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Earth Observations from Space: The First 50 Years of Scientific Achievements 12 Conclusions Just as the invention of the mirror allowed humans to see their own image with clarity for the first time, Earth observations from space have allowed humans to see themselves for the first time living on and altering a dynamic planet. THE EMERGENCE OF INTEGRATED EARTH SYSTEM SCIENCE During the International Geophysical Year (IGY) of 1957-1958, 67 nations cooperated in an unprecedented effort to study the Earth. In an age otherwise characterized by Cold War tensions, the noted geophysicist Sydney Chapman (1888-1972) referred to the IGY as “the common study of our planet by all nations for the benefit of all.” This global effort laid the foundation for the integration of Earth sciences and demanded widespread simultaneous observations. It involved large teams of observers, many of whom were deployed to the ends of Earth—in polar regions, on high mountaintops, and at sea—to study meteorology, oceanography, glaciology, ionospheric physics, aurora and airglow, seismology, gravity, geomagnetism, solar radiation, and cosmic rays. Even in 1957 it was recognized that satellite data would bring observations of Earth that no amount of ground-based observations could achieve. Hundreds of sounding rockets were launched into the upper atmosphere and near space during the IGY, and the “space age” officially began with geophysical satellites, although still in their infancy, playing an important role (Chapter 2). During the IGY the Soviet Union launched the world’s first satellite, Sputnik, in October 1957. The United States launched its first satellite, Explorer 1, shortly thereafter in January 1958. Over the course of the next five decades, the United States and its international partners have launched an array of satellites that fundamentally altered our understanding of the planet. A half-century later, Earth scientists can acquire global satellite data with orders of magnitude greater coverage than obtained during the intensive field expeditions of the IGY from the comfort of their desktops. The advent of satellites revolutionized the Earth sciences. They provided the first complete global record of biological, physical, and chemical parameters such as cloud cover, winds, and ice cover. They provided consistency of coverage not available with ground measurements. Time series data revealed large-scale processes and features that could not have been discovered by other ways. Prior to the availability of satellite-based observations, scientists seeking global perspectives from largely ground-based observations were required to develop international collaborations and launch large-scale field campaigns. Piecing together data points required interpolation and extrapolation to fill data gaps, particularly for remote locations. In addition, large-scale sampling efforts involved extensive logistics and advance planning, which prohibited frequent repetition. Because the rate of change of many parameters of interest is much greater than the rate at which global maps could be produced in the presatellite era, it was impossible to observe the full dynamics of the system. Therefore, the unique and revolutionary vantage point from space provides scientists with global images and maps of parameters of interest unmatched by any ground-based observing technology in terms of frequency and coverage. Because satellites collect data continuously and allow for daily (or at least monthly averaged) global images, changes can be observed at the relevant temporal and spatial scale required to detect Earth system processes. The full dynamics of the system have only been observed or characterized since the advent of satellite observations and have allowed the study of previously inaccessible phenomena such as stratospheric ozone creation and depletion, the transport of air pollution across entire ocean basins from China to the continental United States (Chapter 5), global energy fluxes (Chapter 4), ice sheet flow (Chapter 7), global primary pro-
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Earth Observations from Space: The First 50 Years of Scientific Achievements ductivity (Chapter 9), ocean currents and mesoscale features (Chapter 8), and global maps of winds (Chapter 8). Prior to the satellite era, even if it was possible to compose a global picture from individual surface observations (e.g., through the World Weather Watch, established in 1963), the coverage and density of the network and lack of vertical resolution left much to be desired. Other geophysical and biological phenomena were sampled much less frequently, often as a partial “snapshot” of an otherwise dynamic set of interacting Earth processes. Discovery of the variability in the velocity of ice sheet flow is another example of how the dynamics of the system went undetected until reliable and repeated satellite observations became available (Chapter 7). This discovery revolutionized the study of ice sheet flow and yielded an important realization: sea-level change due to freshwater input from the