DESC: The image of the Earth on the front cover was acquired from the GOES-8 Imager by the Laboratory for Atmosphere at NASA'S Goddard Space Flight Center, Greenbelt, MD. It was taken on September 2, 1994, using two channels for the Imager to produce a "natural color" effect. The photos shown here on the back cover illustrate the four spheres of the Earth system - the atmosphere, biosphere, hydrosphere, geosphere - together with changes being produced by human activities.