The image of the Earth on the front cover was acquired from the GOES--8 Imager by the Laboratory for Atmosphere at NASAa 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.