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Ice and Snow Study Guide (page 2)

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Updated on Sep 26, 2011

Ice Sheets on the Continents

One very important type of ice remains: continental ice sheets. Only two places on Earth have very large continental ice sheets, but in the past, during ice ages, the ice sheets were much more extensive.

Ice sheets are to glaciers what the urban sprawl of Los Angeles is to a single city street. Today, ice sheets cover almost all of Greenland in the Northern Hemi sphere and Antarctica in the southern. They are massive almost beyond belief, with ice miles in depth (or height, depending on where you start to measure).

Recall that the continental ice sheets of Greenland and Antarctica amount to 2.2% of the world's water (Ice caps are another term for ice sheets because they cap the rock they are on.) . Also recall that the ocean has 97.2% of the world's water.

Let's calculate how much sea level would rise if Greenland and Antarctica melted. For simplicity, assume that all the 2.2% of glaciers and ice caps is in Greenland and Antarctica. That's a good assumption. You'll be walked through the calculation here, just up to the answer, and then be asked to compute the answer for one of the practice questions. If Greenland and Antarctica melted, using the assumption just stated, the ocean would rise in sea level by a fraction of of its current volume. Disregarding the issue of continental shelves, we will assume that the ocean has a constant depth of 4,000 meters (again, not a bad assumption). So, how much would sea level rise if Greenland and Antarctica melted?

One of the great discoveries of the earth sciences over the last hundred years or so is the recognition that long ago, ice sheets covered massive parts of northern North America, Europe, and Russia. This understanding is still being refined today by hundreds or more specialists around the world who study Earth's past ice ages. Such intensive study is required not only because earth's history is fascinating, but because Earth's past might give us clues to Earth's future.

The earliest evidence for past ice ages is in the form of geological formations left by the great ice sheets (mountain glaciers leave some of these, too, but the results are smaller). Examples of this geological evidence include moraines, which are piles of rubble left by the ice sheet (or glacier) as the ice pushes for ward across the land. The final, farthest rubble pile left by the advance of an ice sheet (or a glacier) is called the terminal moraine. Much of Long Island, New York State, is a giant terminal moraine from a past ice age.

Streams of meltwater that flow from a glacier or in a tunnel under an ice sheet can create a ridge of gravel called an esker. When some of the finely ground glacial material is blown by the prevailing winds into an elongated hill (or series of hills), we have what is called a drumlin.

Geological features like these, found in the United States, presented the early modern geologists with evidence that giant ice sheets had once covered most of Canada and a good deal of the northern United States. We now know that the ice sheets during the last ice age were very thick. New York City, for example (or Manhattan Island, because New York City did not exist), as recently as 20,000 year ago was covered with an ice sheet a mile thick. How do we know the thickness? The geological evidence can provide some sense of the lateral extent of the ice, but what evidence do we have for its thickness?

Scientists can get numbers for the ice's volume on the continents during the last ice age by using oxygen isotopes. We have already discussed the science behind the radioactive isotope of carbon (C-14) and other radioisotopes (such as potassium-40) that are used for dating the buildings of ancient cliff dwellings and the birth of igneous rocks. In Lesson 8, the stable isotope of carbon (C-13) was briefly noted. Many elements have stable isotopes as well as radioactive ones. Ordinary oxygen is O-16, with eight protons and eight neutrons in its nucleus. Oxygen also has a stable isotope, O-18, with eight protons and ten neutrons in the nucleus.

Water (H2O) contains both O-16 and O-18. The oxygen in water is mostly O-16, but small fractions of the water molecules contain O-18. This "heavy" water is everywhere, in the ocean, in the rain, in your body. When water evaporates from the ocean, the heavy water stays behind with just a little more statistical propensity than the water that contains O-16. That means that water vapor in the sky contains a higher percentage of O-16 water than water in the ocean. That means, in turn, that rain or snow contains a higher percentage of O-16 water than water in the ocean.

When reservoirs of water are all in balance, as they are most of the time, the lighter water in the rain or snow returns to the ocean, so the ocean is not permanently altered. But during the development of an ice age, in which ice sheets on the continents are growing, the water that is locked up as ice, which fell as snow from the sky, is lighter (it has more O-16) than the water in the ocean. During the ice age, so much water was locked up in the ice sheets that the ocean's ratio of O-18 to O-16 was changed. If there was more O-16 locked up in the ice sheets, can you tell whether the ratio of O-18 to O-16 in the ocean increased or decreased?

The ratio of O-18 to O-16 in the ocean increased. The ocean's water became slightly "heavier." That ratio can be measured today, even though the ancient ocean is history. Organisms that make calcium carbonate shells in the ocean use carbonate from the ocean water. The carbonate (CO32) is formed by chemical reactions between carbon dioxide and water molecules in the ocean, and therefore, the oxygen isotopes in the carbonate molecules have the same ratio of O-18 to O-16 as does the ocean. The carbonate shells fall to the bottom when the creatures that make them die. For the most part, the shells are preserved in the ocean bottom sediments. Scientists drill into and retrieve these sediments, take them back to their laboratories, and measure the isotope contents of the shells to determine the isotopes of the ancient ocean at various times.

By measuring the ratio of O-18 and O-16 in the shells, which equals that of the ancient ocean in which the shells grew, it is possible to calculate how much ice (with a reduced ratio of O-18 to O-16) would have been on the continents to increase the ocean water's ratio to the value that scientists measure. This technique has shown that so much ice was on land during the middle of the last ice age that sea level was down by about 150 meters! This means that the continents went further out than they are now, out to the edges of the continental shelves in many places.

When did the ice age occur? From studies of geological "calling cards" of the ice sheets, as noted previously, as well as the all-important modern methods of oxygen isotopes (and other techniques), it is known that the ice sheets waxed and waned in a large cycle that lasted about 100,000 years in length. In between the ice ages were warmer times, like the one we live in now. The most recent ice age ended about 10,000 to 12,000 years ago.

Many scientists of climate (climatologists) say we are just about at the start of another ice age and claim that our in-between warm period is coming to an end. Other climatologists say that because we are now increasing the greenhouse gas carbon dioxide, the earth will not be following the same rules that determined its climate in the past. Still other climatologists point to the fact that although most of the warm intervals between ice ages last 10,000 to 12,000 years, some times those warm intervals are longer, up to 20,000 years, and that might be the natural case for us now. Only time will tell.

Practice problems of this concept can be found at: Ice and Snow Practice Questions

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