 Alaska's Glacier Bay has fascinated naturalists and glaciologists for centuries. Named a national monument in 1925, and a national park in 1980, Glacier Bay today attracts more than 250,000 visitors a year. Just 200 years ago, the entire bay was covered by glacier ice. With the help of ground and satellite measurements, study of the recession of the glaciers of Glacier Bay, as well as in other areas, is providing us with important clues about changes in global climate. A glacier is a dynamic system consisting of snow, ice, and often rock debris that transports material from higher elevations, where snow accumulates to lower elevations where snow and ice melt. Snow becomes glacier ice over time, when the pressure of increasing layers of snow accumulating year after year transforms the snow, first into fern, which is very dense snow, and eventually into ice. Today there are over 200 separate smaller glaciers in Glacier Bay National Park, to the delight of park visitors who view them from cruise ships, private boats, and kayaks. Glaciers at the head of tar inlet are the most spectacular. Glaciers include the Grand Pacific and the Marjorie. Together they comprise three kilometers of ice front which is calving into the sea. The Johns Hopkins and Marjorie glaciers are each about one and a half kilometers wide and 22 kilometers long. They are both dwarfed by the Grand Pacific Glacier, an all tower 60 to 90 meters above the water at their termini, with another 120 meters below the water surface. These glaciers calve icebergs into the ocean on a regular basis, thrilling visitors with a splash and thunder. Written records of the glaciers in Glacier Bay began with George van Guren's visit in 1794. Ground observations beginning in the late 1800s and ground based in satellite measurements in recent years have shown that many glaciers in and near Glacier Bay have been receding, some rapidly. When a glacier recedes, water from the melting ice is released into the ocean and sea level rises. Though the greatest potential for sea level rises from melting of the Antarctic and Greenland ice sheets, the small glaciers of the world such as those in Glacier Bay would contribute to a sea level rise of about six tenths of a meter if they were to melt completely. Precise measurements of glacier changes may be made by ground based surveying. The late glaciologist William O'Field began photographing and measuring glaciers in Glacier Bay in 1926. Many of the glacier terminus positions mapped by field and his colleagues are shown on the map of Glacier Bay prepared by the National Park Service. Another work in the early part of this century was even more difficult and hazardous than it is today. Scientists had to roll through iceberg-laden water to reach many of the glacier fronts. Detailed measurements in Glacier Bay, made by field and others, have provided an excellent description of the dates and magnitude of changes in the glaciers there. The once enormous Muir Glacier, located in what is now Muir Inlet in the east arm of Glacier Bay, was named for John Muir, the famous naturalist and explorer who visited Glacier Bay in the late 19th century. In 1905, just 26 years after Muir's first visit, Fremont Morris wrote, Formally the Muir presented a perpendicular front at least 200 feet in height from which huge birds were detached at frequent intervals. The sight and sound of one of these vast masses falling from the cliff or suddenly appearing from the submarine icefoot was something which once witness was not to be forgotten. It was grand and impressive beyond description. Unfortunately, the recent changes in the Muir have not increased its impressiveness from a scenic standpoint. Instead of the imposing cliff of ice, the front is sloping and seems to be far less active than formerly. The Muir Glacier is now but a small remnant of its former glory and has nearly retreated up out of the ocean. In fact, many of the large tidewater glaciers that John Muir first observed in 1879 have been reduced to small glaciers that terminate on land. Land that is uncovered as glaciers recede from its plant and animal life to appear and flourish. Beginning with the launch of the first Landsat satellite in 1972, we have, with a spatial resolution of up to 30 meters, been able to acquire detailed satellite images of the glaciers of Glacier Bay and measure changes in those glaciers. Two satellite sensors, the multi-spectral scanner and the Muir Advanced Thematic Mapper, allow us to measure glaciers without having to visit the area. Maps drawn by earlier explorers of Glacier Bay can be registered to or overlaying on satellite images using computer techniques. Common, stable points between the map and the images are located and digitally overlaying using image processing techniques. One image is stretched in relationship to the other. Once the images are registered, we can measure changes in Glacier Terminus positions from the time of the earliest maps to the present. Using computer techniques, we can register elevation data to satellite images. This allows us to obtain a three-dimensional perspective and actually appear to fly by an area enabling more realistic visualization. Measurements of the Muir Glacier have shown that the Terminus retreated up the Muir Inlet at a rate of about four-tenths of a kilometer per year between 1794 and 1892 for a total retreat of more than 40 kilometers. Record measurements derived from images acquired in 1973, 1980, 1983, and 1986 show that the Muir Glacier Terminus went back more than seven kilometers between 1973 and 1986, with most of that retreat occurring between 1973 and 1983. McBride Glacier is the most active glacier in Muir Inlet. It is still receding, and over the last two decades has formed an impressive inlet. As recorded by satellite data, the Terminus of the McBride Glacier retreated almost three kilometers between 1973 and 1986, and the nearby Burles Glacier shrank considerably, filling lakes with its meltwater. Note the increase in the amount of vegetation in the vicinity of Burles Glacier in 1986. The Burles is now a dying ice field, having been cut off from a source of nourishment. Still, other glaciers have been in equilibrium or advancing. Some, like the Johns Hopkins, have stopped their retreat and have advanced during the period covered by the satellite record. Also, the Lamplew, Reed, Marjorie, and Grand Pacific glaciers are all advancing at this time. The nearby Brady Glacier has been advancing since 1794. However, the major feature of Glacier Bay is the large-scale retreat of its tidewater glaciers in both Muir and Tar Inlets. Tidewater glaciers, like the Muir and the Marjorie, terminate in the sea. Tidewater glaciers follow their own cycles that are independent of short-term climate changes. There are also non-tidewater glaciers near Glacier Bay, many of which are receding. Recession of non-tidewater glaciers in the area is probably due to amelioration of the regional climate. Meteorological data from the nearby stations at Juno, Sitka, and Yakutat show a tendency toward an increase in average summer air temperature since about 1940 when the meteorological record began. Combining the extensive records of explorers beginning in the late 18th century with more recent efforts of glaciologists and computer scientists, a wealth of information has been obtained about the deglaciation of Glacier Bay, Alaska. As these glaciers and others continue to change, or if they remain stable over time, the satellite record will permit assessment of the impact of changing climate on the glaciers. And many of the non-tidewater glaciers are excellent indicators of regional climate and climate change.