in using satellite data was gained using extant satellite systems such as the Thematic Mapper (TM), the Systeme Probatoire pour l' Observatión de la Terre (SPOT), and the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) systems. These data were successfully combined with algorithm development and evaluation in field campaigns such as FIFE, BOREAS, and the Hydrological and Atmospheric Pilot Experiment.26 The intensive, decade-long effort in this area resulted in the experimental design of two major EOS satellites (AM-1 and PM-1) being optimized for synergistic measurements of land surface variables. The primary platform, AM-1, orbits with a morning overpass time, chosen to minimize cloud contamination for land surface imaging, and includes a synergistic combination of three instruments: the Moderate-Resolution Imaging Spectroradiometer (MODIS), the Multi-Angle Imaging Spectroradiometer (MISR), and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). MODIS has capability in the visible and near infrared for remote sensing of vegetation characteristics with high time resolution. MISR takes multiangle measurements that allow determination of albedo and better constrain so-called vegetation indices related to conductance to water and photosynthesis. 27 ASTER and LANDSAT provide high spatial resolution information on land cover. The proposed remote sensing strategy for the physical climate system and carbon cycle studies is shown in Figure 2.3.

Research on land surface processes exemplifies a constructive partnership among many groups: global climate modelers, organism- to ecosystem-oriented bio- and microclimatologists, ecologists, and remote sensing scientists, as well as geographers, plant physiologists, soil scientists, and hydrologists. Having defined the need for improved satellite algorithms early on, the community carried out the necessary theoretical, modeling, and empirical demonstrations of such a capability. These science requirements are now largely executed on the EOS AM-1 spacecraft. AM-1 is a large and expensive mission, but the community has confidence in the quality of its land surface mission. This forms one component of AM-1's full scientific agenda, which also includes cloud, aerosol, atmospheric chemistry, and oceanographic experiments.

Human Use and Modification of Ecosystems

At the beginning of carbon cycle research, the carbon cycle appeared to be roughly in balance, with fossil emissions balanced approximately by ocean uptake and atmospheric accumulation. Beginning in the 1970s, ecologists led by George Woodwell began to make the case that emissions from land use, largely deforestation, had to contribute significant inputs to the atmosphere. As this hypothesis became better and better documented, it became clear that to balance land-use emissions an additional sink process, dubbed the “missing sink,” was required. The significance of land use in the carbon cycle was recognized prior to the USGCRP and the IGBP and has been a major focus of both programs since their



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