Earth Observations from Space: The First 50 Years of Scientific Achievements (2008)

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Summary Just as the invention of the mirror allowed humans to see their A Unique Vantage Point own image with clarity for the first time, Earth observations from space have allowed humans to see themselves for the The 1957-1958 International Geophysical Year (IGY), first time living on and altering a dynamic planet. with 67 participating nations, was an unprecedented effort referred to by noted geophysicist Sydney Chapman (1888- Observing Earth from space over the past 50 years has 1972) as “the common study of our planet by all nations for fundamentally transformed the way people view our home the benefit of all.” Teams of observers were deployed around planet. The image of the “blue marble” (Figure S.1) is taken the globe—some to the ends of the Earth in polar regions, for granted now, but it was revolutionary when taken in 1972 on high mountaintops, and at sea—to study Earth processes. by the crew on Apollo 17. Since then the capability to look The effort in Antarctica alone involved hundreds of people at Earth from space has grown increasingly sophisticated and in logistically complex and expensive expeditions. Even has evolved from simple photographs to quantitative measure- in 1957 it was recognized that satellite data would provide ments of Earth properties such as temperature, concentrations observations of the Earth system that no amount of ground- of atmospheric trace gases, and the exact elevation of land based observations could achieve. During the IGY the Soviet and ocean. Imaging Earth from space has resulted in major Union launched the world’s first satellite, Sputnik, in October scientific accomplishments; these observations have led to 1957 and transformed the Earth science endeavor. Shortly new discoveries, transformed the Earth sciences, opened new thereafter, the United States launched its first satellite, avenues of research, and provided important societal benefits Explorer 1, in January 1958. Over the course of the next five by improving the predictability of Earth system processes. decades, an array of satellites have been launched that have This report highlights the scientific achievements made fundamentally altered our understanding of the planet. Today possible by the first five decades of Earth satellite observa- from the comfort of their desks, Earth scientists can acquire tions by space-faring nations. It follows on a recent report global satellite data with orders of magnitude greater spatial from the National Research Council (NRC) entitled Earth and temporal coverage than obtained during the intensive Science and Applications from Space: National Imperatives field expeditions of the IGY. for the Next Decade and Beyond, also referred to as the The global view obtained routinely by observations from “decadal survey.” Recognizing the increasing need for space space is unmatched in its ability to resolve the dynamics observations, the decadal survey identifies future directions and the variability of Earth processes. Ship-based observa- and priorities for Earth observations from space. This com- tions, for example, cannot provide the spatial and temporal panion report was requested by the National Aeronautics and information to detect the dynamic nature of the ocean. Space Administration (NASA) to highlight, through selected Similarly, aircraft and weather balloon measurements alone examples, important past contributions of Earth observations cannot resolve the details required to understand the complex from space to our current understanding of the planet. dynamics of ozone depletion. Space observations provide detailed quantitative information on many atmospheric, oceanic, hydrologic, cryospheric, and biospheric processes. Because satellite information is gathered at regular intervals,  National Research Council (NRC). 2007. Earth Science and Applica- it provides, like a movie, a view of changes over time. For tions from Space: National Imperatives for the Next Decade and Beyond. the first time, satellites make it possible to track a tropical The National Academies Press, Washington, D.C.  

 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS is to watch the weather in motion in a sequence of satellite images (e.g., http:// www.goes.noaa.gov). These images are captured by geostationary satellites, first positioned over the equator in the mid-1960s. They collect frequent pho- tographs at various wavelengths, from which the moving pictures of weather can be assembled. Geostationary satel- lites rapidly became a major source of data for weather services worldwide, which is now essential to air traffic management, disaster preparedness, agriculture, and many other everyday applications. Fundamental Contributions to Science This report describes many exam- ples of scientific accomplishments from satellite observations that have trans- formed the Earth sciences, some of which are highlighted in this summary (Table S.1). Few are as transformative as the advances in space geodesy over the past five decades, particularly with the ubiquitous introduction of Global Figure S.1  The blue marble as seen by the crew of Apollo 17. Image (AS17-148-22727) Positioning System (GPS) devices, courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center. which have brought geodetic position- SOURCE: http://eol.jsc.nasa.gov. ing to everyday life. At the time of the IGY, the geolocation of most points at the surface of Earth entailed errors that reached hundreds of meters in remote cyclone from its gestation over the ocean to landfall and to areas. Today, scientists rely on an International Earth Ref- observe the ever-fluctuating intensity of the storm. erence Frame from which geographical positions can be Over the past two decades, this dynamic global view has described relative to the geocenter, in three-dimensional radically transformed our understanding of ice sheets. Before Cartesian coordinates to centimeter accuracy or better—a satellites, Antarctica’s and Greenland’s ice sheet mass bal- two to three orders-of-magnitude improvement compared ance was assumed to be controlled by the difference between to 50 years ago. This is even more remarkable considering ice melting and accumulation rates, and the rate of ice dis- it is accomplished on an active planet whose surface is con- charge into the ocean was assumed to be constant. Satellite stantly in motion. The change in position of the rotation axis radar images from RADARSAT revealed that (1) the velocity (the poles) is determined daily to centimeter accuracy, and of ice sheet flow is highly variable, (2) there exist complex the changes in length of day are determined to millisecond networks of ice streams, and (3) the velocity of ice stream accuracy within a few hours. Inexpensive GPS receivers flow toward the sea has increased measurably in response are now taken for granted by consumers who are rapidly to climate change. The collapse of the Larsen B Ice Shelf becoming accustomed to GPS navigation on the road, on in Antarctica in 2002—captured only because of frequent the water, or in the air without realizing the enormous body coverage by satellite imagery—dramatically illustrated the of science behind this technological achievement: accurate dynamics of ice sheets on astonishingly short timescales position information of the satellites, very stable clocks, and (Figure S.2). These revelations carry weighty implications: well-calibrated atmospheric corrections. the rapid transfer of ice from the continental ice sheets to the Satellite-derived global maps of air pollution caused a sea could result in a significant rise of sea level. major change in concepts of pollution control by demonstrat- One of the most effective ways to illustrate the impact ing its transport between nations and continents that. The that observations from space have had on weather forecasting first tropospheric ozone maps from space in the 1980s drew 

