The increasing extent of human actions involving direct interaction with the global land surface has prompted concern about the effects on the global environment. Natural scientists have taken up the study of the physical, chemical, and biological processes associated with changes in global land cover. Social scientists are now being challenged to understand the motivations for and resultant types of human activities affecting global land cover--an important contributor to global environmental change.
The evolving technology of satellite remote sensing offers potential for contributing to human dimensions' studies. Satellite remote sensing is defined as the use of satellite-borne sensors to observe, measure, and record the electromagnetic radiation reflected or emitted by the Earth and its environment for subsequent analysis and extraction of information. As a frame of reference, satellite image data provide global context information on the location and extent of human activities, which can assist in the planning and coordination of global change research. Satellite image data also offer capabilities for deriving information on the effects of human behavior within the biosphere, providing site-specific details about land cover, humans' activities in changing it, and the associated rates of change. For example, figure 1 shows the progression of forest alteration in the Amazon; figure 2 shows the lowering of the Aral Sea due to diversion of inflowing rivers for agricultural irrigation; figure 3 illustrates increased use of groundwater for agricultural irrigation in the U.S. Central Great Plains; figure 4 shows use of groundwater for agricultural irrigation in Saudi Arabia; and figure 5 illustrates expansion of the eastern metropolitan region of Phoenix, Arizona, from 1976 to 1992.
Most early satellite sensors acquired data for meteorological purposes. The introduction of earth resources satellite sensors--those with a primary objective of mapping and monitoring land cover--occurred when the first Landsat satellite was launched in July 1972. Now Landsat data provide the longest and most extensive archive of satellite image data for monitoring the global land surface. Efforts are ongoing to reconfigure selected portions of the historical global Landsat data archive to make it more useful for global change research.
Several satellite sensors currently provide direct observation of the global land and ocean surface, with the data of each sensor having unique characteristics of spectral, spatial, and temporal resolution. These data collectively offer a range of possibilities for addressing specific inquiries associated with monitoring the effects of natural events and human actions on land cover and the oceans. Addressing these questions may require high frequency of coverage, specific combinations of spectral information, or high spatial resolution. In many cases, the data of two or more sensors offer complementary characteristics, making joint use of data advantageous.
The evolution of major satellite data acquisition programs for global change research purposes includes an emphasis on the development of Data and Information Systems (DIS). The purpose of DIS are to archive, manage, and distribute the enormous volumes of data generated by Earth-orbiting satellites efficiently to make such data known to and accessible by global change researchers. Several major programs have also been initiated to develop satellite image data sets from historical and current data to provide research-quality, consistently structured time-series data sets that will be useful for global change research.