Ice sheet and satellite altimetry

Since 1991, the altimeters of the ERS European Satellites allow the observa tion of 80% of the Antarctica ice sheet and the whole Greenland ice sheet: They thus offer for the first time a unique vision of polar ice caps. Indee d, surface topography is an essential data thanks to its capacity to highli ght the physical processes which control the surface shape, or to test mode ls. Moreover, the altimeter is also a radar which makes it possible to esti mate the snow surface or subsurface characteristics, such as surface roughn ess induced by the strong katabatic wind or ice grain size. The polar ice c aps may not be in a stationary state, they continue to respond to the clima tic warming of the beginning of the Holocene, that is 18000 years ago, and possibly start to react to present climatic warming: the altimeter offers t he unique means of estimating the variations of volume and thus the contrib ution of polar ice caps to present sea level change.