Milankovitch proposed in the early twentieth
century that the ice ages were caused by variations
in the Earth’s orbit around the Sun. For some
time this theory was considered untestable,
largely because there was no sufficiently precise
chronology of the ice ages with which the orbital
variations could be matched.
To establish such a chronology it is necessary
to determine the relative amounts of land ice that
existed at various times in the Earth’s past. A recent
discovery makes such a determination possible:
relative land-ice volume for a given period can be
deduced from the ratio of two oxygen isotopes,
16 and 18, found in ocean sediments. Almost
all the oxygen in water is oxygen 16, but a few
molecules out of every thousand incorporate the
heavier isotope 18. When an ice age begins, the
continental ice sheets grow, steadily reducing the
amount of water evaporated from the ocean that
will eventually return to it. Because heavier isotopes
tend to be left behind when water evaporates
from the ocean surfaces, the remaining ocean
water becomes progressively enriched in oxygen
18. The degree of enrichment can be determined
by analyzing ocean sediments of the period,
because these sediments are composed of calcium
carbonate shells of marine organisms, shells that
were constructed with oxygen atoms drawn from
the surrounding ocean. The higher the ratio of
oxygen 18 to oxygen 16 in a sedimentary specimen,
the more land ice there was when the sediment
was laid down.
As an indicator of shifts in the Earth’s climate,
the isotope record has two advantages. First, it is
a global record: there is remarkably little variation
in isotope ratios in sedimentary specimens taken
from different continental locations. Second, it is
a more continuous record than that taken from
rocks on land. Because of these advantages,
sedimentary evidence can be dated with sufficient
accuracy by radiometric methods to establish a
precise chronology of the ice ages. The dated
isotope record shows that the fluctuations in
global ice volume over the past several hundred thousand years have a pattern: an ice age occurs
roughly once every 100,000 years. These data have
established a strong connection between variations
in the Earth’s orbit and the periodicity of the ice
ages.
However, it is important to note that other
factors, such as volcanic particulates or variations
in the amount of sunlight received by the Earth,
could potentially have affected the climate. The
advantage of the Milankovitch theory is that it
is testable; changes in the Earth’s orbit can be
calculated and dated by applying Newton’s laws
of gravity to progressively earlier configurations
of the bodies in the solar system. Yet the lack of
information about other possible factors affecting
global climate does not make them unimportant.
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71. The author of the passage would be most likely to
agree with which of the following statements about the
Milankovitch theory?
(A) It is the only possible explanation for the ice
ages.
(B) It is too limited to provide a plausible explanation
for the ice ages, despite recent research
findings.
(C) It cannot be tested and confirmed until further
research on volcanic activity is done.
(D) It is one plausible explanation, though not the
only one, for the ice ages.
(E) It is not a plausible explanation for the ice ages,
although it has opened up promising possibilities
for future research.
Application Th e author’s reaction to the statements about the
Milankovitch theory must be based on how the
author treats the theory in the passage. Th e fi rst,
second, and third paragraphs describe the theory
and the use of a new research method to test the
theory. Th e passage states that data from these
tests have established a strong connection between
variations in the Earth’s orbit and the periodicity of
the ice ages, suggesting that the author of the
passage believes the theory is plausible. In the
fi nal paragraph, the author points to other factors
that might be involved, suggesting that the theory
might not provide a complete explanation.
