Other nations have also made significant contributions to the capacity to observe Earth from space. This multinational investment has enabled much international collaboration among satellite projects.

A prime example of an interdisciplinary research endeavor is the study of the global carbon cycle, which employs a wide range of research approaches such as ground and satellite observations, modeling studies, and laboratory experiments. The well-known Keeling curve was obtained from in situ observations and revealed atmosphere-biosphere interactions, as well as the long-term trend of increasing atmospheric carbon dioxide (Keeling et al. 1976). These findings launched major efforts in understanding the role of the terrestrial and oceanic biosphere in carbon uptake through photosynthesis and the impact of increased carbon dioxide levels on global climate. However, primary productivity is controlled by geophysical processes; thus, understanding the interconnections, such as the effect of a changing climate and hydrologic cycle on the global biosphere and vice versa, required observations at a global scale of land-cover changes (from Landsat and AVHRR [Advanced Very High Resolution Radiometer]; see Chapter 11), biomass estimates and primary productivity (AVHRR, CZCS [Coastal Zone Color Scanner], SeaWiFS [Sea-viewing Wide Field-of-view Sensor], and MODIS [Moderate Resolution Imaging Spectroradiometer]; see Chapters 9 and 10), changes in the hydrologic cycle (Landsat, AVHRR, MODIS, and Topography Experiment (TOPEX)/Poseidon; see Chapters 6 and 7) and climate (AVHRR, MODIS, and SeaWiFS). Once the data were available, major scientific advances came from assimilating them into three-dimensional coupled modeling of the atmosphere, land, ocean, and cryosphere (Fung 1986, Heiman and Keeling 1986, Fung et al. 1987, Keeling et al. 1989).

Equally interdisciplinary in nature is climate change research. In fact, many of the accomplishments highlighted in this report have contributed to the improved understanding of the climate system and laid the groundwork modeling for projecting climate change. One notable example is the long-term observations of Earth’s radiation budget, which revealed the role of the ocean and atmosphere in transporting heat and the role of aerosols from the volcanic eruption of Mount Pinatubo in cooling the climate (Chapter 4). With the understanding of the importance of aerosols to the climate system comes the need to observe continuously both natural and anthropogenic sources of aerosols (Chapter 4). Satellite observations have also been central in revealing the role of important gases, such as water vapor and ozone, in the climate system (Chapters 4 and 5).

Long-term observations of water in each phase are central to understanding the climate system: sea ice contributes to Earth’s albedo and its decrease not only indicates a warmer climate but is also a positive feedback (Chapter 7); melting of continental ice sheets contributes to sea-level rise (Chapter 7); the availability of liquid water is important in controlling the productivity of the terrestrial ecosystem, which in turn affects the amount of carbon dioxide (CO2) uptake (Chapter 9); and water vapor is important as a greenhouse gas and in heat exchange processes between the ocean, land, and atmosphere (Chapters 3, 4, 8, and 9). Due to water’s relatively high specific heat capacity and its large-scale circulation, the ocean plays a central role in storing and transporting Earth’s heat content (Chapter 8). In fact, more than 80 percent of Earth’s heat is stored in the ocean. Improving our understanding of ocean circulations and consequently the transport of heat is a major challenge to more accurate climate models and predictions. Lastly, the above-mentioned advances in understanding the global carbon cycle further the ability to predict future atmospheric CO2 levels.

The long-term observations obtained during the past 50 years of Earth science from space combined with advances in data assimilation, computer models, and ground-based process studies brought climate scientists to the point at which they could begin to project how climate change will affect weather and natural resources at the regional level, the scale at which the information is of greatest societal relevance (NRC 2001a).

This comes at a time when improved understanding of the climate system is central to the viability of our economy, as seasonal-to-interannual climate fluctuations strongly influence agriculture, the energy sector, and water resources (CCSP 2003). However, important scientific challenges—for example, cloud-water feedback in climate models—must be conquered with the aid of continuous satellite data before the appropriate seasonal-to-interannual climate information can be made readily available at the appropriate scale (NRC 2007a). The Earth science community has built over the past decades the capacity to incorporate all the pieces into an integrated systems perspective, thanks to ever more sophisticated models. As the community is now poised to make major advances in climate science and predicting climate changes at various scales, the ability to provide sustained multidecadal global measurements is crucial (NRC 1999, 2001b, 2007a).

The ability to observe and predict El Niño/La Niña conditions in advance of their full manifestation based on satellite and in situ data illustrates the significant breakthrough climate scientists have made in providing important regional climate information to resource managers (Box 12.1, Figure 12.1).

As many accomplishments have shown, the length and continuity of a given data record often yield additional scientific benefits beyond the initial research results of the mission and beyond the monitoring implications for operational agencies. For example, the effect of aerosols from a volcanic eruption (Mount Pinatubo) on the global climate would have gone undetected without the continuous observations of the Earth Radiation Budget Experiment (ERBE, Chapter 4). Thus, maintaining well-calibrated long-term data sets is likely to yield important scientific advances in understanding the Earth system, in addition to contributing to societal appli-



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