SUMMARY  attention to human impacts on the atmosphere, especially in bromide in sea ice and sea salt particles) and nitrogen and the tropics where agricultural fires and land-use changes alter sulfur oxides (from urban regions, power plants, and smelt- ozone in the lower atmosphere. Newer satellites show plumes ing operations). In combination with numerical models, the of ozone, aerosols, and gases such as carbon monoxide span- global sources of these gases can be mapped and tracked. ning oceans and linking continents. Therefore, pollution is Climate science has also advanced spectacularly through now viewed as a global, not a local, phenomenon. Quantita- satellite observations. The radiometer flown on Explorer 7 tive information from satellites provides data for modeling from 1959 to 1961 made it possible for the first time to efforts to predict coupled atmospheric chemical and climate directly measure the energy entering and leaving Earth. changes with greater confidence. Orbital sensors precisely This and follow-on missions enabled scientists to mea- locate sources of ozone-destroying bromine monoxide (from sure Earth’s energy balance with much greater confidence a 31 Jan 2002 b 17 Feb 2002 c 23 Feb 2002 d 05 Mar 2002 Figure S.2  Collapse of the Larsen B Ice Shelf in Western Antarctica, January-March 2002. Two thousand square kilometers of the ice shelf disintegrated in just 2 days. SOURCE: National Snow and Ice Data Center. 7-3 a,b,c,d 

