Geoscience Reference
In-Depth Information
ice age cycles
h e relatively rapid climate l uctuations of the past 11,000 years are super-
imposed on more gradual changes in the orbit of the earth around the sun.
h e earth's climate has l uctuated in and out of ice ages over the past two
and a half million years, as described in chapters 5 and 6. What caused these
recurring ice ages, how were they discovered, and how have they af ected
climate during the Holocene?
h e theory of the nature and causes of these longer-term variations in
climate has evolved over the past two centuries with the help of a distin-
guished group of astronomers, geologists, mathematicians, climatologists,
and physicists. One of the earliest and most unlikely, and thus remarkable,
contributors to our understanding of these cycles was James Croll. He grew
up in Scotland in the early nineteenth century and was forced to drop out of
school at the age of thirteen to work on the family farm. At er his long and
arduous days in the i elds, Croll returned home to study physical science late
into the night. He had a passion for understanding the forces governing the
natural world. His family could not af ord a formal university education, so,
when Croll reached his late teens, he was forced to embark on a series of jobs
in which he had little interest or aptitude, including millwright, carpenter,
salesman, shopkeeper, hotel keeper, and life insurance salesman. Only in his
spare time could he study his true passion—earth science.
A major turning point for Croll came in middle age, when he accepted a
job as a janitor at the Andersonian College and Museum during the 1860s.
His salary was meager, but he had access to an extensive scientii c library
where he spent evenings studying physics and geology. He was drawn to a
hotly debated topic of the day: the features and causes of past ice ages. He
was particularly interested in the hypothesis that past ice ages were related
to changes in the earth's orbit around the sun.
h e shape of the orbit itself changes on long timescales, pulsing from near-
circular to slightly more elliptical. h e gradual distortion of the earth's orbit
is caused by the gravitational pull of other nearby planets. When the orbit is
elliptical, the earth receives more sunlight than at other times because of the
slight change in its distance from the sun. Today, the earth's orbit is very close
to circular, and there is only a small dif erence in the amount of radiation
received during its yearlong revolution.
Croll calculated that, for each revolution of the earth around the sun, the
amount of solar radiation received on the earth's surface is slightly dif erent
