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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12 Conclusions coverage than obtained during the intensive field expeditions Just as the invention of the mirror allowed humans to see their own image with clarity for the first time, Earth observations of the IGY from the comfort of their desktops. from space have allowed humans to see themselves for the The advent of satellites revolutionized the Earth sci- first time living on and altering a dynamic planet. ences. 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 THE EMERgENCE OF INTEgRATED coverage not available with ground measurements. Time EARTH SYSTEM SCIENCE series data revealed large-scale processes and features that During the International Geophysical Year (IGY) of could not have been discovered by other ways. Prior to the 1957-1958, 67 nations cooperated in an unprecedented effort availability of satellite-based observations, scientists seek- to study the Earth. In an age otherwise characterized by ing global perspectives from largely ground-based observa- Cold War tensions, the noted geophysicist Sydney Chapman tions were required to develop international collaborations (1888-1972) referred to the IGY as “the common study of and launch large-scale field campaigns. Piecing together our planet by all nations for the benefit of all.” This global data points required interpolation and extrapolation to fill effort laid the foundation for the integration of Earth sci- data gaps, particularly for remote locations. In addition, ences and demanded widespread simultaneous observations. large-scale sampling efforts involved extensive logistics It involved large teams of observers, many of whom were and advance planning, which prohibited frequent repetition. deployed to the ends of Earth—in polar regions, on high Because the rate of change of many parameters of interest mountaintops, and at sea—to study meteorology, oceanog- is much greater than the rate at which global maps could be raphy, glaciology, ionospheric physics, aurora and airglow, produced in the presatellite era, it was impossible to observe seismology, gravity, geomagnetism, solar radiation, and the full dynamics of the system. cosmic rays. Even in 1957 it was recognized that satellite Therefore, the unique and revolutionary vantage point data would bring observations of Earth that no amount of from space provides scientists with global images and maps ground-based observations could achieve. of parameters of interest unmatched by any ground-based Hundreds of sounding rockets were launched into the observing technology in terms of frequency and coverage. upper atmosphere and near space during the IGY, and the Because satellites collect data continuously and allow for “space age” officially began with geophysical satellites, daily (or at least monthly averaged) global images, changes although still in their infancy, playing an important role can be observed at the relevant temporal and spatial scale (Chapter 2). During the IGY the Soviet Union launched the required to detect Earth system processes. The full dynamics world’s first satellite, Sputnik, in October 1957. The United of the system have only been observed or characterized States launched its first satellite, Explorer 1, shortly thereafter since the advent of satellite observations and have allowed in January 1958. Over the course of the next five decades, the the study of previously inaccessible phenomena such as United States and its international partners have launched an stratospheric ozone creation and depletion, the transport of array of satellites that fundamentally altered our understand- air pollution across entire ocean basins from China to the ing of the planet. A half-century later, Earth scientists can continental United States (Chapter 5), global energy fluxes acquire global satellite data with orders of magnitude greater (Chapter 4), ice sheet flow (Chapter 7), global primary pro- 

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 CONCLUSIONS ductivity (Chapter 9), ocean currents and mesoscale features summer ice over the past decades (Chapter 7). Satellite (Chapter 8), and global maps of winds (Chapter 8). Prior to observations have become available and matured as scientific the satellite era, even if it was possible to compose a global data at a time when they are critically important in helping picture from individual surface observations (e.g., through society manage planetary-scale resources and environmental the World Weather Watch, established in 1963), the coverage challenges. Although many scientific challenges remain, it is and density of the network and lack of vertical resolution undeniable that satellite observations have allowed scientists left much to be desired. Other geophysical and biological to improve the ability to monitor and predict changes in the phenomena were sampled much less frequently, often as a Earth system and manage life on Earth (NRC 2007a). partial “snapshot” of an otherwise dynamic set of interacting It is