Glaciers: Incredible Frozen Time Machines
by Ellen Butts and Joyce Schwartz
When you drink a glass of water or take a shower, think of glaciers. Why?
Glaciers contain at least 75 percent of Earth's fresh water—much more than all our planet's lakes and rivers combined. If you're flying in a plane on a summer day and see ice glistening below, think of glaciers. Why? They're so large and thick that they stay frozen all year round. And next time you're eager for a time-travel adventure, think of glaciers. Why? Ice cores drilled from glaciers allow scientists to peer back into Earth's past.
A thick mass of slow-moving ice that has formed on land is called a glacier. Ice sheets are the vast glaciers that sprawl over the entire continent of Antarctica and the island of Greenland. The latter's ice sheet is only the size of Mexico, but Antarctica's is twice as big as the continent of Australia and has been accumulating snow and ice for at least 40 million years! Together, they contain more than 99 percent of Earth's ice.
Glaciers form by the accumulation, compression, and recrystallization of snow. They require very specific conditions of climate and geography, which means that they are found in polar or high mountain regions where snowfall is heavy in winter, temperatures stay below freezing for long periods, and summers are cool. Freshly fallen snowflakes are light and fluffy, but as new layers of snow fall, their weight compresses the snow underneath into small, dense grains that grow to be the size of rock salt. Scientists call these enlarged crystals firn. Firn contains many air bubbles, but over thousands or millions of years the individual grains of firn enlarge, the pockets of air are closed off, and the firn is compressed into slabs of deep glacier ice.
“Hot” ice! Is that possible? Actually, ice is one of the hottest solids in existence, because it's close to its melting point. Glaciers are always moving, but because ice is “hot,” they move like liquids rather than solids. Glaciers slide and “creep.” They slide over the ground on meltwater, a very thin layer of water from melted ice, and “creep” when their icy layers glide over one another because of their weight. Different parts of the same glacier slide or creep at different speeds. The center moves more rapidly than the sides; the surface moves more rapidly than the bottom, because the sides and bottom are restricted by friction. Most glaciers move several feet per year, while others “race” a few miles.
Glaciers carry, grind, or crush everything in their path—huge rocks, forests, and hills. As rocks are carried, they are broken down into smaller and smaller pieces. Sometimes rock fragments are left behind in piles called moraines. Glaciers even reshape mountains, leaving pointed peaks and jagged ridges. Wherever they slowly sweep Earth's surface, they completely remodel the landscape.
Ice Cores and Global Climate Change
Far below a glacier's surface, information about Earth's history and climate is locked away in layers of ice. Ice cores are the key to this treasure. They are cylinders of ice about 12 centimeters (4 to 5 inches) in diameter that are extracted from glaciers with a drill. By dating cores from around the world and studying the gases and other contaminants trapped in them, glaciologists have been able to create a picture of global climates reaching back over 100,000 years. The more we understand about past climate changes, the better we'll be at predicting future ones.
Glaciers grow by adding a new layer of snow each year. It's easy for scientists to see the annual layers in an ice core by lighting it from beneath. They can then count the layers to determine the age of any section, much like you can count tree rings to determine a tree's age.
Dr. Richard Alley, an expert on glaciers and climate, spent five summers removing a 3,000-meter-long (2-mile) core from the Greenland Ice Sheet. Because it was impossible to remove it in one piece, his team drilled out the core in 1-meter (3-foot) segments. Annual layers in the Greenland ice core are clearly defined back to depths dating to 50,000 years ago, but deeper layers have been thinned and deformed by the glacier's flow. An experienced glaciologist like Dr. Alley knows how to interpret the problem layers and can get an accurate count as far back as 100,000 years.
Glaciologists check their count of the layers by testing them for contaminants from volcanic eruptions and other catastrophic events whose dates are known. For example, finding volcanic ash from the eruption of Mt. St. Helens in 1980 in the right layer means that it has been correctly counted and dated.
After dating the cores, scientists look for evidence of climate change frozen in the ice. Anything that can be carried by the wind (dust from faraway deserts, salt from the ocean, and pollen, for example) has been found in ice cores. Each of these substances provides a clue to past climates. Increasing amounts of dust and ocean salt in a series of layers signal a cooler, dryer climate. The amount of pollen from warm-weather plants compared to the amount from cold-weather plants in a layer also helps scientists figure out what the global climate was like in the past.
Carbon dioxide gas has also been found trapped in bubbles inside ice cores. (You may be familiar with it as the greenhouse gas that comes from burning fossil fuels like oil and coal.) Although no one is sure whether increased amounts of carbon dioxide cause global warming, glaciologists have found that warm climates and larger amounts of carbon dioxide go together.
From their work on ice cores, glaciologists know that the history of Earth's climate is one of constant change. For the past 10,000 years, the global climate has been unusually stable, but in the last few decades, average temperatures have risen to their highest levels in over 100 years. Scientists are concerned that human activity may push the climate into another period of drastic change. They hope to use what they've learned from ice cores to help us understand how climate works and avoid upsetting its delicate balance.
- An amount that has been gathered together.
- The act or process of being squeezed or pressed together.
- A scientist who studies how glaciers flow, and how they interact with the atmosphere, ocean, and land.
- Formation of crystals again.
- What is firn?
[anno: Firn is a consolidation of snow crystals that have become enlarged as they grow under the weight of layers of snow. Firn has air bubbles that become sealed off over time.]
- What have glaciologists found in the ice core samples they have drilled out of glaciers?
[anno: Scientists have found gases, such as carbon dioxide, dust, salt, and pollen.]
- What have glaciologists learned from the ice core samples they have drilled out of glaciers?
[anno: Glaciologists have learned how the climate of Earth has changed over time. Answers may also include that they have learned about old plant life and possible old animal life.]
- Why is it important for glaciologists to study the ice cores? What might scientists be able to do with their findings?
[anno: Answers may vary but could include that scientists, by understanding how the climate has changed in the past, might be able to predict what the climate will do in the future.]