Introduction

The GEWEX Radiation Panel (GRP) was organized to bring together theoretical and experimental insights into the radiative interactions and climate feedbacks associated with cloud processes, including the effects of water vapor within the atmosphere and at the Earth's surface. The most fundamental scientific question the GRP must answer is: How sensitive is the Earth's climate to changes in radiative and other forcings? Answering this question will enable improved prediction of transient natural climate variations, such as El Niņo, and provide better understanding of the consequences of natural and human-induced climate changes.

Changes in atmospheric water vapor, precipitation, clouds and aerosols affect the energy balance of the Earth and since these processes are intertwined, complex and simultaneous, considerable uncertainty and controversy remain concerning the quantitative impact of these feedback processes. As a result, the treatment of this problem is intricate and requires coordinated observations, modeling and process studies. To observe globally these interacting phenomena over the large range of space and time scales at which they occur, satellite observations must be employed. Hence, radiation processes must also be understood to allow for accurate quantitative interpretation of satellite observations.