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Earth Observations from Space: The First 50 Years of Scientific Achievements
Summary
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.
Observing Earth from space over the past 50 years has fundamentally transformed the way people view our home planet. The image of the “blue marble” (Figure S.1) is taken for granted now, but it was revolutionary when taken in 1972 by the crew on Apollo 17. Since then the capability to look at Earth from space has grown increasingly sophisticated and has evolved from simple photographs to quantitative measurements of Earth properties such as temperature, concentrations of atmospheric trace gases, and the exact elevation of land and ocean. Imaging Earth from space has resulted in major scientific accomplishments; these observations have led to new discoveries, transformed the Earth sciences, opened new avenues of research, and provided important societal benefits by improving the predictability of Earth system processes.
This report highlights the scientific achievements made possible by the first five decades of Earth satellite observations by space-faring nations. It follows on a recent report from the National Research Council (NRC) entitled EarthScience and Applications from Space: National Imperativesfor the Next Decade and Beyond,1 also referred to as the “decadal survey.” Recognizing the increasing need for space observations, the decadal survey identifies future directions and priorities for Earth observations from space. This companion report was requested by the National Aeronautics and Space Administration (NASA) to highlight, through selected examples, important past contributions of Earth observations from space to our current understanding of the planet.
A UNIQUE VANTAGE POINT
The 1957-1958 International Geophysical Year (IGY), with 67 participating nations, was an unprecedented effort referred to by noted geophysicist Sydney Chapman (1888-1972) as “the common study of our planet by all nations for the benefit of all.” Teams of observers were deployed around the globe—some to the ends of the Earth in polar regions, on high mountaintops, and at sea—to study Earth processes. The effort in Antarctica alone involved hundreds of people in logistically complex and expensive expeditions. Even in 1957 it was recognized that satellite data would provide observations of the Earth system that no amount of ground-based observations could achieve. During the IGY the Soviet Union launched the world’s first satellite, Sputnik, in October 1957 and transformed the Earth science endeavor. Shortly thereafter, the United States launched its first satellite, Explorer 1, in January 1958. Over the course of the next five decades, an array of satellites have been launched that have fundamentally altered our understanding of the planet. Today from the comfort of their desks, Earth scientists can acquire global satellite data with orders of magnitude greater spatial and temporal coverage than obtained during the intensive field expeditions of the IGY.
The global view obtained routinely by observations from space is unmatched in its ability to resolve the dynamics and the variability of Earth processes. Ship-based observations, for example, cannot provide the spatial and temporal information to detect the dynamic nature of the ocean. Similarly, aircraft and weather balloon measurements alone cannot resolve the details required to understand the complex 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, it provides, like a movie, a view of changes over time. For the first time, satellites make it possible to track a tropical
1
National Research Council (NRC). 2007. Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. The National Academies Press, Washington, D.C.
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Earth Observations from Space: The First 50 Years of Scientific Achievements
Summary
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.
Observing Earth from space over the past 50 years has fundamentally transformed the way people view our home planet. The image of the “blue marble” (Figure S.1) is taken for granted now, but it was revolutionary when taken in 1972 by the crew on Apollo 17. Since then the capability to look at Earth from space has grown increasingly sophisticated and has evolved from simple photographs to quantitative measurements of Earth properties such as temperature, concentrations of atmospheric trace gases, and the exact elevation of land and ocean. Imaging Earth from space has resulted in major scientific accomplishments; these observations have led to new discoveries, transformed the Earth sciences, opened new avenues of research, and provided important societal benefits by improving the predictability of Earth system processes.
This report highlights the scientific achievements made possible by the first five decades of Earth satellite observations by space-faring nations. It follows on a recent report from the National Research Council (NRC) entitled Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond,1 also referred to as the “decadal survey.” Recognizing the increasing need for space observations, the decadal survey identifies future directions and priorities for Earth observations from space. This companion report was requested by the National Aeronautics and Space Administration (NASA) to highlight, through selected examples, important past contributions of Earth observations from space to our current understanding of the planet.
