Since well before global warming became a heated political issue, scientists have been trying to determine the rate at which our planet’s temperature is increasing. While placing many thermometers around the world would appear to be the solution, local temperatures can vary widely across regions and from one year to the next. Instead, researchers have found they can obtain a measure of average global temperatures by using satellites to monitor heat-sensitive objects on the ground. Of these objects, glaciers are among the most reliable indicators of climate change.
Despite typical glaciers’ massive sizes, monitoring them is not always an easy task. Only specific types of small glaciers are good measures of climate change. Some glaciers are too large to measure accurately, and others are simply too unpredictable. Once scientists find a suitable glacier, they must take satellite images of the ice for a minimum of five years and compare the results. They then have to look closely at the outside edge of the glacier (the glacier’s terminus). If a large percentage of the glacier’s edge is receding then the area around the ice is growing warmer, and if a large percentage is expanding then the area is growing cooler. When enough measurements from many different parts of the world have been gathered, the researchers can determine whether the earth is growing warmer or cooler.
Types of Glaciers
Glaciers form when snow accumulates on a patch of land over tens to hundreds of years. The snow eventually becomes so thick that it collapses under its own weight and forms dense glacial ice. When enough of the ice is compacted together it succumbs to gravity and begins to flow downhill or spread out across flat lands (Williams and Hall, 1993). There are many different types of glaciers, and not all of them are good indicators of climate change. "Glaciers that do tend to be good climate indicators are small land-based, non-surge type glaciers. They respond directly to both regional temperature and precipitation [snow]," said Dorothy Hall, a hydrospheric scientist at NASA’s Goddard Flight Space Center. She and a team of scientists from around the globe have used satellites over the past 25 years to measure changes in glaciers in Europe, Iceland and Alaska.
More than 90 percent of the 33 million cubic kilometers of glacier ice in the world is locked up in the gigantic Greenland and Antarctic ice sheets. Because they are so massive and exist in such frigid latitudes, large-scale changes are very difficult to track and verify, said Hall.
In addition to the ice sheets, there are two types of small glaciers that make for bad climate gauges. "Both surge glaciers and tidewater glaciers have their own cycles of advance and retreat. These cycles are certainly related to climate, but we are not exactly sure how," said Hall. Even if the climate changes in the region, these glaciers would most likely maintain their distinctive patterns of behavior.
Surge glaciers can sluice down a valley at rates of up to a few kilometers a day, said Hall. Once they attain their final destination, they stagnate or gradually retreat for the next ten-to-fifty years. Tidewater glaciers, on the other hand, advance for roughly a thousand years before reaching their destination, explained Hall. When they encounter the sea, they calve and drop icebergs into the water as they continue to make their way out into the ocean. They then pull back and retreat over the course of one or two hundred years.
The Muir tidewater glacier in Alaska’s Glacier Bay has been observed for nearly 200 years. Explorers in the mid-1700s recorded observations of Muir at its peak. They noticed that about two hundred years ago it began to recede. Recent measurements show that the glacier has withdrawn more than 90 kilometers (Hall et. al., 1994).
Challenges to detecting glacial change
Most small glaciers in the world are not as exciting as surge and tidewater glaciers. They may form on the tops of mountains and slowly make their way over hundreds of years through valleys and across plains. A change in the yearly temperature around a glacier can cause it to expand or contract. Hall said, "By measuring the terminus of the glacier periodically, scientists can tell if the local climate is changing. To get an indication of whether the global climate is changing, researchers must monitor these small glaciers across the planet, and for many years to decades"
Before man-made satellites were invented, monitoring enough glaciers to get a measure of global climate change would have been impossible, said Hall. Some glaciers tend to extend in many different directions. As they melt, one part may retract and another part may stay put. Between 1973 and 1987, many outlet glaciers of the Vatnajökull ice cap in Iceland has been steadily receding. Yet, parts of the glacier haven't moved at all (Hall et al. 1992). If scientists were to travel to the glacier and measure just this section, they could be deceived. Measurements have to be made regularly at every extension of the ice cap to see if the glacier as a whole is melting, Hall said. While this may be feasible for one glacier, it is nearly impossible to measure a hundred glaciers this way.
Satellite sensors such as those on Landsat 5 and Landsat 7 allow scientists to measure the entire rim of any glacier on an annual basis, cloud cover permitting. These satellites each have seven different types of light detectors (photoreceptors) on board, which acquire images of different wavelengths of sunlight being reflected off of the Earth. One light detector records only the blue light coming off the Earth (band 1). Another observes all the yellow-green light (band 2) and still another picks up on all the near-infrared light (band 4). The satellites move in circular orbits, very nearly from pole-to-pole, around the Earth and scan strip after strip of our spinning planet. The satellites’ images are then beamed back to the surface, where Hall and other researchers can examine them.
