Global issues related to the earth's environment and to the sustainability of current development practices have been at the forefront of scientific and political debates for the last decade. As it often touches upon basic societal values the topic has sometimes been the cause of heated arguments. The study of how the earth is influenced by and affects human activities is now an emerging field of science. It requires the setting up of most complex scientific experiments and analyses. The importance of the topic is fully recognised in the European Union's Fourth Framework Programme for Research and Technological Development (1994-98) which will provide a strong support to European research on global change.

To advance the scientific understanding of the earth's functioning it is well accepted that long-term, comprehensive and global observations on a series of key geo- and biophysical parameters must be collected, archived and analysed. Vegetation and land cover, among others, play a critical role in climate and atmospheric chemistry and are at the core of human interactions with the biosphere . Because of the scale of the investigations necessary to address the global dimension satellite borne sensors are playing an indispensable role in fulfilling the needs for comprehensive and repetitive data acquisition.

The need for the compilation of a global land cover map at a resolution of 1 km has been expressed by various scientific organisations and steps have already been taken to lay the ground work for such activity. The IGBP-Data and Information System Activity for example has stressed the urgency of compiling such type of information; a global 1 km satellite data set derived from the AVHRR instrument has been assembled under its auspices and is now being analysed.

Because of its unique technical and operational characteristics the VEGETATION instrument will bring a significant contribution in the study of two important characteristics of the biosphere. The first relates to the role of vegetation in the carbon budget and the second to the general dynamics of land cover. It is well known that together with soils the vegetation represents a major factor of uncertainty in global carbon budget. Modelling efforts have been pursued for several years to reduce such uncertainty. Whether to estimate biomass, carbon fluxes or evapotranspiration the modelling of vegetation processes acquires its full value only if it leads to geographically explicit assessments and predictions. For initialisation, calibration and validation of models, one must rely on an appropriate knowledge of the patterns of vegetation distribution at the surface of the earth and on a quantitative understanding of the associated processes. Improvements in radiometric measurements of canopy characteristics and in the development of physiological plant functioning models will contribute to such objectives.

As an example, the objective of the project "STEM-VGT: Satellite measurements and terrestrial ecosystem modeling using VEGETATION instrument" is to develop process models of vegetation functioning that can be used at regional and global scale for predicting carbon and water exchanges between land surface and the atmosphere

Finally, it is now recognised that quantitative predictions about changes in the earth environment necessitate new developments in our capability to predict future states of land cover. Modelling land cover and land use over large and heterogeneous regions is a difficult exercise because of the wide range of driving forces influencing the processes of change themselves. Accurate and constantly updated information on the state of the land cover must be available to identify initial conditions, validate the predictions and adjust for unexpected events. In such perspective the capability to permanently monitor the evolution of land cover over the entire surface of the earth is likely to represent one of the most significant development in assessing global change in the biosphere. The availability of advanced instruments supported by an efficient operation for data processing and distribution is expected to stimulate further research in this field.

The VEGETATION programme should, therefore, be considered as a significant contribution toward improving the performance and operation of global change research.