widely known that satellite data, particularly from Earth processes. the southern hemisphere, have contributed to improvements Discovery of the variability in the velocity of ice sheet in weather prediction, resulting in protection of human lives flow is another example of how the dynamics of the sys- and infrastructure (Chapter 3). Since the availability of sat- tem went undetected until reliable and repeated satellite ellite images, no tropical cyclone has gone undetected, and observations became available (Chapter 7). This discovery the advance warning allows crucial time to prepare. In fact, revolutionized the study of ice sheet flow and yielded an the advent of satellites has been heralded as unquestionably important realization: sea-level change due to freshwater “the greatest single advancement in observing tools for input from the continental ice sheets was not a function of tropical meteorology” (Sheets 1990). Furthermore, because the balance between ice sheet melting and precipitation at satellite data give access to the largely undersampled ocean, higher elevation, but a function of the flow dynamics. The hurricane track forecasts have improved dramatically, help- increasing velocity of continental ice flow into the ocean in ing save lives and property every year (Considine et al. response to climate change and the collapse of the Larsen B 2004). Other aspects of human welfare have and will also Ice Shelf emphasized the sensitivity of ice sheet dynamics benefit from satellite observations. For example, it is also to a changing climate. unlikely that a famine early warning system would be avail- Satellite sensors provide a panoptic viewpoint, yet his- able to assist in planning aid distribution without the ability torically they suffered from poor resolution and calibration to observe vegetation cover and the availability of water problems. On the other hand, ground-based instruments, resources from space (Chapter 10). Given the projected although more precise and better calibrated, are limited to climate change and associated sea-level rise, having global their particular locales, and problems arise since they must be satellite coverage available in the future will serve crucial coordinated and intercalibrated with other ground stations. societal needs unmet by any other observing system. As satellite sensors and data processing have become more Conclusion 1: The daily synoptic global view of sophisticated, equaling or surpassing those for ground-based Earth, uniquely available from satellite observations, has measurements, scientists have obtained not only images but revolutionized Earth studies and ushered in a new era of also quantitative global measurements of unprecedented multidisciplinary Earth sciences, with an emphasis on precision. Intercalibration proved particularly challenging in dynamics at all accessible spatial and temporal scales, putting together global maps of marine primary productiv- even in remote areas. This new capability plays a criti- ity from shipboard measurements (Chapter 9). Estimating cally important role in helping society manage planetary- marine primary productivity requires sample manipulation scale resources and environmental challenges. and measurements of 14C uptake rates at each location, which are sensitive to variations in sampling techniques and methods. Although global marine primary productivity INTEgRATED gLOBAL VIEW OF THE CARBON CYCLE estimates had been attempted before the satellites era, they AND CLIMATE SYSTEM were flawed because of intercalibration issues. More impor- tantly, because it takes years to obtain global coverage of The global view of Earth from satellites has imparted ground-based marine primary productivity measurements, the understanding that everything is connected—land, ocean, satellites allowed for the first time observation of global and atmosphere. Interdisciplinary teams of researchers have marine primary productivity on a monthly and annual basis explored these connections to better understand the Earth and detection of decadal-scale trends. as a system beyond the sum of its elements. The concept Satellite observations also provide access to otherwise of studying the Earth as an integrated system at a national virtually inaccessible regions, such as polar regions, the upper level was led by the National Aeronautics and Space atmosphere, and the open oceans. Quantitative assessment Administration (NASA), inspired by NASA’s “Ride report” and monitoring of the sea ice extent in the Arctic has only (NASA 1987), and intended as the U.S. component to the been possible since routine satellite observations became International Geosphere-Biosphere Program. Consequently, available. Without satellite images, it is unlikely that trends in NASA launched its mission to planet Earth to study the decreasing Arctic summer sea ice would have been detected Earth’s geosphere and biosphere as an integrated system as readily, demonstrating univocally the drastic decline in instead of discrete but interrelated components (CRS 1990).

