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Testing Climate Models with Geodetic Observations

PI: Olivier de Viron and Jean-Paul Boy
Institution: Royal Observatory of Belgium
Abstract:
The Earth is a complex system, composed of a solid elastic inner-core, a liquid core, a solid mantle, and superficial fluid layers (atmosphere, ocean and continental water storage). The models show that the changes induced in the dynamics of the climate system by the anthropogenic increase of the CO2 affects the atmospheric and oceanic circulation, and the cryosphere (in particular the amount of mass in the polar ice sheet and the sub polar glaciers). Variations of the global circulation and the mass repartition at the Earth's surface induce significant changes to be observed with geodetic techniques. In particular, the gravity field, as measured for instance by the Gravity Recovery and Climate Experiment (GRACE) mission, is directly sensitive to the mass repartition at the Earth surface. The Earth rotation rate is rather sensitive to the global zonal circulation in the atmosphere and in the ocean. The shape of the Earth, as measured by the deformation of global scale network of GPS (and soon Galileo) stations, is also perturbed by the variable mass repartition at the Earth surface. Those quantities are measured by geodesists with an incredible precision: the variation of the Earth orientation in space, and thus its rotation, are measured at the sub-centimeter level. The geoid, derived from gravity field, is measured at the millimetre level, and a millimetric position can be achieved for the station position. The geodesy can thus be a useful tool to study global scale changes in the climate system. It has several advantages with "classical" climatic observation:

(1) it is global, and can thus test the results where only few observational data are available,
(2) it is very precise, for instance GRACE is able to "feel" a 1mm of water anomaly over a few hundred kilometer scales,
(3) it is independent, as it does not use any of the data that have been, so far, assimilated in atmospheric or climate models.

This project aims to test what would be the consequences on the geodetic observables of the expected changes in the climate system induced by the CO2 increase, as predicted by the different coupled models. This will allow us to know which observables are different from one model to another, and thus which can be used in order to test the models against the independent geodetic observations.
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