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"  Glaciers, at least in the popular imagination, are one of the great bellwethers of climate change: If the earth heats up, the glaciers melt and sea levels rise. But not all of these huge ice sheets react to a changing planet in the same way. A few recent studies have suggested that, while the overriding global trend in glaciers shrinking, a few oddballs in the Himalayas are actually growing for reasons unknown.  Since glaciers can&amp;#039;t exactly hop onto a scale to see whether they&amp;#039;re growing or shrinking, how do scientists know in the first place? Glaciologists have come up with a variety of ways to measure these icy behemoths. Over the past few decades, modern technologies such as satellites, aircraft, and even submarines have made monitoring glaciers easier.  Picture This Before scientists could spy on glaciers from space, they had to rely on low-tech methods of glacier measurement, including mapping the ice on foot. With the advent of early aviation, on-the-ground study gave way to aerial photography, and those early pictures are a treasure for modern glaciologists. In a study published last month in Nature Geoscience, Anders Bj\u00F8rk, a PhD fellow at the National History Museum of Denmark, used recently rediscovered photographs from a 1932 expedition to Greenland to show that glaciers shrink and grow on a much shorter time scale than we&amp;#039;d ever imagined. In his study he showed that glaciers were affected by small changes in climate over years or decades. &amp;quot;It has previously been thought that the ice sheet responds on a much longer timescale, hundreds to thousands of years, but our results show that it is possible for glaciers to shift more rapidly,&amp;quot; Bj\u00F8rk says. &amp;quot;This has been demonstrated earlier with modern data on retreating glaciers in warming events, but now we can also see it in cooling events where the glaciers advance.&amp;quot; The pictures were from the Seventh Thule Expedition, a mapping expedition led by the Danish explorer Knud Rasmusson. &amp;quot;At his time he was a superstar,&amp;quot; Bj\u00F8rk says. This superstar wasn&amp;#039;t interested in glaciers\u2014his mission was designed primarily to exert a Danish presence over Greenland, which was also claimed by the Norwegians at the time. But the explorers did have some foresight. One report of the expedition, published in 1935, read: &amp;quot;The expedition contributed also in another way to glaciological research, in that the air-photographs give an excellent picture of the conditions at the margin of the inland-ice; moreover the instantaneous record contained in the air and terrestrial photographs of the present state of the glaciers of the entire coast is in itself of great value for future researchers.&amp;quot; Old-school photography isn&amp;#039;t the most reliable thing in the world, and there isn&amp;#039;t much of it. &amp;quot;In Greenland in particular, information older that the satellite era is extremely rare, and this a very good data set that provides both a good regional coverage as well as high-quality imagery.&amp;quot; Bj\u00F8rk says. But to get more information about the inner workings of the icy behemoths, scientists need more modern methods. IceBridge Graham Cogley, a professor at Canada&amp;#039;s Trent University, says that when glaciers were first being measured and mapped, the elevation and the extent of the ice were recorded on paper maps, which were eventually input into computer systems. At first these electronic maps were rudimentary, but as the technology developed, scientists started to add better location details to glacial maps via GPS, and began traversing the glaciers on snowmobiles and sled dogs to get accurate readings of the thickness of the glacier using ground-penetrating radar. Now glaciologists are taking mapping a step further, and building computer models of glacial behavior from the information gathered from the land or the air.  Glaciologist Ian Joughin and PhD candidate Twila Moon, both of the University of Washington, study more than 200 glaciers on the Greenland ice sheet. Though they travel to Greenland occasionally, most of their data comes from satellites and aircraft. One program called Operation IceBridge, run by NASA, uses radar to measure glaciers in the Arctic and the Antarctic. The radar instruments are mounted in an aircraft that flies over ice caps and glaciers, not only taking pictures, but also measuring the ice&amp;#039;s topography, area, and speed from above. Before they start scanning the ice, the researchers draw up a specific grid of the area they are looking at, and gradually building up a library of radar images of a single area of ice. These can be compiled into maps that show how quickly or slowly glaciers are moving over time.  IceBridge is actually a stopgap measure, Moon says. NASA&amp;#039;s ICESat (Ice, Cloud, and Land Elevation Satellite) finally failed in 2010, but ICESat 2 is not expected until 2016. &amp;quot;[IceBridge is] making up for some data that we are not at this point collecting from satellites,&amp;quot; Moon says.  Mass Balance One of the main things that scientists like Joughin and Moon monitor with this equipment is the mass balance of glaciers around the world. A glacier will stay the same size if the amount of snow added onto its top is equal to the amount of ice melting or breaking off in a given year. During cool years with lots of snowfall, glaciers grow. But during warmer years, when the summer sun melts more ice than snow falls in the winter, the glacier shrinks. If the glacier speeds up, then that shrinking can accelerate. &amp;quot;They&amp;#039;re like rivers of ice speeding into the ocean&amp;quot; Joughin says. Many glaciologists describe glaciers as working like a conveyer belt, gradually flowing downhill. &amp;quot;If you speed up your conveyor belt, you have the potential to move more ice,&amp;quot; Moon says. &amp;quot;We don&amp;#039;t understand what the controls on that velocity are.&amp;quot; It could be driven by more water flowing between the ice and the ground it rests on, the temperature of the ocean, the amount of precipitation, the amount of melt on the surface, or some combination of these factors.  Speed and area can be measured by plane, but not volume. Figuring out how a glacier&amp;#039;s true mass\u2014and how it&amp;#039;s changing over time\u2014is done from space. Two Gravity Recovery And Climate Experiment (GRACE) satellites orbit the Earth, taking constant measurements of the globe&amp;#039;s gravitational field. Gravity is affected by mass, and so GRACE can measure changes in mass here on earth, including shifts in the distribution of water and ice around the planet.  GRACE gives scientists some idea of how glaciers are moving now, but glaciologists hope that newer, better tools will be available soon to to aid in making their predictions more accurate. For now, they&amp;#039;ve seen that while a few glaciers are growing steadily in isolated, snow-laden areas, the majority of the time the momentum is going the other way\u2014&amp;quot;98 percent of the time we notice they&amp;#039;re getting thinner,&amp;quot; Cogley says.  The biggest problem glacier scientists face is the fact that good data just doesn&amp;#039;t go back very far. &amp;quot;It turns out you need all the computing power in the universe to do the calculation faster than the glacier,&amp;quot; Cogley says. But as Bj\u00F8rk&amp;#039;s study shows, even over short time spans it&amp;#039;s now possible to see big changes in these big ice sheets. &amp;quot;When I started working on these kinds of methods, people suggested that once you mapped the glacier, you were done, because glaciers didn&amp;#039;t change that much, and instead we see they are changing on a timescale of months or years, not decades or centuries,&amp;quot; Joughin says. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/science/environment/a7805/how-do-we-know-how-much-glaciers-are-shrinking-10118704/>