A UNIQUE VANTAGE POINT
The 1957-1958 International Geophysical Year (IGY), with 67 participating nations, was an unprecedented effort referred to by noted geophysicist Sydney Chapman (1888-1972) as “the common study of our planet by all nations for the benefit of all.” Teams of observers were deployed around the globe—some to the ends of the Earth in polar regions, on high mountaintops, and at sea—to study Earth processes. The effort in Antarctica alone involved hundreds of people in logistically complex and expensive expeditions. Even in 1957 it was recognized that satellite data would provide observations of the Earth system that no amount of ground-based observations could achieve. During the IGY the Soviet Union launched the world’s first satellite, Sputnik, in October 1957 and transformed the Earth science endeavor. Shortly thereafter, the United States launched its first satellite, Explorer 1, in January 1958. Over the course of the next five decades, an array of satellites have been launched that have fundamentally altered our understanding of the planet. Today from the comfort of their desks, Earth scientists can acquire global satellite data with orders of magnitude greater spatial and temporal coverage than obtained during the intensive field expeditions of the IGY.
The global view obtained routinely by observations from space is unmatched in its ability to resolve the dynamics and the variability of Earth processes. Ship-based observations, for example, cannot provide the spatial and temporal information to detect the dynamic nature of the ocean. Similarly, aircraft and weather balloon measurements alone cannot resolve the details required to understand the complex 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, it provides, like a movie, a view of changes over time. For the first time, satellites make it possible to track a tropical
1
National Research Council (NRC). 2007. Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. The National Academies Press, Washington, D.C.
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Earth Observations from Space: The First 50 Years of Scientific Achievements
FIGURE S.1 The blue marble as seen by the crew of Apollo 17. Image (AS17-148-22727) courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center. SOURCE: http://eol.jsc.nasa.gov.
cyclone from its gestation over the ocean to landfall and to observe the ever-fluctuating intensity of the storm.
Over the past two decades, this dynamic global view has radically transformed our understanding of ice sheets. Before satellites, Antarctica’s and Greenland’s ice sheet mass balance was assumed to be controlled by the difference between ice melting and accumulation rates, and the rate of ice discharge into the ocean was assumed to be constant. Satellite radar images from RADARSAT revealed that (1) the velocity of ice sheet flow is highly variable, (2) there exist complex networks of ice streams, and (3) the velocity of ice stream flow toward the sea has increased measurably in response to climate change. The collapse of the Larsen B Ice Shelf in Antarctica in 2002—captured only because of frequent coverage by satellite imagery—dramatically illustrated the dynamics of ice sheets on astonishingly short timescales (Figure S.2). These revelations carry weighty implications: the rapid transfer of ice from the continental ice sheets to the sea could result in a significant rise of sea level.
One of the most effective ways to illustrate the impact that observations from space have had on weather forecasting 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 photographs at various wavelengths, from which the moving pictures of weather can be assembled. Geostationary satellites 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 examples of scientific accomplishments from satellite observations that have transformed 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 Positioning System (GPS) devices, which have brought geodetic positioning 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 areas. Today, scientists rely on an International Earth Reference Frame from which geographical positions can be described relative to the geocenter, in three-dimensional Cartesian coordinates to centimeter accuracy or better—a two to three orders-of-magnitude improvement compared to 50 years ago. This is even more remarkable considering it is accomplished on an active planet whose surface is constantly in motion. The change in position of the rotation axis (the poles) is determined daily to centimeter accuracy, and the changes in length of day are determined to millisecond accuracy within a few hours. Inexpensive GPS receivers are now taken for granted by consumers who are rapidly becoming accustomed to GPS navigation on the road, on the water, or in the air without realizing the enormous body of science behind this technological achievement: accurate position information of the satellites, very stable clocks, and well-calibrated atmospheric corrections.
Satellite-derived global maps of air pollution caused a major change in concepts of pollution control by demonstrating its transport between nations and continents that. The first tropospheric ozone maps from space in the 1980s drew
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Earth Observations from Space: The First 50 Years of Scientific Achievements
attention to human impacts on the atmosphere, especially in the tropics where agricultural fires and land-use changes alter ozone in the lower atmosphere. Newer satellites show plumes of ozone, aerosols, and gases such as carbon monoxide spanning oceans and linking continents. Therefore, pollution is now viewed as a global, not a local, phenomenon. Quantitative information from satellites provides data for modeling efforts to predict coupled atmospheric chemical and climate changes with greater confidence. Orbital sensors precisely locate sources of ozone-destroying bromine monoxide (from bromide in sea ice and sea salt particles) and nitrogen and sulfur oxides (from urban regions, power plants, and smelting operations). In combination with numerical models, the global sources of these gases can be mapped and tracked.
