Earth's radiation budget
The Earth exchanges energy with its spatial environment almost exclusively through radiation (gain from solar energy and loss by infrared emission). In fact, each region of our planet receives from the sun an energy flux depending on the latitude and season through a radiation in the shortwave domain (0.2-4 mm). A portion is reflected to space. The remainder is absorbed by the Earth and its atmosphere and feeds the thermal engine of the atmospheric and oceanic circulation. As a result the solar heat is redistributed from the Equator to the Poles. To balance the incoming shortwave energy, the surface–atmosphere system ultimately emits radiation to space in the infrared or longwave domain (4-100 mm). The Earth’s radiation balance (ERB) is the difference between the absorbed solar radiation and the emitted infrared radiation and is equal to zero on an annual and long-term equilibrium basis. Temporarily (over a few years) the ERB can be slightly unbalanced, indicative of warming or cooling according to the sign of the unbalance. The geographical and temporal ERB distributions on the contrary are largely variable (figure). The ERB determination is essential to access the atmospheric modelling and climate studies. A major research problem is how clouds affect the radiation energy balance of the planet, and react to the greenhouse gas forcing.
Important for climate studies, the ERB estimation is also the most direct method of the Earth observation, since it is deduced almost directly from the radiometric measurements (in spite of the angular and space-time sampling issues). That explains why ERB was studied from space very early at the beginning of the spatial era (Tiros, 1962-1965), and then be continued through the Nimbus, ERBE , ScaRaB , CERES and GERB programs. All have improved the radiometric accuracy and the time-space extrapolation procedures.
Member of the ERBE and CERES programs and in charge for ScaRaB (a third instrument should be launched in 2009 on Megha-Tropiques ), the LMD/IPSL has been strongly associated to these efforts.