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00 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS Other nations have also made significant contributions to which in turn affects the amount of carbon dioxide (CO2) the capacity to observe Earth from space. This multinational uptake (Chapter 9); and water vapor is important as a investment has enabled much international collaboration greenhouse gas and in heat exchange processes between the among satellite projects. ocean, land, and atmosphere (Chapters 3, 4, 8, and 9). Due A prime example of an interdisciplinary research to water’s relatively high specific heat capacity and its large- endeavor is the study of the global carbon cycle, which scale circulation, the ocean plays a central role in storing and employs a wide range of research approaches such as ground transporting Earth’s heat content (Chapter 8). In fact, more and satellite observations, modeling studies, and laboratory than 80 percent of Earth’s heat is stored in the ocean. Improv- experiments. The well-known Keeling curve was obtained ing our understanding of ocean circulations and consequently from in situ observations and revealed atmosphere-biosphere the transport of heat is a major challenge to more accurate interactions, as well as the long-term trend of increasing climate models and predictions. Lastly, the above-mentioned atmospheric carbon dioxide (Keeling et al. 1976). These find- advances in understanding the global carbon cycle further the ings launched major efforts in understanding the role of the ability to predict future atmospheric CO2 levels. terrestrial and oceanic biosphere in carbon uptake through The long-term observations obtained during the past 50 photosynthesis and the impact of increased carbon dioxide years of Earth science from space combined with advances levels on global climate. However, primary productivity is in data assimilation, computer models, and ground-based controlled by geophysical processes; thus, understanding the process studies brought climate scientists to the point at interconnections, such as the effect of a changing climate which they could begin to project how climate change will and hydrologic cycle on the global biosphere and vice versa, affect weather and natural resources at the regional level, required observations at a global scale of land-cover changes the scale at which the information is of greatest societal (from Landsat and AVHRR [Advanced Very High Resolu- relevance (NRC 2001a). tion Radiometer]; see Chapter 11), biomass estimates and This comes at a time when improved understanding of primary productivity (AVHRR, CZCS [Coastal Zone Color the climate system is central to the viability of our economy, Scanner], SeaWiFS [Sea-viewing Wide Field-of-view Sen- as seasonal-to-interannual climate fluctuations strongly sor], and MODIS [Moderate Resolution Imaging Spectrora- influence agriculture, the energy sector, and water resources diometer]; see Chapters 9 and 10), changes in the hydrologic (CCSP 2003). However, important scientific challenges—for cycle (Landsat, AVHRR, MODIS, and Topography Experi- example, cloud-water feedback in climate models—must be ment (TOPEx)/Poseidon; see Chapters 6 and 7) and climate conquered with the aid of continuous satellite data before (AVHRR, MODIS, and SeaWiFS). Once the data were avail- the appropriate seasonal-to-interannual climate information able, major scientific advances came from assimilating them can be made readily available at the appropriate scale (NRC into three-dimensional coupled modeling of the atmosphere, 2007a). The Earth science community has built over the past land, ocean, and cryosphere (Fung 1986, Heiman and Keel- decades the capacity to incorporate all the pieces into an ing 1986, Fung et al. 1987, Keeling et al. 1989). integrated systems perspective, thanks to ever more sophis- Equally interdisciplinary in nature is climate change ticated models. As the community is now poised to make research. In fact, many of the accomplishments highlighted major advances in climate science and predicting climate in this report have contributed to the improved understand- changes at various scales, the ability to provide sustained ing of the climate system and laid the groundwork modeling multidecadal global measurements is crucial (NRC 1999, for projecting climate change. One notable example is the 2001b, 2007a). long-term observations of Earth’s radiation budget, which The ability to observe and predict El Niño/La Niña revealed the role of the ocean and atmosphere in transporting conditions in advance of their full manifestation based on heat and the role of aerosols from the volcanic eruption of satellite and in situ data illustrates the significant break- Mount Pinatubo in cooling the climate (Chapter 4). With the through climate scientists have made in providing impor- understanding of the importance of aerosols to the climate tant regional climate information to resource managers system comes the need to observe continuously both natural (Box 12.1, Figure 12.1). and anthropogenic sources of aerosols (Chapter 4). Satellite As many accomplishments have shown, the length and observations have also been central in revealing the role of continuity of a given data record often yield additional sci- important gases, such as water vapor and ozone, in the cli- entific benefits beyond the initial research results of the mis- mate system (Chapters 4 and 5). sion and beyond the monitoring implications for operational Long-term observations of water in each phase are agencies. For example, the effect of aerosols from a volcanic central to understanding the climate system: sea ice contrib- eruption (Mount Pinatubo) on the global climate would utes to Earth’s albedo and its decrease not only indicates a have gone undetected without the continuous observations warmer climate but is also a positive feedback (Chapter 7); of the Earth Radiation Budget Experiment (ERBE, Chapter melting of continental ice sheets contributes to sea-level 4). Thus, maintaining well-calibrated long-term data sets is rise (Chapter 7); the availability of liquid water is important likely to yield important scientific advances in understanding in controlling the productivity of the terrestrial ecosystem, the Earth system, in addition to contributing to societal appli-