 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS Table S.1  Examples of the Scientific Accomplishments of Earth Observations and Landmark Satellites That Have Contributed to Each Accomplishment Satellite Monitoring global stratospheric ozone depletion, including Antarctica and Arctic regions TIROS series, Nimbus 4 and 7, ERS 1, Envisat Detecting tropospheric ozone Nimbus 7, ERS 2, Envisat, Aqua, Aura, MetOp Measuring the Earth’s radiation budget Explorer 7, TIROS, and Nimbus Generating synoptic weather imagery TIROS series, ATS, SMS Assimilating data for sophisticated numerical weather prediction Numerous weather satellites, including the TIROS series and NOAA’s GOES and POES Discovering the dynamics of ice sheet flows in Antarctica and Greenland RadarSat, InSAR, Landsat, Aura, and Terra Detecting mesoscale variability of ocean surface topography and its importance in ocean mixing Topex/Poseidon Observing the role of the ocean in climate variability TIORS-N and NOAA series Monitoring agricultural lands (a contribution to the Famine Early Warning System) Landsat Determining the Earth reference frame with unprecedented accuracy Lageos, GPS c ­ ompared to earlier indirect estimates resulting in improved thus have revolutionized our understanding of basin-scale climate models. Over the years, as radiometers improved, interannual variability, such as El Niño events. Altimetry these measurements achieved the precision, spatial resolu- observations also improved our understanding of mean tion, and global coverage necessary to observe directly the ocean circulation. The newly discovered prevalence of ocean perturbations in Earth’s global energy budget associated with eddies revolutionized the way oceanographers think about short-term events such as major volcanic eruptions or the El the mesoscale energy sources for deep-ocean mixing. The Niño-Southern Oscillation (ENSO). In addition, radiometers new paradigm is that of a very dynamic, turbulent system, in orbit nearly continuously since the 1960s directly measure with the energy primarily provided by winds and tides that the equator-to-pole heat transport by the climate system, the are variable on many timescales. greenhouse effect of atmosphere trace gases, and the effect of clouds on the energy budget of Earth. These observa- Societal Applications of Satellite DATA tions advance our understanding of the climate system and improve climate models. The most broadly used products from satellites are Another important contribution to climate science was weather observations that enable forecasts. Since satellite made by the long-term record of sea surface temperature images have become readily available, no tropical cyclone (SST) from the Advanced Very High Resolution Radiom- (hurricane or typhoon) has gone undetected, which provides eter (AVHRR) flown on the Television Infrared Observation affected coastal areas with advance warning and crucial time Satellite series (TIROS-N) and the National Oceanic and to prepare. This exemplifies not only how satellite observa- Atmospheric Administration (NOAA) satellite series. As the tions have transformed the Earth sciences but also how the longest oceanographic data record from remote sensing, it improved predictability of Earth processes can provide direct had broad impact. The SST record exposed the role of the societal benefits. Weather forecasts more than a few hours ocean in regional and global climate variability and revealed into the future are made with the aid of numerical weather important details about ocean currents. Trend analysis of the prediction models. By assimilating satellite observations, SST record provided evidence for global warming as 80 per- which yield dramatically improved and continually updated cent of the excess heat is entering the ocean and also helped knowledge of the state of the atmosphere, meteorologists can improve understanding of the important climate-atmosphere devise models that project the weather into the future with feedbacks in the tropics that are also responsible for ENSO much improved accuracy compared to presatellite forecasts. events. Understanding the increase in SST and anthropogenic Consequently, 7-day forecasts have improved significantly in heat input to the surface ocean also has important ramifica- accuracy over the past decades, particularly for the relatively tions for quantifying and predicting sea-level rise in response data-sparse southern hemisphere. Needless to say, these to global warming. improvements in forecast skills are saving countless human Very accurate measurements of sea surface heights by lives and have an enormous economic value (saving the the Topographic Experiment (Topex)/Poseidon altimeter energy sector alone hundreds of millions of dollars). have revolutionized our understanding of ocean dynamics. The ability to detect land-cover changes at all spatial These observations allow scientists to characterize the scales and energy of mesoscale features at a global scale and  Anomaly correlation of 500 hPa height forecasts for medium-range forecasts improved from 30 to 70 percent in the southern hemisphere (~45 to 70 percent in the northern hemisphere) between 1981 and 2006 (Simmons  In the size range of 10-100 km. and Hollingsworth 2002). 

SUMMARY  scales from space has also produced extraordinary societal series (1980s) provided the first global maps of ozone benefits. The phenomenal advantage brought by satellite depletion by man-made chlorine- and bromine-containing information in monitoring and, more importantly, enabling compounds released to the atmosphere. These observations forecasts of the productivity of large-area crops was dem- combined with field studies, were critical to the develop- onstrated in the early 1970s. Since then federal agencies ment and adaptation of the Montreal Protocol, opened to such as the U.S. Department of Agriculture have routinely signatures in 1987, and subsequent amendments designed used multispectral satellite imagery—offered by the Landsat to phase out ozone-destroying halogenated compounds. series and other missions—in crop commodity forecasting. Since then satellite observations from newer platforms (Aura A particularly noteworthy application is the Famine Early series) have continued to verify model predictions of the Warning System Network, which was initially set up in Sub- amounts and distributions of the causative agents. They also Saharan Africa and now operates in other arid environments track variations in the size and depth of the annual Antarctic of the developing world. This system uses satellite images ozone hole (Figure S.3) and the more subtle but dangerous in conjunction with ground-based information to predict and losses of ozone over heavily populated midlatitudes. Recent mitigate famines. satellite observations show a decrease in chlorine-containing Another example of an important societal benefit gained gases and the apparent beginning of an ozone recovery at from satellite observations is the continuous observation of midlatitudes, yielding increased confidence that the Montreal stratospheric ozone. Satellite observations from the ­Nimbus Protocol is indeed achieving its goal. 21 September 1991 CIO O3 FIGURE S.3  Chlorine monoxide (ClO; left panel) and stratospheric ozone (O3; right panel) columns over the southern hemi- sphere ­ measured by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) for days during the austral springs of 1991 and 1992. These images show that high ClO concentrations coincide in space and time with low O3 con- 20 September 1992 centrations confirming ground-based mea- surements and the proposed mechanisms for ozone depletion. The white circle over CIO (1019 molecules m-2) the pole indicates the area where no data is 0.0 0.5 1.0 1.5 2.0 2.5 3.0 available. SOURCE: Waters et al. (1993). O3 (Dobson Units) Reprinted with permission from Macmillian 120 140 160 180 200 220 240 260 280 300 320 340 Publishers Ltd., copyright 1993. 

