Encyclopedia of Evolution.pdf - Online Reading Center

0 ice ages
modern computer at least a few minutes or hours, by hand,
over the course of several years, even while he was on vacation.
The calculations were difficult because he took into
account not only changes in Earth’s orbit but also processes
such as changes in tilt (the angle between the ecliptic,
or plane of Earth’s revolution around the Sun, and the
angle of the axis of Earth’s revolution; currently about 23
degrees) and precession (how the Earth spins like a top,
with true north pointing to different parts of the sky at
different times; true north currently points to Polaris, the
North Star) (see figure). During World War I, Milanković
was under house arrest. This allowed him to devote full
time to the calculations; writer Bill Bryson calls him
“the happiest prisoner of war in history.” The dates that
emerged from Milanković’s calculations, for when ice ages
were most likely to have occurred, did not correspond to
what was then believed to be the times of glaciation. Therefore
by the time he died he did not have the satisfaction
of seeing his theory vindicated. Now that scientists have
radiometric dates of several of the glaciations, they see that
Milanković was largely correct. Modern evidence corroborates
Milanković’s belief that recent glaciations occurred
about once every 100,000 years. Oxygen isotope ratios in
the shells of microscopic fossils reflect the amount of water
in the oceans. Evaporation removes the lighter isotope of
water and the water is stored in glaciers; the remaining
ocean water therefore is enriched in the heavier isotope (see
isotopes). Peak levels of oxygen isotope ratios occur about
once every 100,000 years (see figure at right).
Variations in Earth’s angle and orbit affect the ice ages
because they shift the balance of solar energy going into and
out of the planet. Short, cool summers resulting from less solar
energy—because of a combination of distance from the Sun
and angle of the sunlight—reduce snowmelt. The increased
area covered with snow reflects more light into outer space,
increasing the albedo (reflected light) of the planet. This further
tilts the balance of climate toward an ice age. The accumulation
of ice and snow in the northern latitudes eventually
pushes the ice southward in great rivers of snow (glaciers). If
the summers in the latitudes further south are cool enough
that the ice does not melt as much as it advances, then the
result is an advancing glacier. The rate of advance has been
variable but has seldom exceeded three feet (1 m) a year. The
most recent maximum glacial extent ended about 14,000
years ago. The Arctic Ocean was relatively ice-free during the
glaciations and was the source of evaporated water for much
of the snow that caused them. Extensive glaciations did not
occur in the Southern Hemisphere because it contains mostly
ocean, which does not change temperature as readily as land.
Glaciers covered and still cover Antarctica, but there is virtually
no land surface between Antarctica and the tip of South
America. Therefore Antarctica produced, and produces, separate
icebergs rather than advancing glaciers.
The Milanković cycle explains why cyclical glaciations
occur today but does not explain why they did not occur
prior to the Pleistocene. Continental drift may have been
primarily responsible for beginning the Pleistocene-Holocene
cycle of ice ages. The land that is now North and South
Ice ages result from the convergence of three cycles: changes in the tilt of the axis (currently about . degrees from vertical), the wobble of the axis,
and the shape of the orbit. This convergence is called the Milanković cycle. (Modified from Bonnicksen)