"The circulation of geophysical fluids (atmosphere, oceans, continental hydrology) at the Earth’s surface induces global mass redistribution, and therefore gravity changes, as well as global deformations. Since its launch in April 2002, the GRACE (Gravity Recovery And Climate Experiment) mission is measuring time-variable Earth’s gravity field, with spatial and temporal resolutions of a few hundreds of kilometres and 10 to 30 days, respectively, and allows the recovery of global mass variations within the entire climate system. The ESA mission GOCE (Gravity field and steady-state Ocean Circulation Experiment), to be launched at the end of 2007, will improve the knowledge of the smaller wavelengths of the Earth’s gravity field. Thanks to several decades of radar altimetry, elevations changes of the oceans and ice-sheets are mapped with a high precision (at the cm level) and a high sampling rate (from 10 days with Jason-1 to 35-days for ENVISAT). However these missions only allow the retrieval of volume changes rather than mass. For example, sea level variations are not only due to fresh water inputs (from ice-sheet or glacier melting) but also to thermal expansion (steric effects). Continental water storage (soil moisture, groundwater, snow, etc.) cannot be completely mapped from space, as only the top soil layer (few centimetres) can be deduced from radiometer measurements. The analysis of time-variable gravity field (in addition to other remote sensing techniques) will allow closing the mass balance of the climatic system, both on a global scale and regional scales (river or oceanic basins). This project involves the collaboration between two well recognized institutions, with complementary expertises, the Institut de Physique du Globe, hosted at the University Louis Pasteur of Strasbourg in France, and the NASA Goddard Space Flight Center in the USA."