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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 CONTRIBUTIONSTO 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