continental ice sheets was not a function of the balance between ice sheet melting and precipitation at higher elevation, but a function of the flow dynamics. The increasing velocity of continental ice flow into the ocean in response to climate change and the collapse of the Larsen B Ice Shelf emphasized the sensitivity of ice sheet dynamics to a changing climate. Satellite sensors provide a panoptic viewpoint, yet historically they suffered from poor resolution and calibration problems. On the other hand, ground-based instruments, although more precise and better calibrated, are limited to their particular locales, and problems arise since they must be coordinated and intercalibrated with other ground stations. As satellite sensors and data processing have become more sophisticated, equaling or surpassing those for ground-based measurements, scientists have obtained not only images but also quantitative global measurements of unprecedented precision. Intercalibration proved particularly challenging in putting together global maps of marine primary productivity from shipboard measurements (Chapter 9). Estimating marine primary productivity requires sample manipulation and measurements of 14C uptake rates at each location, which are sensitive to variations in sampling techniques and methods. Although global marine primary productivity estimates had been attempted before the satellites era, they were flawed because of intercalibration issues. More importantly, because it takes years to obtain global coverage of ground-based marine primary productivity measurements, satellites allowed for the first time observation of global marine primary productivity on a monthly and annual basis and detection of decadal-scale trends. Satellite observations also provide access to otherwise virtually inaccessible regions, such as polar regions, the upper atmosphere, and the open oceans. Quantitative assessment and monitoring of the sea ice extent in the Arctic has only been possible since routine satellite observations became available. Without satellite images, it is unlikely that trends in decreasing Arctic summer sea ice would have been detected as readily, demonstrating univocally the drastic decline in summer ice over the past decades (Chapter 7). Satellite observations have become available and matured as scientific data at a time when they are critically important in helping society manage planetary-scale resources and environmental challenges. Although many scientific challenges remain, it is undeniable that satellite observations have allowed scientists to improve the ability to monitor and predict changes in the Earth system and manage life on Earth (NRC 2007a). It is widely known that satellite data, particularly from the southern hemisphere, have contributed to improvements in weather prediction, resulting in protection of human lives and infrastructure (Chapter 3). Since the availability of satellite images, no tropical cyclone has gone undetected, and the advance warning allows crucial time to prepare. In fact, the advent of satellites has been heralded as unquestionably “the greatest single advancement in observing tools for tropical meteorology” (Sheets 1990). Furthermore, because satellite data give access to the largely undersampled ocean, hurricane track forecasts have improved dramatically, helping save lives and property every year (Considine et al. 2004). Other aspects of human welfare have and will also benefit from satellite observations. For example, it is also unlikely that a famine early warning system would be available to assist in planning aid distribution without the ability to observe vegetation cover and the availability of water resources from space (Chapter 10). Given the projected climate change and associated sea-level rise, having global satellite coverage available in the future will serve crucial societal needs unmet by any other observing system. Conclusion 1: The daily synoptic global view of Earth, uniquely available from satellite observations, has revolutionized Earth studies and ushered in a new era of multidisciplinary Earth sciences, with an emphasis on dynamics at all accessible spatial and temporal scales, even in remote areas. This new capability plays a critically important role in helping society manage planetary-scale resources and environmental challenges. INTEGRATED GLOBAL VIEW OF THE CARBON CYCLE AND CLIMATE SYSTEM The global view of Earth from satellites has imparted the understanding that everything is connected—land, ocean, and atmosphere. Interdisciplinary teams of researchers have explored these connections to better understand the Earth as a system beyond the sum of its elements. The concept of studying the Earth as an integrated system at a national level was led by the National Aeronautics and Space Administration (NASA), inspired by NASA’s “Ride report” (NASA 1987), and intended as the U.S. component to the International Geosphere-Biosphere Program. Consequently, NASA launched its mission to planet Earth to study the Earth’s geosphere and biosphere as an integrated system instead of discrete but interrelated components (CRS 1990).
