The anthropogenic increase of greenhouse gas will probably induce significant changes of the atmospheric and oceanic global circulation. We have evaluated the variation of the Earth's gravity field, geocenter motion and rotational variations using the available atmospheric and oceanic outputs from coupled general circulation models participating to Couple Model Intercomparison Project (CMIP 2+) and the corresponding ice sheets (Antarctica and Greenland) changes according to Huybrechts et al. (2003). We discuss on the possible detection of these geodetic effects, especially with new space gravity missions such as GRACE.