various Earth system state and flux variables in order to aid short- and long-term prediction of polar motion and changes in the length-of-day, and might ultimately foster the incorporation of Earth Orientation Parameters into contemporaneous global re-analyses of the Earth System by means of data assimilation.

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