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Earth Observations from Space: The First 50 Years of Scientific Achievements 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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Earth Observations from Space: The First 50 Years of Scientific Achievements cations. The importance of stable, accurate, intercalibrated, long-term climate data records is universally recognized, and strategies on how to collect and maintain such data streams have been provided in many previous reports (NRC 1985, 2000, 2001b, 2003, 2004). Important elements to successful long-term climate data from satellites include a long-term strategy to guarantee that follow-on missions overlap to allow for cross-calibrations, leadership in data stewardship and management, and strong interagency collaborations. Follow-on missions maximize the return on previously made investments in technology development, including sensors and data analysis tools. Missions designed for process studies of initially short durations may provide significant scientific value by continuing a given data record in the context of global change research. The value of a continuous data record increases significantly through the development of uninterrupted follow-on missions, particularly if careful cross-calibrations between subsequent generations of satellite sensors are undertaken (NRC 2004). The long-term data records from Landsat and AVHRR exemplify the scientific value of such carefully maintained data streams (Chapters 9 and 10). Conclusion 2: To assess global change quantitatively, synoptic data sets with long time series are required. The value of the data increases significantly with seamless and intercalibrated time series (NRC 2004), which highlight the benefits of follow-on missions. Further, as these time series lengthen, historical data sets often increase in scientific and societal value. MAXIMIZING THE RETURN ON INVESTMENT IN EARTH OBSERVATIONS FROM SPACE As scientists have gained experience in studying Earth through satellite observations, they have defined new technology needs, helped drive technology development to provide more quantitative and accurate measurements, and advanced more sophisticated methods to interpret satellite data (Chapter 2). Many scientific accomplishments have resulted from rapid satellite technology development that responded to scientific needs and provided capabilities that enabled major advances in the Earth sciences. The value of satellite observations from space grows dramatically as new, more accurate instruments are developed. Initially, satellites provided a means for acquiring pictures. Now, satellite image acquisition and interpretation provide quantitative geophysical or biological variables by transforming measurements of reflected or emitted electromagnetic radiation into desired parameters. For many applications such as ocean and land topography, ice sheet dynamics, and concentrations of atmospheric gases, observations are scientifically valuable if they can be made with great accuracy, which has driven technology evolution. For example, the Williamstown report (NASA 1970) outlines the need for satellite sensors to measure the geopotential and mean sea level to determine the general circulation of the oceans and resolve the spatial variations of the gravity field as a goal for geophysics and physical oceanography. NASA responded to this challenge by launching three satellites within 9 years following the Williamstown conference, with Seasat—the third and most advanced satellite—providing accurate ocean elevation with a precision to tens of centimeters. For the first time the bathymetry of the ocean floor could be observed from space, revealing the large mid-Atlantic ridges and trenches (Chapter 11). As the precision of altimetry data further increased the importance of eddies in the mixing of the open ocean was discovered (Chapter 8). It is common for any given satellite or instrument in space to supply data that may be used in multiple fields of Earth science by design or serendipitously (see Table A.1). Although Landsat was designed to observe changes on land, including the terrestrial ecosystem, assembling the approximately 5,000 individual images for a global time series proved to be too computationally intensive. Instead, it was AVHRR data—designed to monitor the atmosphere—that turned out to be invaluable to producing global terrestrial primary productivity estimates. Due to careful intercalibrations between the different sensors, the AVHRR data record now extends over 20 years (Chapter 8) and has allowed the detection of trends in terrestrial primary productivity (Chapter 9). In fact, data from AVHRR have also been used in many other fields to study processes such as snow cover, sea surface temperature, cloud optical properties, and global land-cover change (Chapters 6, 8, and 10). The design of MODIS illustrates the potential for using a single instrument to serve many applications. Its spectral bands were designed to serve a diversity of user communities in the Earth sciences, allowing observations of the following parameters: land, cloud, and aerosol properties; ocean color and marine biogeochemistry; atmospheric water vapor; surface and cloud temperature; cloud properties; cirrus cloud