Climate science has also advanced spectacularly through satellite observations. The radiometer flown on Explorer 7 from 1959 to 1961 made it possible for the first time to directly measure the energy entering and leaving Earth. This and follow-on missions enabled scientists to measure Earth’s energy balance with much greater confidence
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.
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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
compared to earlier indirect estimates resulting in improved climate models. Over the years, as radiometers improved, these measurements achieved the precision, spatial resolution, and global coverage necessary to observe directly the perturbations in Earth’s global energy budget associated with short-term events such as major volcanic eruptions or the El Niño-Southern Oscillation (ENSO). In addition, radiometers in orbit nearly continuously since the 1960s directly measure the equator-to-pole heat transport by the climate system, the greenhouse effect of atmosphere trace gases, and the effect of clouds on the energy budget of Earth. These observations advance our understanding of the climate system and improve climate models.
Another important contribution to climate science was made by the long-term record of sea surface temperature (SST) from the Advanced Very High Resolution Radiometer (AVHRR) flown on the Television Infrared Observation Satellite series (TIROS-N) and the National Oceanic and Atmospheric Administration (NOAA) satellite series. As the longest oceanographic data record from remote sensing, it had broad impact. The SST record exposed the role of the ocean in regional and global climate variability and revealed important details about ocean currents. Trend analysis of the SST record provided evidence for global warming as 80 percent of the excess heat is entering the ocean and also helped improve understanding of the important climate-atmosphere feedbacks in the tropics that are also responsible for ENSO events. Understanding the increase in SST and anthropogenic heat input to the surface ocean also has important ramifications for quantifying and predicting sea-level rise in response to global warming.
Very accurate measurements of sea surface heights by the Topographic Experiment (TOPEX)/Poseidon altimeter have revolutionized our understanding of ocean dynamics. These observations allow scientists to characterize the scales and energy of mesoscale2 features at a global scale and thus have revolutionized our understanding of basin-scale interannual variability, such as El Niño events. Altimetry observations also improved our understanding of mean ocean circulation. The newly discovered prevalence of ocean eddies revolutionized the way oceanographers think about the mesoscale energy sources for deep-ocean mixing. The new paradigm is that of a very dynamic, turbulent system, with the energy primarily provided by winds and tides that are variable on many timescales.
SOCIETAL APPLICATIONS OF SATELLITE DATA
The most broadly used products from satellites are weather observations that enable forecasts. Since satellite images have become readily available, no tropical cyclone (hurricane or typhoon) has gone undetected, which provides affected coastal areas with advance warning and crucial time to prepare. This exemplifies not only how satellite observations have transformed the Earth sciences but also how the improved predictability of Earth processes can provide direct societal benefits. Weather forecasts more than a few hours into the future are made with the aid of numerical weather prediction models. By assimilating satellite observations, which yield dramatically improved and continually updated knowledge of the state of the atmosphere, meteorologists can devise models that project the weather into the future with much improved accuracy compared to presatellite forecasts. Consequently, 7-day forecasts have improved significantly in accuracy over the past decades, particularly for the relatively data-sparse southern hemisphere.3 Needless to say, these improvements in forecast skills are saving countless human lives and have an enormous economic value (saving the energy sector alone hundreds of millions of dollars).
The ability to detect land-cover changes at all spatial
2
In the size range of 10-100 km.
3
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 and Hollingsworth 2002).
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Earth Observations from Space: The First 50 Years of Scientific Achievements
scales from space has also produced extraordinary societal benefits. The phenomenal advantage brought by satellite information in monitoring and, more importantly, enabling forecasts of the productivity of large-area crops was demonstrated in the early 1970s. Since then federal agencies such as the U.S. Department of Agriculture have routinely used multispectral satellite imagery—offered by the Landsat series and other missions—in crop commodity forecasting. A particularly noteworthy application is the Famine Early Warning System Network, which was initially set up in Sub-Saharan Africa and now operates in other arid environments of the developing world. This system uses satellite images in conjunction with ground-based information to predict and mitigate famines.