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0 CONCLUSIONS cations. The importance of stable, accurate, intercalibrated, to measure the geopotential and mean sea level to determine long-term climate data records is universally recognized, and the general circulation of the oceans and resolve the spatial strategies on how to collect and maintain such data streams variations of the gravity field as a goal for geophysics and have been provided in many previous reports (NRC 1985, physical oceanography. NASA responded to this challenge 2000, 2001b, 2003, 2004). Important elements to successful by launching three satellites within 9 years following the long-term climate data from satellites include a long-term Williamstown conference, with Seasat—the third and most strategy to guarantee that follow-on missions overlap to advanced satellite—providing accurate ocean elevation allow for cross-calibrations, leadership in data stewardship with a precision to tens of centimeters. For the first time the and management, and strong interagency collaborations. bathymetry of the ocean floor could be observed from space, Follow-on missions maximize the return on previously revealing the large mid-Atlantic ridges and trenches (Chap- made investments in technology development, including sen- ter 11). As the precision of altimetry data further increased sors and data analysis tools. Missions designed for process the importance of eddies in the mixing of the open ocean was studies of initially short durations may provide significant discovered (Chapter 8). scientific value by continuing a given data record in the It is common for any given satellite or instrument in context of global change research. The value of a continuous space to supply data that may be used in multiple fields of data record increases significantly through the development Earth science by design or serendipitously (see Table A.1). of uninterrupted follow-on missions, particularly if careful Although Landsat was designed to observe changes on land, cross-calibrations between subsequent generations of satel- including the terrestrial ecosystem, assembling the approxi- lite sensors are undertaken (NRC 2004). The long-term data mately 5,000 individual images for a global time series records from Landsat and AVHRR exemplify the scientific proved to be too computationally intensive. Instead, it was value of such carefully maintained data streams (Chapters 9 AVHRR data—designed to monitor the atmosphere—that and 10). turned out to be invaluable to producing global terrestrial primary productivity estimates. Due to careful intercalibra- Conclusion 2: To assess global change quantitatively, tions between the different sensors, the AVHRR data record synoptic data sets with long time series are required. The now extends over 20 years (Chapter 8) and has allowed value of the data increases significantly with seamless and the detection of trends in terrestrial primary productivity intercalibrated time series (NRC 2004), which highlight (Chapter 9). In fact, data from AVHRR have also been used the benefits of follow-on missions. Further, as these time in many other fields to study processes such as snow cover, series lengthen, historical data sets often increase in sci- sea surface temperature, cloud optical properties, and global entific and societal value. 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 MAxIMIZINg THE RETURN ON INVESTMENT IN bands were designed to serve a diversity of user commu- EARTH OBSERVATIONS FROM SPACE nities in the Earth sciences, allowing observations of the As scientists have gained experience in studying Earth following parameters: land, cloud, and aerosol properties; through satellite observations, they have defined new tech- ocean color and marine biogeochemistry; atmospheric water nology needs, helped drive technology development to vapor; surface and cloud temperature; cloud properties; provide more quantitative and accurate measurements, and cirrus cloud water vapor; atmospheric temperature; ozone; advanced more sophisticated methods to interpret satellite and cloud top altitude. It has led to scientific breakthroughs data (Chapter 2). Many scientific accomplishments have such as discovery of the brown clouds (Chapter 4), measur- resulted from rapid satellite technology development that ing marine primary productivity annually (Chapter 9), and responded to scientific needs and provided capabilities that observation of optical depth and effective particle radius in enabled major advances in the Earth sciences. The value of low clouds (Chapter 4). Because of the potential to design satellite observations from space grows dramatically as new, missions with spectral bands that can serve many different more accurate instruments are developed. Initially, satel- scientific user communities, creating follow-on missions lites provided a means for acquiring pictures. Now, satellite that continue measurements—and thus ensure the long- image acquisition and interpretation provide quantitative term climatic data records discussed above—does not have geophysical or biological variables by transforming measure- to come at an increased cost or at the cost of research and ments of reflected or emitted electromagnetic radiation into development missions. desired parameters. For many applications such as ocean In addition, the measurement of a given variable, in and land topography, ice sheet dynamics, and concentra- some cases from multiple sensors, often contributes to tions of atmospheric gases, observations are scientifically several fields of Earth science. For example, few scientific valuable if they can be made with great accuracy, which has accomplishments are as “transformative” as the advances in driven technology evolution. For example, the Williamstown space geodesy over the past five decades (Chapter 11). This report (NASA 1970) outlines the need for satellite sensors breakthrough has not only transformed the field of geodesy

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0 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 ter- restrial 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 re- search 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. 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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0 CONCLUSIONS a b Mapped – 1997 Mapped – 1998 c d 12-1 a,b,c,d