water vapor; atmospheric temperature; ozone; and cloud top altitude. It has led to scientific breakthroughs such as discovery of the brown clouds (Chapter 4), measuring marine primary productivity annually (Chapter 9), and observation of optical depth and effective particle radius in low clouds (Chapter 4). Because of the potential to design missions with spectral bands that can serve many different scientific user communities, creating follow-on missions that continue measurements—and thus ensure the long-term climatic data records discussed above—does not have to come at an increased cost or at the cost of research and development missions. In addition, the measurement of a given variable, in some cases from multiple sensors, often contributes to several fields of Earth science. For example, few scientific accomplishments are as “transformative” as the advances in space geodesy over the past five decades (Chapter 11). This breakthrough has not only transformed the field of geodesy
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Earth Observations from Space: The First 50 Years of Scientific Achievements BOX 12.1 El Niño-Southern Oscillation El Niño is a condition that has been known for well over a century. In some years waters off the west coast of South America would become warmer than usual, and the fish populations normally found there would disappear, bringing hardship to fishermen in the region. It occurs periodically around Christmastime and thus was named “El Niño”—the Spanish term referring to the Christ Child. Much of the groundwork for understanding and describing the El Niño-Southern Oscillation (ENSO) as a coupled atmosphere-ocean phenomenon was laid in the 1970s and 1980s and based on in situ data and modeling studies (e.g., Rowntree 1972, Wyrtki 1975, Rasmusson and Carpenter 1982, Zebiak 1982, Shukla and Wallace 1983, Cane 1984). However, satellite data confirmed observations and model efforts and revealed the global impact of ENSO (Friedler 1984). The improved understanding of the atmosphere-ocean connection has improved the ability to predict ENSO conditions and has advanced our understanding of the teleconnections and impacts on the marine and terrestrial biosphere (Barber and Chavez 1983). In normal years winds blow from east to west, causing warm surface waters to “pile up” in the western tropical Pacific. During an El Niño, the winds relax and the warm surface waters flow back toward the eastern Pacific. Wind-driven upwellings do not reach deep enough to bring nutrients from below the thermocline. Without the supply of nutrients, phytoplankton do not thrive and this creates a chain reaction in the marine ecosystem. The major El Niño event of 1982 revealed its impacts not only on the ocean but also on global weather patterns, which invigorated research efforts to improve ENSO predictions. Because ENSO events are accompanied typically by drought conditions in Indonesia and Australia and heavier-than-normal rainfall in South America, their effects can be seen in virtually every form of Earth observations from space. By piecing together the different observations (sea surface temperature [SST], winds, sea surface height, biological productivity, rainfall, and land cover), scientists are working to develop theories to explain what triggers an El Niño and to predict consequences once an El Niño has developed. Satellite observations of SST and winds combined with in situ data are also used to predict El Niño events up to a year in advance. Figure 12.1 illustrates how the physical and biological properties of the Pacific are related during an El Niño and the opposite, La Niña, condition.
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Earth Observations from Space: The First 50 Years of Scientific Achievements FIGURE 12.1 These images of the Pacific Ocean show conditions during an El Niño (1997) and La Niña (1998). The upper images were produced using sea surface height measurements made by the U.S.-French TOPEX/Poseidon satellite. They show variations in sea surface height relative to normal conditions as an indicator of the amount of heat stored in the ocean. The two lower images show variability in chlorophyll concentration relative to normal levels as a measure of phytoplankton biomass. These were produced using data from SeaWiFS. In 1997 the warm surface water in the eastern Pacific (shown in white in the upper figure) was 14 to 32 cm (6 to 13 in.) higher than normal and about 10 cm (4 in.) above normal in the red areas. The same waters were abnormally low in chlorophyll (shown in blue in the lower image) because the supply of nutrients from upwelling was greatly reduced. This El Niño condition results in the well-known absence of fish off the west coast of South America. The images for 1998 show the low sea level or a cold pool of water (shown in purple in the upper image) during the La Niña phase. The lower figure shows higher-than-average chlorophyll (yellow) associated with this cold pool. During La Niña, nutrients were upwelled into the cold pool, resulting in an extensive phytoplankton bloom at the equator that lasted for several months. SOURCE: NASA Jet Propulsion Laboratory (top row); provided by J. Campbell and based on data from SeaWiFS Project, NASA Goddard Space Flight Center, and GeoEye (bottom row).
