Aided by the recent ability to analyze samples of air trapped in

1991 10 SECTION A 17-23

Aided by the recent ability to analyze samples of air trapped in glaciers, scientists now have a clearer idea of the relationship between atmospheric composition and global temperature change over the past 160,000 years. In particular, determination of atmospheric composition during periods of glacial expansion and retreat (cooling and warming) is possible using data from the 2,000 meter Vostok ice core drilled in Antarctica. The technique involved is similar to that used in analyzing cores of marine sediments, where the ratio of the two common isotopes of oxygen, 18O and 16O, accurately reflects past temperature changes. Isotopic analysis of oxygen in the Vostok core suggests mean global temperature fluctuations of up to 10 degrees centigrade over the past 160,000 years.

Data from the Vostok core also indicate that the amount of carbon dioxide has fluctuated with temperature over the same period: the higher the temperature, the higher the concentration of carbon dioxide and the lower the temperature, the lower the concentration. Although change in carbon dioxide content closely follows change in temperature during periods of deglaciation, it apparently lags behind temperature during periods of cooling. The correlation of carbon dioxide with temperature, of course, does not establish whether changes in atmospheric composition caused the warming and cooling trends or were caused by their.

The correlation between carbon dioxide and temperature throughout the Vostok record is consistent and predictable. The absolute temperature changes, however, are from 5 to 14 times greater than would be expected on the basis of carbon dioxide’s own ability to absorb infrared radiation, or radiant heat. This reaction suggests that, quite aside from changes in heat-trapping gases, commonly known as greenhouse gases, certain positive feedbacks are also amplifying the temperature change. Such feedbacks might involve ice on land and sea, clouds, or water vapor, which also absorb radiant heat.

Other data from the Vostok core show that methane gas also correlates closely with temperature and carbon dioxide. The methane concentration nearly doubled, for example, between the peak of the penultimate glacial period and the following interglacial period. Within the present interglacial period it has more than doubled in just the past 300 years and is rising rapidly. Although the concentration of atmospheric methane is more than two orders of magnitude lower than that of carbon dioxide, it cannot be ignored: the radiative properties of methane make it 20 times more effective, molecule for molecule, than carbon dioxide in absorbing radiant heat. On the basis of a simulation model that climatological researchers have developed, methane appears to have been about 25 percent as important as carbon dioxide in the warming that took place during the most recent glacial retreat 8,000 to 10,000 years ago.

1. ​​​The primary purpose of the passage is to

(A) interpret data
(B) explain research methodology
(C) evaluate a conclusion
(D) suggest a new technique
(E) attack a theory

2. ​​​According to the passage, which of the following statements about methane is true?

(A) Methane is found in marine sediments.
(B) Methane is more effective than carbon dioxide in absorbing radiant heat.
(C) The Earth’s atmosphere now contains more than twice as much methane as it does carbon dioxide.
(D) The higher the concentration of carbon dioxide in the Earth’s atmosphere, the lower the concentration of methane.
(E) Most of the global warming that has occurred during the past 10 years has been associated with increased methane concentration.

3. ​​​According to the passage, which of the following statements best describes the relationship between carbon dioxide and global temperature?

(A) Carbon dioxide levels change immediately in response to changes in temperature.
(B) Carbon dioxide levels correlate with global temperature during cooling periods only.
(C) Once carbon dioxide levels increase, they remain high regardless of changes in global temperature.
(D) Carbon dioxide levels increase more quickly than global temperature does.
(E) During cooling periods, carbon dioxide levels initially remain high and then decline.

4. ​​​​​​The author mentions “certain positive feedbacks” (Highlighted) in order to indicate that

(A) increased concentration of carbon dioxide in the Earth’s atmosphere is responsible for global temperature increase
(B) some climate simulation models have produced useful information
(C) greenhouse gases alone do not account for global temperature increase
(D) variables that benefit life are causing global temperature to increase
(E) beneficial substances that are not heat-trapping gases and that contribute to global temperature increase have been found in the Vostok ice core

5. ​​It can be inferred from the passage that a long-term decrease in the concentration of carbon dioxide in the Earth’s atmosphere would

(A) increase methane concentration in the Earth’s atmosphere
(B) accompany a period of glaciation
(C) encourage the formation of more oxygen isotopes in the Earth’s atmosphere
(D) promote the formation of more water in the Earth’s global environment
(E) increase the amount of infrared radiation absorbed by the Earth’s atmosphere

6. ​​​​​The passage suggests that when the methane concentration in the Earth’s atmosphere decreases, which of the following also happens?

(A) Glaciers melt faster.
(B) The concentration of carbon dioxide increases.
(C) The mean global temperature decreases.
(D) Carbon dioxide absorbs more radiant beat.
(E) More clouds form in the Earth’s atmosphere.

7. ​​​​​In the fourth paragraph, the author is primarily concerned with

(A) restating the main idea of the passage
(B) using research findings to develop a simulation model
(C) outlining the direction of future reserves
(D) providing an additional example of a phenomenon
(E) introducing a conflicting hypothesis