 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS Infrastructure requirements to Based on its review of important scientific accomplish- AdvancE Science ments, the committee concludes the following (for a detailed description, see Chapter 12): Earth observations from space demonstrate the success­ ful synergy between science and technology. As scientists 1. The daily synoptic global view of Earth, uniquely have gained experience in studying Earth through satel- available from satellite observations, has revolutionized lite observations, they have defined new technological Earth studies and ushered in a new era of multidisci- needs, helped drive technological development to provide plinary Earth sciences, with an emphasis on ­dynamics at more quantitative and accurate measurements, and have all accessible spatial and temporal scales, even in remote advanced more sophisticated methods to interpret satellite areas. This new capability plays a critically important data. To ­ capitalize fully on the investment made in Earth- role in helping society manage planetary-scale resources o ­ rbiting observing platforms and make the best use of these and environmental challenges. observations, satellite data require careful calibration and 2. To assess global change quantitatively, synoptic sophisticated analysis and assimilation tools. Optimal data data sets with long time series are required. The value of p ­ rocessing can be undertaken only if a suitably trained the data increases significantly with seamless and inter- workforce is in place to develop these tools and interpret the calibrated time series, which highlight the benefits of observations. In this respect, full and open access to satellite follow-on missions. Further, as these time series lengthen, data is crucial because training and maintaining the required historical data sets often increase in scientific and societal workforce is possible only if the data are continuously acces- value. sible to the broad scientific community. 3. The scientific advances resulting from Earth The concept of open data access was adopted by the IGY observations from space illustrate the successful synergy when establishing the World Data Center System 50 years between science and technology. The scientific and com- ago, and it is even more meaningful today than at the time mercial value of satellite observations from space and of the Cold War. This does not preclude commercialization their potential to benefit society often increase dramati- of some aspects of useful data product development, but the cally as instruments become more accurate. portion of carefully calibrated low-level data that is properly 4. Satellite observations often reveal known a public good should be made available to all stakeholders at p ­ henomena and processes to be more complex than previ- no more than the cost of reproduction. The Landsat story is a ously understood. This brings to the fore the indisputable case in point: wholesale commercialization of the data led to benefits of multiple synergistic observations, including a precipitous drop in their use for both scientific and commer- orbital, suborbital, and in situ measurements, linked with cial applications, which recovered upon return to the earlier the best models available. open data access policy. Only when academic, government, 5. The full benefits of satellite ­observations of Earth and commercial scientists are given liberal access to the data, are realized only when the essential infrastructure, such and when a sufficient number of scientists are trained in the as models, computing facilities, ground networks, and effective use of these data, will the analysis tools mature to trained personnel, is in place. the benefit of all parties. Our 50-year experience with passive 6. Providing full and open access to global data to (e.g., optical) and active (e.g., radar or lidar [light detection an international audience more fully capitalizes on the and ranging]) surface imagery, weather satellites, and plan- investment in satellite technology and creates a more etary field measurements shows that the maturation process interdisciplinary and integrated Earth science commu- of these tools requires decades. nity. International data sharing and collaborations on satellite missions lessen the burden on individual nations conclusions to maintain Earth observational capacities. 7. Over the past 50 years, space observations of The first 50 years of Earth observations from space the Earth have accelerated the cross-disciplinary inte- imparted the fundamental lessons that everything—land, gration of analysis, interpretation, and, ultimately, our ocean, and atmosphere—is intricately intertwined and that understanding of the dynamic processes that govern the the Earth is a complex and dynamic system. In addition, planet. Given this momentum, the next decades will bring “each [satellite] mission taught scientists not only something more remarkable discoveries and the capability to pre- new about the Earth system, but also something new about dict Earth processes, critical to protect human lives and how to create, operate, and improve the technology for property. However, the nation’s commitment to Earth observing the Earth from space.” satellite missions must be renewed to realize the potential of this fertile area of science. Because the critical infrastructure to make the best use NASA Earth Observatory, http://earthobservatory.nasa.gov/Study/­  Nimbus 1. of satellite data takes decades to build and is now in place, 

SUMMARY  the scientific community is poised to make great progress observations from space be renewed. Resources will be toward understanding and predicting the complexity of the required to maintain the current momentum and not risk Earth system. However, building a predictive capability relies losing the workforce and infrastructure built over the past strongly on the availability of intercalibrated long-term data decades. Given the many scientific challenges ahead, we records, which can only be maintained if subsequent genera- have seen only the beginning of an era of Earth observations tions of satellite sensors overlap with their predecessors. As from space. A report in 50 years will present many more the decadal survey points out, the capability to observe Earth significant achievements and discoveries and highlight how from space is jeopardized by delays in and lack of funding satellites played a vital role in observing the dynamics of the for many critical satellite missions. Earth system and in guiding our nation and others in meeting The decadal survey and this committee both recom- the challenges posed by global changes. mend that the nation’s commitment to continue Earth