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Earth Observations from Space: The First 50 Years of Scientific Achievements but also provided vital information for studying global sea-level change, earthquakes, and volcanoes. Furthermore, Earth scientists from all disciplines rely on an International Earth Reference Frame from which geographical positions can be accurately described relative to the geocenter, in three-dimensional Cartesian coordinates to centimeter accuracy or better—a 2 to 3 orders of magnitude improvement compared to 50 years ago. Measured by AVHRR and SAGE (Stratospheric Aerosol and Gas Experiment), aerosols represent a geophysical variable important to Earth’s radiation budget, air quality forecasts, cloud formation affecting weather forecasts, and hydrologic applications (Chapter 4). Thus, a scientific accomplishment in one field can lead to major advances in other fields and drive interdisciplinary research efforts. The advances in understanding and predicting El Niño-Southern Oscillation (ENSO) conditions exemplify the advantage of studying the Earth as an integrated system and the benefit of combining in situ and satellite observations with modeling studies. Conclusion 3: The scientific advances resulting from Earth observations from space illustrate the successful synergy between science and technology. The scientific and commercial value of satellite observations from space and their potential to benefit society often increase dramatically as instruments become more accurate. The observational vantage point from space added a new appreciation for the complexity of many previously known Earth science processes. Because of the problem of spatial and temporal undersampling by ground-based observing tools, composing a synoptic view required interpolation across data gaps. Consequently, more complex features were averaged out through the interpolation process and not revealed until satellites observed these features directly. Similarly, the frequency of synoptic views available from daily satellite overflights made an unprecedented temporal resolution available. As altimetry measurements became accurate to the centimeter scale, they revealed how highly time dependent and essentially turbulent the ocean was, which is in contrast to the presatellite view that the ocean was primarily in steady state with slowly changing, large-scale circulation (Chapter 8). This resulted in a paradigm shift with implications for climate change research that have yet to be fully understood (Wunsch 2007). In the case of many scientific accomplishments, significant results are not solely based on satellite data but include in situ data and model components. In fact, the value of space-based observations increases with well-coordinated ground-based observations, suborbital observations, and/or cross-calibration among satellites with complementary instruments. Ground-based observations also provide an important “surface validation” for satellite data and are used to calibrate spaceborne instruments. Such surface validations become increasingly important in pushing satellite sensors to provide more quantitative and accurate measurements. Ocean buoys and drifters as well as shipboard observations have been used extensively to validate sea surface temperature, ocean color, and wind observations from satellites (Chapter 8). In addition, as satellite data have become more quantitative and more readily used by the broader research community, they have contributed to field campaigns and altered the scientific endeavor. For example, ground-based campaigns are more effectively planned and guided because of the information made available from satellite observations. Just as the synergy between satellite and ground-based observations yields new insights, so does the combination of satellite observations from different instruments. Thus, to capitalize fully on some investments in satellite sensors, simultaneous measurements are necessary. The recent analysis of the merged altimetry data set from TOPEX/Poseidon and the European Remote Sensing Satellite (ERS) revealed the prevalence of westward-propagating eddies not seen from individual sensors (Chapter 8). This discovery would not have been possible without merging the two data sets from the individual sensors. Conclusion 4: Satellite observations often reveal known phenomena and processes to be more complex than previously understood. This brings to the fore the indisputable benefits of multiple synergistic observations, including orbital, suborbital, and in situ measurements, linked with the best models available. The greatest benefit of Earth observations from space is gained when data are integrated into state-of-the-art models, combined with ground-based observation network and process studies, and analyzed with sophisticated methods. Model development has aided in developing an interdisciplinary thinking in the Earth sciences. Building sophisticated models and data analysis tools often involves long lead times and requires training of a skilled workforce. Consequently, the major scientific breakthrough might follow years after the satellite data have first become available. To capitalize fully on the investment, satellite data also require careful calibration (NRC 2004). In addition, building long-term data records for climate research requires cross-and intercalibration between various sensors and follow-on missions, data processing and archiving, and maintenance of the metadata (NRC 2004). To develop the aforementioned infrastructure and data assimilation and analysis tools, scientists need to be trained in using and analyzing satellite data. Thus, investment in training and supporting a remote sensing community is important to guaranteeing scientific advances from satellite data (NRC 2007a). Attracting young scientists to the field of remote sensing is made easier by the prospect of stability in the satellite data supply. In contrast, data gaps may result in the loss of a highly specialized workforce (NRC 2007a). The