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0 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS but also provided vital information for studying global sea- increasingly important in pushing satellite sensors to provide level change, earthquakes, and volcanoes. Furthermore, more quantitative and accurate measurements. Ocean buoys Earth scientists from all disciplines rely on an International and drifters as well as shipboard observations have been Earth Reference Frame from which geographical positions used extensively to validate sea surface temperature, ocean can be accurately described relative to the geocenter, in three- color, and wind observations from satellites (Chapter 8). In dimensional Cartesian coordinates to centimeter accuracy or addition, as satellite data have become more quantitative and better—a 2 to 3 orders of magnitude improvement compared more readily used by the broader research community, they to 50 years ago. have contributed to field campaigns and altered the scientific Measured by AVHRR and SAGE (Stratospheric Aerosol endeavor. For example, ground-based campaigns are more and Gas Experiment), aerosols represent a geophysical effectively planned and guided because of the information variable important to Earth’s radiation budget, air quality made available from satellite observations. forecasts, cloud formation affecting weather forecasts, Just as the synergy between satellite and ground-based and hydrologic applications (Chapter 4). Thus, a scientific observations yields new insights, so does the combination accomplishment in one field can lead to major advances in of satellite observations from different instruments. Thus, other fields and drive interdisciplinary research efforts. The to capitalize fully on some investments in satellite sensors, advances in understanding and predicting El Niño-Southern simultaneous measurements are necessary. The recent analy- Oscillation (ENSO) conditions exemplify the advantage of sis of the merged altimetry data set from TOPEx/Poseidon studying the Earth as an integrated system and the benefit of and the European Remote Sensing Satellite (ERS) revealed combining in situ and satellite observations with modeling the prevalence of westward-propagating eddies not seen from studies. individual sensors (Chapter 8). This discovery would not have been possible without merging the two data sets from Conclusion 3: The scientific advances resulting from the individual sensors. Earth observations from space illustrate the successful synergy between science and technology. The scientific Conclusion 4: Satellite observations often reveal and commercial value of satellite observations from known phenomena and processes to be more complex space and their potential to benefit society often increase than previously understood. This brings to the fore the dramatically as instruments become more accurate. indisputable benefits of multiple synergistic observations, including orbital, suborbital, and in situ measurements, linked with the best models available. 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 The greatest benefit of Earth observations from space and temporal undersampling by ground-based observing is gained when data are integrated into state-of-the-art tools, composing a synoptic view required interpolation models, combined with ground-based observation net- across data gaps. Consequently, more complex features work and process studies, and analyzed with sophisticated were averaged out through the interpolation process and methods. Model development has aided in developing an not revealed until satellites observed these features directly. interdisciplinary thinking in the Earth sciences. Building Similarly, the frequency of synoptic views available from sophisticated models and data analysis tools often involves daily satellite overflights made an unprecedented temporal long lead times and requires training of a skilled workforce. resolution available. As altimetry measurements became Consequently, the major scientific breakthrough might accurate to the centimeter scale, they revealed how highly follow years after the satellite data have first become avail- time dependent and essentially turbulent the ocean was, able. To capitalize fully on the investment, satellite data also which is in contrast to the presatellite view that the ocean was require careful calibration (NRC 2004). In addition, building primarily in steady state with slowly changing, large-scale long-term data records for climate research requires cross- circulation (Chapter 8). This resulted in a paradigm shift with and intercalibration between various sensors and follow-on implications for climate change research that have yet to be missions, data processing and archiving, and maintenance of fully understood (Wunsch 2007). the metadata (NRC 2004). In the case of many scientific accomplishments, signifi- To develop the aforementioned infrastructure and data cant results are not solely based on satellite data but include assimilation and analysis tools, scientists need to be trained in situ data and model components. In fact, the value of in using and analyzing satellite data. Thus, investment in space-based observations increases with well-coordinated training and supporting a remote sensing community is ground-based observations, suborbital observations, and/or important to guaranteeing scientific advances from satellite cross-calibration among satellites with complementary instru- data (NRC 2007a). Attracting young scientists to the field of ments. Ground-based observations also provide an important remote sensing is made easier by the prospect of stability in “surface validation” for satellite data and are used to calibrate the satellite data supply. In contrast, data gaps may result in spaceborne instruments. Such surface validations become the loss of a highly specialized workforce (NRC 2007a). The

