Milankovitch Cycles and Glaciation
Although the history of glaciation during the Pleistocene epoch (2 million to 10,000 years ago) is well established, we do not know with complete certainty why glaciation takes place. For over a century, geologists and climatologists have struggled with this problem, but it remains unsolved.
It is long known that Earth's orbit around the Sun changes periodically, cyclically affecting the way solar radiation strikes the Earth, but the idea that these changes affect climate was first advanced by James Croll in the late 1800s. Later, Milutin Milankovitch elaborated the theory with calculations that convincingly argued that the cycles, now known as Milankovitch cycles, could cause climatic variations.
The Milankovitch cycles emerge from the way three cyclic changes in Earth's orbit combine. One characteristic of Earth's orbit is its eccentricity, the degree to which the orbit is an ellipse rather than a circle. Changes in the eccentricity of Earth's orbit occur in a cycle of about 96,000 years. The inclination, or tilt, of Earth's axis also varies periodically, moving between 22 degrees and 24.5 degrees. The tilt of Earth's axis, toward the Sun at some times of the year and away from the Sun at other times, is responsible for the annual cycle of seasons. The greater the tilt, the greater the contrast between summer and winter temperatures. Changes in the tilt occur in a cycle 41,000 years long. Also, Earth wobbles as it spins, like a slightly unsteady top. The wobble cycle is completed once every 21,700 years. Changes in eccentricity, tilt and wobble do not affect the total amount of solar radiation Earth receives in a year, but they do affect how evenly or unevenly this radiation is disturbed over the course of a year. According to the Milankovitch theory, about every 40,000 years the three separate cycles combine in
such a way that the difference between summer and winter temperatures is at a minimum. At this point winter temperatures are milder but so too are summer temperatures. As a result, less ice is melted in the summer than is formed in the winter, so glaciers build up and a period of glaciation results.
Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that a detailed chronology of the Pleistocene temperature changes was determined that could test the predictions of this theory. A correspondence between Milankovitch cycles and climate fluctuations of the last 65 million years seems clear. Furthermore, studies of rock samples drilled from the deep-sea floor and the fossils contained in them indicate that the fluctuation of climate during the past few hundred thousand years is remarkably close to that predicted by Milankovitch.
A problem with Milankovitch's explanation of glaciation arises from the fact that the variations in Earth's orbit, and hence the Milankovitch cycles, have existed for billions of years. Thus, we might expect that glaciation would have been a cyclic event throughout geologic time. In fact, periods of glaciation are rare. So there must be another factor acting together with the Milankovitch cycles that causes periods of glaciation. Once this additional factor makes the temperature low enough, the cyclic variations of the Milankovitch cycles will force the planet into and out of glacial epochs with a fixed regularity.
Many hypotheses have been proposed for the additional cooling factor. Some suggest that variations in the Sun's energy output could account for the ice ages. However, our present understanding of the Sun's luminosity holds that it should have progressively increased, not decreased, over the course of Earth's history. Still others argue that volcanic dust injected into the atmosphere shields Earth from the Sun's rays and initiates an ice age. However, no correlation has been found between volcanic activity and the start of the last ice age. An increasingly attractive theory holds that decreases in atmospheric carbon dioxide starts the cooling trend that leads to glaciation. Carbon dioxide traps solar energy reflected from the Earth's surface. If carbon dioxide levels decrease, less heat is trapped and Earth's surface cools. Recent studies of the carbon dioxide content of gas bubbles preserved in the Greenland ice cap do in fact show that high carbon dioxide levels are associated with warm interglacial periods, and low levels with cold glacial periods.
【Paragraph 1】
It is long known that Earth's orbit around the Sun changes periodically, cyclically affecting the way solar radiation strikes the Earth, but the idea that these changes affect climate was first advanced by James Croll in the late 1800s. Later, Milutin Milankovitch elaborated the theory with calculations that convincingly argued that the cycles, now known as Milankovitch cycles, could cause climatic variations.
1. The word "elaborated" in the passage is closet in meaning to
A. corrected
B. defended
C. studied
D. developed
【Paragraph 3】
The Milankovitch cycles emerge from the way three cyclic changes in Earth's orbit combine. One characteristic of Earth's orbit is its eccentricity, the degree to which the orbit is an ellipse rather than a circle. Changes in the eccentricity of Earth's orbit occur in a cycle of about 96,000 years. The inclination, or tilt, of Earth's axis also varies periodically, moving between 22 degrees and 24.5 degrees. The tilt of Earth's axis, toward the Sun at some times of the year and away from the Sun at other times, is responsible for the annual cycle of seasons. The greater the tilt, the greater the
contrast between summer and winter temperatures. Changes in the tilt occur in a cycle 41,000 years long. Also, Earth wobbles as it spins, like a slightly unsteady top. The wobble cycle is completed once every 21,700 years. Changes in eccentricity, tilt
and wobble do not affect the total amount of solar radiation Earth receives in a year, but they do affect how evenly or unevenly this radiation is disturbed over the course of a year. According to the Milankovitch theory, about every 40,000 years the three
separate cycles combine in such a way that the difference between summer and winter temperatures is at a minimum. At this point winter temperatures are milder but so too are summer temperatures. As a result, less ice is melted in the summer than is
formed in the winter, so glaciers build up and a period of glaciation results.