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Earth Observations from Space: The First 50 Years of Scientific Achievements full benefit of satellite data is only realized when a robust scientific community is trained to use the data to address fundamental and applied research questions. The Landsat story, described in numerous accounts (e.g., NRC 2002), is a case in point: wholesale commercialization of the data led to a precipitous drop in their use for science and commercial applications, which recovered upon return to the earlier policy that made data access affordable. Only when academic, government, and commercial scientists are given liberal access to data and a sufficient number are trained in the effective use of these data will the analysis tools mature to the benefit of all parties. Similarly, obtaining the maximum benefit from weather satellites required a decade-long process of improving methods of radiance data assimilation (Lord 2006; see Chapter 3). Conclusion 5: The full benefits of satellite observations of Earth are realized only when the essential infrastructure, such as models, computing facilities, ground networks, and trained personnel, is in place. NASA’s open and free data policy has created a worldwide linked community of Earth scientists. This open-access policy encourages use of the data for scientific purposes and maximizes the potential societal benefits of the observations. The long list of accomplishments is unlikely to have materialized without this open data policy that encouraged the growth of the field (NRC 2004). As previously mentioned, when the Landsat program was privatized during the late 1980s and early 1990s, the data became so costly that it severely hampered the research program (Malakoff 2000), illustrating the importance of maintaining free or affordable data streams. Open access also increases the societal benefits of the data by allowing nations without the observational capabilities of the developed world to gain access to important environmental observations. The Famine Early Warning System Network, although developed by a U.S. agency, is an example of such an application that aids developing nations in resource management without having to first build the ground-based observational capabilities. Consequently, data sharing among agencies and other countries leads to more than the sum of its parts, particularly if nations with Earth-orbiting satellites collaborate on an international strategy regarding the important satellite missions and data needs to observe the Earth system (NRC 2007a). Conclusion 6: Providing full and open access to global data to an international audience more fully capitalizes on the investment in satellite technology and creates a more interdisciplinary and integrated Earth science community. International data sharing and collaborations on satellite missions lessen the burden on individual nations to maintain Earth observational capacities. OPPORTUNITIES FOR THE FUTURE OF EARTH OBSERVATIONS FROM SPACE Fifty years from now a report similar to this one is likely to describe many more astounding discoveries about the Earth system, if the commitment to satellite observations from space is sustained. Although this report provides an extensive sampling of important accomplishments enabled by Earth satellite data, many scientific questions and societal challenges remain unresolved, including improving 10-day weather forecasts, more accurately forecasting hurricane intensity, increasing resolution of earthquake fault systems and volcanoes to detect precursors of events, mitigating climate change impacts, and protecting natural resources (NRC 2007a). Because the critical infrastructure to make the best use of satellite data takes decades to build and is now in place, the scientific community is poised to make significant progress toward understanding and predicting the complexity of the Earth system. However, building a predictive capability relies strongly on the availability of seamlessly intercalibrated long-term data records, which can only be maintained if subsequent generations of satellite sensors overlap with their predecessors. Unfortunately, the current capability to observe Earth from space is jeopardized by delays in and lack of funding for many critical satellite missions (NRC 2007a). Because important climate data records and important Earth-observing missions are at risk of suffering detrimental data gaps or of being cut altogether, the committee strongly agrees with the following recommendation by the decadal survey (NRC 2007a): The U.S. government, working in concert with the private sector, academe, the public, and its international partners, should renew its investment in Earth-observing systems and restore its leadership in Earth science and applications. To sustain the rate of scientific discovery and advances, committing to the maintenance of long-term observing capacities and to innovation in observing technology is equally important. Because past observations taught scientists that the Earth is a highly dynamic system and not as predictable as initially assumed, long-term observations are required if humans wish to understand and predict future changes. Future advances will be associated with tremendous societal benefits, given the current challenges presented, for example, by climate change and loss of biodiversity. One can envision the availability of regional annual climate predictions to assist in water resource management, an infectious disease early warning system, operational use of air pollution maps, and improved ability to foresee volcanic eruptions or earthquakes (NRC 2001a, 2007a). The committee strongly agrees with the following lines from the interim report of the decadal survey (NRC 2005):
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Earth Observations from Space: The First 50 Years of Scientific Achievements Understanding the complex, changing planet on which we live, how it supports life, and how human activities affect its ability to do so in the future is one of the greatest intellectual challenges facing humanity. It is also one of the most important challenges for society as it seeks to achieve prosperity, health, and sustainability. If the nation’s commitment to continue Earth observations from space is renewed, we have seen just the beginning of an era of Earth observations from space, and a report in 50 years will be able to highlight many more valuable scientific achievements and discoveries. Conclusion 7: Over the past 50 years, space observations of the Earth have accelerated the cross-disciplinary integration of analysis, interpretation, and, ultimately, our understanding of the dynamic processes that govern the planet. Given this momentum, the next decades will bring more remarkable discoveries and the capability to predict Earth processes, critical to protect human lives and property. However, the nation’s commitment to Earth satellite missions must be renewed to realize the potential of this fertile area of science.