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05 CONCLUSIONS OPPORTUNITIES FOR THE FUTURE OF full benefit of satellite data is only realized when a robust EARTH OBSERVATIONS FROM SPACE scientific community is trained to use the data to address fundamental and applied research questions. Fifty years from now a report similar to this one is The Landsat story, described in numerous accounts (e.g., likely to describe many more astounding discoveries about NRC 2002), is a case in point: wholesale commercialization the Earth system, if the commitment to satellite observations of the data led to a precipitous drop in their use for science from space is sustained. Although this report provides an and commercial applications, which recovered upon return extensive sampling of important accomplishments enabled to the earlier policy that made data access affordable. Only by Earth satellite data, many scientific questions and societal when academic, government, and commercial scientists challenges remain unresolved, including improving 10-day are given liberal access to data and a sufficient number are weather forecasts, more accurately forecasting hurricane trained in the effective use of these data will the analysis intensity, increasing resolution of earthquake fault systems tools mature to the benefit of all parties. Similarly, obtain- and volcanoes to detect precursors of events, mitigating ing the maximum benefit from weather satellites required a climate change impacts, and protecting natural resources decade-long process of improving methods of radiance data (NRC 2007a). assimilation (Lord 2006; see Chapter 3). Because the critical infrastructure to make the best use of satellite data takes decades to build and is now in place, the Conclusion 5: The full benefits of satellite observa- scientific community is poised to make significant progress tions of Earth are realized only when the essential infra- toward understanding and predicting the complexity of the structure, such as models, computing facilities, ground Earth system. However, building a predictive capability relies networks, and trained personnel, is in place. strongly on the availability of seamlessly intercalibrated long-term data records, which can only be maintained if NASA’s open and free data policy has created a world- subsequent generations of satellite sensors overlap with wide linked community of Earth scientists. This open-access their predecessors. Unfortunately, the current capability to policy encourages use of the data for scientific purposes and observe Earth from space is jeopardized by delays in and lack maximizes the potential societal benefits of the observations. of funding for many critical satellite missions (NRC 2007a). The long list of accomplishments is unlikely to have mate- Because important climate data records and important Earth- rialized without this open data policy that encouraged the observing missions are at risk of suffering detrimental data growth of the field (NRC 2004). As previously mentioned, gaps or of being cut altogether, the committee strongly agrees when the Landsat program was privatized during the late with the following recommendation by the decadal survey 1980s and early 1990s, the data became so costly that it (NRC 2007a): severely hampered the research program (Malakoff 2000), illustrating the importance of maintaining free or affordable The U.S. government, working in concert with the data streams. private sector, academe, the public, and its international Open access also increases the societal benefits of the partners, should renew its investment in Earth-observing data by allowing nations without the observational capa- systems and restore its leadership in Earth science and bilities of the developed world to gain access to important applications. environmental observations. The Famine Early Warning System Network, although developed by a U.S. agency, is an To sustain the rate of scientific discovery and advances, example of such an application that aids developing nations committing to the maintenance of long-term observing in resource management without having to first build the capacities and to innovation in observing technology is ground-based observational capabilities. Consequently, data equally important. Because past observations taught scien- sharing among agencies and other countries leads to more tists that the Earth is a highly dynamic system and not as than the sum of its parts, particularly if nations with Earth- predictable as initially assumed, long-term observations are orbiting satellites collaborate on an international strategy required if humans wish to understand and predict future regarding the important satellite missions and data needs to changes. Future advances will be associated with tremendous observe the Earth system (NRC 2007a). societal benefits, given the current challenges presented, for example, by climate change and loss of biodiversity. One can Conclusion 6: Providing full and open access to global envision the availability of regional annual climate predic- data to an international audience more fully capitalizes tions to assist in water resource management, an infectious on the investment in satellite technology and creates a disease early warning system, operational use of air pollution more interdisciplinary and integrated Earth science com- maps, and improved ability to foresee volcanic eruptions or munity. International data sharing and collaborations on earthquakes (NRC 2001a, 2007a). satellite missions lessen the burden on individual nations The committee strongly agrees with the following lines to maintain Earth observational capacities. from the interim report of the decadal survey (NRC 2005):

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0 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS Conclusion 7: Over the past 50 years, space observa- Understanding the complex, changing planet on which we live, how it supports life, and how human tions of the Earth have accelerated the cross-disciplinary activities affect its ability to do so in the future is one of integration of analysis, interpretation, and, ultimately, the greatest intellectual challenges facing humanity. It is our understanding of the dynamic processes that govern also one of the most important challenges for society as it the planet. Given this momentum, the next decades will seeks to achieve prosperity, health, and sustainability. bring more remarkable discoveries and the capability to predict Earth processes, critical to protect human lives If the nation’s commitment to continue Earth observa- and property. However, the nation’s commitment to tions from space is renewed, we have seen just the beginning Earth satellite missions must be renewed to realize the of an era of Earth observations from space, and a report in 50 potential of this fertile area of science. years will be able to highlight many more valuable scientific achievements and discoveries.