A New Perspective | |||
The challenge facing Abdalati and other researchers was one of perspective. Figuring out whether the entire ice sheet is losing or gaining mass based on data collected at a handful of sites is like crawling on your belly while gazing at individual blades of grass through a microscope to decide if your lawn needs mowing. How difficult is it to interpret the meaning of individual readings or to resolve discrepancies between readings? How would you convert the information you saw through the microscope into a decision to mow or not mow? How many spots must you measure to arrive at a meaningful conclusion? If you stand up and look at the yard, you know in a single glance whether or not the grass needs to be cut. Scientists need this kind of elevated perspective of Greenland. The challenge was developing and launching satellites into orbit that were capable of collecting the data scientists need to measure the mass balance of the entire ice sheet. Abdalati explains that the problem was that the satellites in orbit a decade ago could only provide small pieces of the equation of the ice sheet’s mass balance. Those earlier data were collected over short time spans and therefore could not be pieced together to tell the whole story of what was happening to the ice sheets. For example, Landsat data provided excellent detail of the ice margin, and a means of determining how fast the ice was flowing, but without information on the thickness of the ice sheet scientists could not relate flow rates to how much ice was being discharged from the sheet’s margin. |
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Scientists could use microwave instruments to determine areas of melt because wet snow reflects microwave signals differently than dry snow. So, data from the Special Sensor Microwave/Imager (SSM/I) allowed scientists to map the extent of the wet snow on Greenland every summer, but they had no way to convert these maps of surface “wetness” to a measurement of the net loss of snow. Moreover, there was no reliable means of estimating how much snow was falling on the ice, nor could scientists determine how snowfall was varying over time, except from a limited sampling of ice cores at various locations on the ice sheet. To figure out whether the ice sheet was growing or shrinking, and by how much, scientists needed more. They needed a means of integrating all of these observations over long periods of time and, more critically, they needed to be able to observe directly how the ice thickness and mass were changing. |
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Abdalati and his colleagues pinned their hopes on two missions that would represent a new generation of spacecraft. The first was the twin satellites of the joint NASA and German Aerospace Center (DLR) Gravity Recovery And Climate Experiment (GRACE), launched in March 2002, which detect minute shifts in Earth’s gravity field based on changes in distance between the two satellites. These shifts indicate the mass below the satellites has changed. Over Greenland, the shifts would reveal whether the ice sheet as a whole gained or lost mass. |
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The second mission was NASA’s Ice, Cloud, and land Elevation Satellite (ICESat), launched in January 2003. ICESat uses a sophisticated laser altimetry system to make extremely accurate measurements of Earth’s surface elevation and how it changes over time. Changes in surface elevation of the ice sheet correlate with changes in its mass. Using these two new streams of space-based data—along with all the other information from earlier satellites, aircraft, and field observations—scientists hoped to finally answer the question: Overall, is Greenland’s ice sheet growing or shrinking? |