CALIPSO is a mission studying the clouds and aerosols radiative impacts which now represent the main uncertainties concerning the climate evolution prediction.

The tri-dimensional numerical models constitute the essencial tools for the global scale process analyse for the climate prediction. Thus they must be the more realistic possible integrating a suitable physical representation of the atmospheric processes and those coupling the atmosphere, the ocean and the biosphere. At present, the parameters developed in the numerical models are subject to larges uncertainties for some parameters ajustement. New observations are necessaries to improve those quantities determination.

It was proved (IPCC report,2001) that the main uncertainties about the climate evolution prediction are associated to the cloud and aerosols radiative impact.

The understanding of the Earth's radiation budget require the radiation flux measure at the top of the atmosphere as well as at different levels in the atmosphere.

The Lidar will bring the characterization of the vertical distributions as well as the multi-layers clouds.

A key stage for the improvement of the clouds involvement in the climate models is to have simultaneous measurements of the atmosphere state, microphysical and optical properites, and also clouds radiative properties.

CALIPSO will supply for the first ime, on a three years observation duration, a data collect of a global geographical cover, with profiles (30 meter accuracy) measurements and aerosols anc clouds optical thickness measurements.