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terrestrial vegetation in pumping moisture into the atmosphere, the role of the ocean in regulating atmospheric CO2, and the effects of clouds. Clouds can either warm or cool the global climate. Low clouds reflect sunlight and tend to cool the earth, while high, thin clouds let the sunlight through but trap the returned infra- red radiation and thus warm the planet. Recent analysis of data taken by NASA's Earth Radiation Budget Satellite, and of earth radiation budget sensors on the NOAA-9 and NOAA-10 satellites, shows that the effect of clouds in April, 1983, was to cool the earth by about 15 degrees Centigrade, or about three times the amount of warming predicted for doubling atmospheric CO2. It is not clear at all, with today's state of knowledge, what the effect of global warming will be on clouds, and thence back on the warming itself. Cloud radiative feedback has sufficient robustness to totally cancel or to aggressively magnify any global warming. 

What is required to answer these scientific questions and to make the models adequate for our needs for predicting future global change? The answer is a dedicated mission to understand our planet, how it functions and how it changes over decadal to 100 year time scales: Mission to Planet Earth. An accurate predictive ability is essential for making sound policy decisions in an era of global change. The potential changes touch every area of human productivity and carry a large economic penalty. The consequences of a headlong, ill-informed rush into policy could be disastrous. 

Mission to Planet Earth, the U.S. Global Change Program, the International Geosphere/Biosphere Program, and the International Space Year

At NASA, the term "Mission to Planet Earth" has come to mean the space observation platforms, data and information system, earth process studies, and global earth system models required to understand the global environmental system and its changes. However, NASA's Mission to Planet Earth is part of the larger. S. Global Change Research Program, which in turn contributes to an international scientific plan of global research organized by the International Council of Scientific Unions (ICSU) and called the International Geosphere Biosphere Program (IGBP). NASA has a crucial role in the U.S. Global Change Program, which is to provide the long-term, global observations from space and the associated data and information system that are absolutely essential for understanding the earth, for detecting global change, and for validating a new generation of predictive GCMs. 

The problems associated with global change are clearly global in nature, and while the method of choice for studying problems are observations from space, NASA alone cannot do it. Remote sensing data from space requires complimentary ground-based studies for validation and for focused process studies, and these studies must be conducted in many places around the world. The cooperation of many countries will be required under the auspices