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Section I

Atmospheric Greenhouse Trapping:
Cloud Radiation Feedback

Prof. V. Ramanathan
University of California, San Diego
Scripps Institute for Oceanography

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Water vapor and clouds are the dominant regulators of radiative heating of the planet. Recent general circulation model (GCM) studies have identified the complex scope of the various cloud and water vapor feedbacks and their significant importance to climate change. We do not understand how these radiative effects respond to a climate change, nor do we understand their feedback effects. Lack of understanding of the nature of this cloud-climate feedback is one of the fundamental sources of uncertainties in climate prediction, be it the climate of the next year, the next decade, or the next century.
The fundamental climate forcing is the net radiative flux at the tropopause and at the surface. The fundamental climate variable is the water vapor distribution from the surface up to 20 kilometers altitude. These first-order observations, which are essential for predictions of global climate change, can only be obtained via continuous, in situ observations within the atmosphere in the altitude range from the surface to 25 km. This demands a careful balance between in situ (aircraft) and remote (satellite) observations during this decade. We consider here the seminal observations that must be obtained using unmanned aircraft to expose the mechanisms responsible for dictating cloud-radiative feedback and water vapor-radiative feedback.
With respect to cloud-climate feedback, we stand at the beginning. However, exciting progress has been made during the last few years. For example, because of breakthroughs in satellite radiation budget observations, we now have a global perspective of how clouds regulate the radiative heating of the planet. GCMs have also advanced to a stage where they are attempting to form clouds from a micro-

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