Using satellites to measure glaciers
The trick to measuring the extent of a glacier using a satellite image lies in distinguishing the glacier’s edge from the surrounding land. Unfortunately, this task isn’t as simple as drawing a line on the image between what looks like a glacier and what looks like land. Both glaciers and the surrounding ground often have the same dark gray coloring and can be easily confused by sight.
To separate glacier from surrounding land more accurately, scientists must look at specific types of light being reflected. As can be seen through a prism, sunlight contains many different individual colors (wavelengths). When sunlight strikes objects, certain colors of the spectrum are absorbed and others are reflected. The reflected wavelengths give an object its color.
Like most white objects, the glacier reflects nearly all the colors of the visible spectrum, including the yellow-green sunlight. Yet, a glacier absorbs near-infrared wavelengths of solar energy (light to the right of red on the color spectrum). The research teams using the Landsat 5 and Landsat 7 satellites differentiate snow from other solid materials by looking at the difference between the infrared and yellow-green wavelengths that are reflected. Whenever the difference is large, the area in question is likely to be snow or glacier ice (Hall et al. 1998). By tracking the edges of a glacier from year to year, scientists are often able to see if it is receding or advancing.
Hall explained, while satellite data are easier to collect than ground measurements, scientists still have to record images for many years before they can be certain a glacier is changing, and it is critical to abtain ground measurements to corroborate results deduced from analysis of satellite data. The most obvious reason for the scientists’ uncertainty has to do with the "resolution" of the satellite images. The Landsat 5 and Landsat 7 satellites have a resolution of 30 meters, which means that each pixel (or picture element) on the image represents a 30-by-30-meter patch of land. "Though the glacier will respond immediately to the changes in climate, it may take five to twenty years before we can see the changes in the glacier from satellites," said Hall.
Implications of glacial shrinkage
In some cases, a glacier that recedes around the edges may be growing thicker in areas near the center. Since visible satellite sensors cannot penetrate the surface of the glacier, scientists may be led to believe, at least in the short term, that the glaciers are losing mass. Thus, a glacier must be measured for many years to detect sustained changes in its edge or "ice front."
"It is very hard to measure volume changes in glaciers. People are required to put stakes in the glacier and come back a year later to see how much of a change there is in the height of the glaciers," said Hall. She said that this is a very labor-intensive activity and is done on very few glaciers in the world. The only way to make sure the glacier is pulling back due to a loss of mass is to study it over a period of many years.
On average, scientists are finding that glaciers across the globe are steadily shrinking, said Hall. These findings confirm suspicions that the world is heating up. Researchers generally believe the warming trend may be the result of natural, cyclical changes of the Earth’s climate, and possibly in combination with effects of the large-scale burning of fossil fuels by humans since the industrial revolution. However, the cause of global warming is currently unknown.
The last time the Earth warmed extensively, 120,000 years ago, the Greenland ice sheet drained into the ocean and the sea rose roughly 20 feet above where it is now (Williams and Hall, 1993). Such an increase today would flood coastal communities and low-lying countries such as Holland and Bangladesh, as well as much of the U.S. state of Florida.
References
- Williams, R.S., and D. K. Hall, Glaciers. Atlas of Satellite Observations Related to Global Change, R. J. Gurney, J.L. Foster and C. L. Parkinson, Cambridge University Press, London, 401-422.
- Hall, D.K., C. S. Benton, and W. O. Field, 1994: Changes of Glaciers in Glacier Bay, Alaska, Using Ground and Satellite Measurements, Physical Geography, 16(1), pp. 27-41.
- Hall, D.K., R. S. Williams, and K. J. Bayr, 1992: Glacier Recession in Iceland and Australia, Eos, Transactions, American Geophysical Union, 73(12), pp. 129, 135, 145.
- Hall, D.K., A. B. Tait, G. A. Riggs, and V. V. Salomonson, 1998: Algorithm Theoretical Basis Document (ATBD) for the MODIS Snow-, Lake Ice- and Sea Ice-Mapping Algorithms, pp. 1-35.
Additional References
- Sharp, R. P., Living Ice, Cambridge University Press, Cambridge, 225 p.
- Paterson, W. S. B., The Physics of Glaciers, 3rd Edition, Pergamon Press, 480 p.
NASA Earth Observatory story by John Weier