Another example of an important societal benefit gained from satellite observations is the continuous observation of stratospheric ozone. Satellite observations from the Nimbus series (1980s) provided the first global maps of ozone depletion by man-made chlorine- and bromine-containing compounds released to the atmosphere. These observations combined with field studies, were critical to the development and adaptation of the Montreal Protocol, opened to signatures in 1987, and subsequent amendments designed to phase out ozone-destroying halogenated compounds. Since then satellite observations from newer platforms (Aura series) have continued to verify model predictions of the amounts and distributions of the causative agents. They also track variations in the size and depth of the annual Antarctic ozone hole (Figure S.3) and the more subtle but dangerous losses of ozone over heavily populated midlatitudes. Recent satellite observations show a decrease in chlorine-containing gases and the apparent beginning of an ozone recovery at midlatitudes, yielding increased confidence that the Montreal Protocol is indeed achieving its goal.
FIGURE S.3 Chlorine monoxide (ClO; left panel) and stratospheric ozone (O3; right panel) columns over the southern hemisphere 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 concentrations confirming ground-based measurements and the proposed mechanisms for ozone depletion. The white circle over the pole indicates the area where no data is available. SOURCE: Waters et al. (1993). Reprinted with permission from Macmillian Publishers Ltd., copyright 1993.
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Earth Observations from Space: The First 50 Years of Scientific Achievements
INFRASTRUCTURE REQUIREMENTS TO ADVANCE SCIENCE
Earth observations from space demonstrate the successful synergy between science and technology. As scientists have gained experience in studying Earth through satellite observations, they have defined new technological needs, helped drive technological development to provide more quantitative and accurate measurements, and have advanced more sophisticated methods to interpret satellite data. To capitalize fully on the investment made in Earth-orbiting observing platforms and make the best use of these observations, satellite data require careful calibration and sophisticated analysis and assimilation tools. Optimal data processing can be undertaken only if a suitably trained workforce is in place to develop these tools and interpret the observations. In this respect, full and open access to satellite data is crucial because training and maintaining the required workforce is possible only if the data are continuously accessible to the broad scientific community.
The concept of open data access was adopted by the IGY when establishing the World Data Center System 50 years ago, and it is even more meaningful today than at the time of the Cold War. This does not preclude commercialization of some aspects of useful data product development, but the portion of carefully calibrated low-level data that is properly a public good should be made available to all stakeholders at no more than the cost of reproduction. The Landsat story is a case in point: wholesale commercialization of the data led to a precipitous drop in their use for both scientific and commercial applications, which recovered upon return to the earlier open data access policy. Only when academic, government, and commercial scientists are given liberal access to the data, and when a sufficient number of scientists are trained in the effective use of these data, will the analysis tools mature to the benefit of all parties. Our 50-year experience with passive (e.g., optical) and active (e.g., radar or lidar [light detection and ranging]) surface imagery, weather satellites, and planetary field measurements shows that the maturation process of these tools requires decades.
CONCLUSIONS
The first 50 years of Earth observations from space imparted the fundamental lessons that everything—land, ocean, and atmosphere—is intricately intertwined and that the Earth is a complex and dynamic system. In addition, “each [satellite] mission taught scientists not only something new about the Earth system, but also something new about how to create, operate, and improve the technology for observing the Earth from space.”4
Based on its review of important scientific accomplishments, the committee concludes the following (for a detailed description, see Chapter 12):
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.
To assess global change quantitatively, synoptic data sets with long time series are required. The value of the data increases significantly with seamless and inter-calibrated time series, which highlight the benefits of follow-on missions. Further, as these time series lengthen, historical data sets often increase in scientific and societal value.
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.
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 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.
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.
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.
Because the critical infrastructure to make the best use of satellite data takes decades to build and is now in place,
4
NASA Earth Observatory, http://earthobservatory.nasa.gov/Study/Nimbus 1.
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Earth Observations from Space: The First 50 Years of Scientific Achievements
the scientific community is poised to make great progress toward understanding and predicting the complexity of the Earth system. However, building a predictive capability relies strongly on the availability of intercalibrated long-term data records, which can only be maintained if subsequent generations of satellite sensors overlap with their predecessors. As the decadal survey points out, the capability to observe Earth from space is jeopardized by delays in and lack of funding for many critical satellite missions.
The decadal survey and this committee both recommend that the nation’s commitment to continue Earth observations from space be renewed. Resources will be required to maintain the current momentum and not risk losing the workforce and infrastructure built over the past decades. Given the many scientific challenges ahead, we have seen only the beginning of an era of Earth observations from space. A report in 50 years will present many more significant achievements and discoveries and highlight how satellites played a vital role in observing the dynamics of the Earth system and in guiding our nation and others in meeting the challenges posed by global changes.
