PBS NOVA Extreme Ice 720p HDTV

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• published: 13 Dec 2015
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A glacier is a year-round mass of ice that originates on land. Its area is usually larger than ~100 square feet (one tenth of a square kilometer). Many experts believe that a glacier must show some type of movement; others believe that a glacier can show evidence of past movement. Glaciers appear on every continent except Australia (although glaciers were present there during the last Ice Age).Glacier and ice stream movement is complex. Although glaciers are solid ice, they are in constant motion. Many glaciers end at the sea, where chunks of ice break off, or calve, into the water.Alpine glaciers, also called mountain glaciers, are found throughout the world’s high mountains. If a mountain glacier increases in size and begins to flow down the valley, it is then described as a valley glacier. Some, such as Alaska’s Columbia Glacier, are literally rivers of ice, flowing down mountains and carving valleys.The largest glaciers are continental ice sheets or icecaps, enormous masses (greater than 50,000 square kilometers [12 million acres]) of ice found only in Antarctica and Greenland. These sheets contain vast quantities of fresh water. If the Greenland ice sheet melted, it would cause sea levels to rise some 20 feet (6 meters) all around the world. If the Antarctic ice sheet melted, sea levels would rise up to about 200 feet (60 meters). This meltwater lake was created by warm summer temperatures on the surface of the Greenland Ice Sheet. The water pooling on the surface of a glacier or ice sheet is heavier and denser than ice. It eventually flows into moulins, or stream channels, which drill their way down through the ice; the water then flows out the base of the glacier into the ocean. This meltwater lubricates the glacier bed and speeds up the flow of ice into the sea. Global warming has caused melting to occur dozens of miles further inland than 20 years ago. The largest single mass of ice on the planet is the Antarctic ice sheet, which stretches over 14 million square kilometers (about 5½ million square miles) and contains more than 60% of all the fresh water in the world. The Antarctic ice sheet covers 98% of the Antarctic continent. Warming has been occurring on the Antarctic peninsula (where the Larsen B ice shelf was located) and increasingly in West Antarctica, where ice extends well below sea level.A glacier is formed from compacted layers of snow. When new layers of snow fall, previous layers compress into ice. When the ice extends into the ocean, the temperature of the water and even tides can influence how the floating ice shelf, or “tongue” (the part that extends into the ocean), responds. The breakup of the Larsen B ice shelf on the Antarctic peninsula in 2002 is an example of the abrupt changes that can occur.All of the phases of water—solid, liquid and vapor—relate to glacier dynamics. Water has unique qualities that come into play in understanding the roles glaciers play in the Earth system. For example, water expands rapidly when it freezes—an increase of about 9% by volume. Also, water enters the atmosphere through the familiar cycle of melting and evaporation, but water can also enter the atmosphere directly from a solid state through a process known as sublimation. Also, water—the only substance that is lighter in its solid (ice) than its liquid state—has more density as salt water than as fresh water. The melting fresh water from glaciers alters the ocean, not only by directly contributing to the global sea level rise, but also because it pushes down the heavier salt water, thereby changing what scientists call the THC, or Thermo (heat) Haline (salt) Circulation, meaning currents in the ocean. This has an immediate effect on the near region, such as the north Atlantic off the coast of Greenland, but ultimately the impacts can ripple far beyond the immediate area and climate. Ice that took centuries to develop can vanish in just a few years. A glacier doesn’t melt slowly and steadily like an ice cube on a table. Once glacial ice begins to break down, the interaction of meltwater and sea water with the glacier’s structure can cause increasingly fast melting and retreat. Today, Earth’s surface is made up of 71% water, 10% ice and 19% land. Most of the world’s ice is in the Arctic and Antarctic, but some of it is scattered around Earth in the form of mountain glaciers.