Since the mid-nineteenth century, scientists and naturalists have closely studied glaciers. In this photograph from 1894, two men approach a yawning crevasse. For safety, they use rope harnesses, attaching themselves to more stable ground further away from the crevasse. Even today, people doing fieldwork on glaciers take these precautions. (H.F. Reid Collection at the World Data Center for Glaciology, Boulder)

Presently, glaciers occupy about 10 percent of the world's total land area, with most located in polar regions like Antarctica and Greenland. Glaciers can be thought as remnants from the last Ice Age, when ice covered nearly 32 percent of the land, and 30 percent of the oceans. An Ice Age occurs when cool temperature endure for extended periods of time, allowing polar ice to advance into lower latitudes. For example, during the last Ice Age, giant glacial ice sheets extended from the poles to cover most of Canada, all of New England, much of the upper Midwest, large areas of Alaska, most of Greenland, Iceland, Svalbard and other arctic islands, Scandinavia, much of Great Britain and Ireland, and the northwestern part of the former Soviet Union.

Within the past 750,000 years, scientists know that there have been eight Ice Age cycles, separated by warmer periods called interglacial periods. Currently, the Earth is nearing the end of an interglacial, meaning that another Ice Age is due in a few thousand years. This is part of the normal climate variation cycle. Greenhouse warming may delay the onset of another glacial era, but scientists still have many questions to answer about climate change. Although glaciers change very slowly over long periods, they may provide important global climate change signals.

How is a glacier formed?

Glaciers begin to form when snow remains in the same area year-round, where enough snow accumulates to transform into ice. Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to re-crystallize, forming grains similar in size and shape to grains of sugar. Gradually the grains grow larger and the air pockets between the grains get smaller, causing the snow to slowly compact and increase in density. After about two winters, the snow turns into firn -- an intermediate state between snow and glacier ice. At this point, it is about half as dense as water. Over time, larger ice crystals become so compressed that any air pockets between them are very tiny. In very old glacier ice, crystals can reach several inches in length. For most glaciers, this process takes over a hundred years.

Why do glaciers move?

Once a mass of compressed ice reaches a critical thickness, around 18 meters thick, it becomes so heavy that it begins to deform and move. The sheer girth of the ice, combined with gravity's influence, causes glaciers to flow very slowly. Ice may flow down mountain valleys, fan across plains, or in some locations, spread out to the sea. Movement along the underside of a glacier is slower than movement at the top due to the friction created as it slides along the ground's surface.

Glaciers periodically retreat or advance, depending on the amount of snow accumulation or albation that occurs. This retreat or advance refers only to the position of the terminus, or snout, of the glacier. Even as it retreats, the glacier still deforms and moves downslope, like a conveyor belt. For most glaciers, retreating and advancing are very slow occurrences, noticeable only over a long time. However, when glaciers retreat rapidly, movement may be visible over a few months or years. For instance, massive glacier retreat has been recorded in Glacier Bay, Alaska. Other glaciers have been photographed at intervals showing dramatic recession.

Alternatively, glaciers may surge, racing forward several meters per day for weeks or even months. In 1986, the Hubbard Glacier in Alaska began to surge at the rate of 10 meters per day across the mouth of Russell Fiord. In only two months, the glacier had dammed water in the fiord and created a lake. This illustrates how quickly a surging glacier can change its surroundings.

Retreat of the Franz Josef Glacier: The following photographs are selected from a series that show the retreat of the Franz Josef Glacier in New Zealand over a period of 14 years. Respectively, they are from the years 1951, 1957, and 1964. (World Data Center for Glaciology, Boulder)

	1951
	1957
	1964