One of the great challenges facing mankind in the 21st century is the threat of global warming due to increased levels of carbon dioxide and other greenhouse gases in the atmosphere. Is the threat real ? Computer models that predict the future climate still differ in some very substantial ways with some models predicting little or no warming while others predict temperature increases that would substantially alter our way of life. Without more data and better understanding of the current climate system, there is little hope that we can understand the differences among our current models and both sides in the environmental debate will continue to press for their viewpoints.

TRMM is a research satellite designed to help our understanding of the water cycle in the current climate system. By covering the tropical and semi-tropical regions of the Earth, TRMM provides much needed data on rainfall and the heat release associated with rainfall. It will not single-handedly provide the solution to the climate change debate - it will, however, contribute to our understanding of how clouds affect climate and how much energy is transported in the global water cycle. In coordination with other satellites in NASA's Mission to Planet Earth, TRMM will begin the process of understanding the interactions between water vapor, clouds and precipitation that is central to regulating the climate system.

The energy balance of the global atmosphere shows that only about one fourth of the energy needed to drive the global atmospheric circulation comes from direct solar energy. The other three fourths of the energy is transferred to the atmosphere by evaporating water - mainly from the ocean. As the water vapor rises from the surface, it carries with it the energy it takes to turn liquid water into water vapor - the "Latent Heat of Evaporation". Most of this latent heat is released into the atmosphere in huge equatorial cloud clusters when the vapor condenses to form cloud- and raindrops. The latent heat energy contained in the clouds cannot be seen or measured directly. Rainfall, however, is the product of the release of this energy, and rainfall can be measured. Responsible for three quarters of the energy that drives the global atmospheric circulation, tropical rainfall can be said to Drive the Climate Machine. Unfortunately, there are still uncertainties of as high as 50% in the amount of tropical rainfall. Unless we can better define the amount of rainfall and the energy released when rain occurs, we stand little chance of putting the climate models through the rigorous tests we need to gain confidence in their predictions.