2. According to paragraph 3, Milankovitch's theory holds that periods of glaciation result from a particular combination of changes in all the following EXCEPT
A. the shape of Earth's orbit
B. the inclination of Earth's axis
C. the wobble of Earth as it spins
D. the amount of time required for Earth's rotation around the Sun
3. According to paragraph 3, Milankovitch's theory predicts that glaciers build up most when
A. the least amount of solar heat is being delivered to Earth
B. winter temperatures are lowest
C. the difference between winter temperatures and summer temperatures is greatest
D. both winter temperatures and summer temperatures are relatively mild
【Paragraph 4】
Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that a detailed chronology of the Pleistocene temperature changes was determined that could test the predictions of this theory. A correspondence between Milankovitch cycles and climate fluctuations of the last 65 million years seems clear. Furthermore, studies of rock samples drilled from the deep-sea floor and the fossils contained in them indicate that the fluctuation of climate during the past few hundred thousand years is remarkably close to that predicted by Milankovitch.
4. Which of the following can be inferred from paragraph 4 about rock samples taken from the sea floor and the fossils they contain?
A. There is a correspondence between the fossils in the samples and climate fluctuations.
B. Milankovitch's theory predicts when the samples will contain fossils.
C. It was not known until the 1970s that fossils were present in rocks taken from the sea floor.
D. There is no fossil record in the samples older than a few hundred thousand years.
【Paragraph 5】
A problem with Milankovitch's explanation of glaciation arises from the fact that the variations in Earth's orbit, and hence the Milankovitch cycles, have existed for billions of years. Thus, we might expect that glaciation would have been a cyclic event throughout geologic time. In fact, periods of glaciation are rare. So there must be another factor acting together with the Milankovitch cycles that causes periods of glaciation. Once this additional factor makes the temperature low enough, the cyclic variations of the Milankovitch cycles will force the planet into and out of glacial epochs with a fixed
regularity.
5. According to paragraph 5, which of the following is a problem with Milankovitch's theory?
A. It assures that the astronomical cycles have been in existence for billions of years.
B. It cannot explain why glaciation has been a relatively rare occurrence in Earth's history.
C. It cannot predict periods of glaciation in Earth's distant past.
D. It assures that astronomical cycles have an effect on Earth's climate even during periods when there is no glaciation.
【Paragraph 6】
Many hypotheses have been proposed for the additional cooling factor. Some suggest that variations in the Sun's energy output could account for the ice ages. However, our present understanding of the Sun's luminosity holds that it should have progressively increased, not decreased, over the course of Earth's history. Still others argue that volcanic dust injected into the atmosphere shields Earth from the Sun's rays and initiates an ice age. However, no correlation has been found between volcanic activity and the start of the last ice age. An increasingly attractive theory holds that decreases in atmospheric carbon dioxide starts the cooling trend that leads to glaciation. Carbon dioxide traps solar energy reflected from the Earth's surface. If carbon dioxide levels decrease, less heat is trapped and Earth's surface cools. Recent studies of the carbon dioxide content of gas bubbles preserved in the Greenland ice cap do in fact show that high carbon dioxide levels are associated with warm interglacial periods, and low levels with cold glacial periods.
6. The author discusses “our present understanding of the Sun’s luminosity” in order to
A. provide evidence that Milankovitch's astronomical cycles cannot explain the occurrence of the ice ages
B. present an objection to a proposed explanation of the cause of ice ages
C. challenge the claim that long-term cooling can account for glaciation
D. introduce a problem that none of the proposed hypotheses about the causes of glaciation can answer
7. According to paragraph 6, in addition to Milankovitch cycles, each of the following has been proposed as a contributing cause of ice ages EXCEPT
A. variations in the composition of the Greenland ice cap
B. variations in the Sun's energy output
C. volcanic dust injected into the atmosphere
D. decreases in atmospheric carbon dioxide
8. It follows from the theory of the role of carbon dioxide discussed in paragraph 6, that
A. the decrease in temperature during the last ice age caused a decrease in atmospheric carbon dioxide
B. the atmosphere had higher carbon dioxide content during the last ice age than it had during the warm period immediately before it
C. the cooling of temperatures that led to the last ice age was brought about by a decrease in atmospheric carbon dioxide
D. there was less carbon dioxide in the atmosphere toward the end of the last ice age than there was at the beginning
【Paragraph 5】
A problem with Milankovitch's explanation of glaciation arises from the fact that the variations in Earth's orbit, and hence the Milankovitch cycles, have existed for billions of years. ■Thus, we might expect that glaciation would have been a cyclic event throughout geologic time. In fact, periods of glaciation are rare. ■So there must be another factor acting together with the Milankovitch cycles that causes periods of glaciation. ■Once this additional factor makes the temperature low enough, the cyclic variations of the Milankovitch cycles will force the planet into and out of glacial epochs
with a fixed regularity. ■
9. Look at the four squares【■】that indicate where the following sentence could be added to the passage.
This factor must precede an ice age and have the effect of slightly lowering Earth's temperature.
Where would the sentence best fit?
10. 【Directions】An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not
belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
In the 1920s and 1930s, Milutin Milankovitch worked out an account of the cause of Earth's glaciations.
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●
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Answer Choices
A. Milankovitch argued that glaciations would result when three cycles that affect characteristics of Ea rt h's orbit combined in a certain way.
B. Milankovitch's predictions have been shown to be in agreement with periods of glaciation over the past 65 million years.
C. Since ice ages are much less frequent than Milankovitch's explanation predicts, some factor, such as low levels of atmospheric carbon dioxide, must also be involved in triggering glaciation.
D. Milankovitch showed that changes in Earth's orbit periodically lengthen cold seasons and shorten warm ones so that more ice forms than melts and glaciers build up.
E. Dust from volcanic eruptions that blocks the Sun's warming radiation has been proposed as an explanation alternative to Milankovitch cycles for the cause of ice ages.
F. Studies of gas bubbles preserved in the Greenland ice cap strongly suggest that Milankovitch cycles may also influence atmospheric carbon dioxide levels.
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