Geoscience Reference
In-Depth Information
than it was the year before. h e dif erence is ini nitesimal but cumulative.
He speculated that these small changes in the earth's orbit could alter the
amount of solar radiation reaching the earth's surface just enough to inl u-
ence the earth's climate. His laborious calculations of the earth's orbit over
the past three million years showed cyclical variations between more circular
and more elliptical shapes, with periods lasting 100,000 years.
Another aspect of the earth-sun relationship also interested Croll. He
hypothesized that maximum ice growth, leading to an ice age, would occur
when the orientation of the earth's axis of rotation was in a certain posi-
tion. h e earth's axis of rotation also changes cyclically; it “wobbles” like a
spinning top. Today, the earth's axis at the North Pole points toward the
North Star. Over time, however, the orientation of the earth's axis slowly
changes, tracing a circle in the sky that takes 26,000 years to complete. Croll
reasoned that the most likely time for the growth of a large ice sheet in the
northern hemisphere (where most of the global landmass is located) is when
the earth's orbit is most elongated and when the axis of rotation is pointing
away from the sun during the winter.
Although Croll's calculations showed that the earth's orbital changes
caused only slight variations in the amount of solar energy reaching its
surface, he surmised that ice sheets, once they begin to grow, rel ect sun-
light back to space, causing even more cooling and resulting in more ice—a
so-called “positive feedback” loop. Croll can be credited with being the
i rst to recognize this important process. He also proposed that growing ice
sheets near the poles would increase the temperature dif erences between
the equator and poles, leading to more vigorous winds and surface currents,
ultimately bringing more moisture toward the poles. h ese processes would
enhance ice sheet growth and are still recognized as important today.
Despite his humble beginnings, his lack of formal education, and his
need to work long days in a variety of jobs ill-suited to him, Croll became a
prominent scientist, publishing a topic entitled
Climate and Time in h eir
Geological Relations: A h eory of Secular Changes of the Earth's Climate.
He
i nally landed a position at the Scottish Geological Survey, and ultimately his
impressive achievements were honored by his election to the Royal Society
of London.
By the late nineteenth century, however, Croll's ice age theory was ques-
tioned and i nally rejected. His proposed date for the end of the last ice age,
some 80,000 years ago, was shown to be of by tens of thousands of years. A
much later date, 10,000 years, was estimated by geologists who studied the
