Ice Ages are periods in the earth's history when a significant, extended
cooling of the atmosphere and ocean took place. The earth last entered
such an ice age about 1.6 million years ago, at the beginning of the
Quaternary period. Although continental ice sheets withdrew from North
America and Europe about 10,000 years ago—at the end of the
Pleistocene epoch—many scientists believe that the Quaternary Ice Age
is not over yet. Evidence of earlier ice ages also exists.
the time of the earliest recorded life on earth (about 3.6 billion years
ago), the planet's average surface temperature has been about 20°C (70°F),
with a range of uncertainty of about 5°C (9°C). For more than 90 per
cent of that time the earth has been free of ice ages, and no large
glaciers have existed except in high mountains. Ice ages occur about
every 150 million years, and last a few million years.
the cause of ice ages is still a subject of controversy, an argument
based on astronomical observations of the galaxy has gained credibility
in recent years. The earth and its solar system are located
asymmetrically within one limb of the Milky Way galaxy. The galaxy
completes one rotation about once every 300 million years, taking the
solar system through denser and thinner regions of interstellar dust and
through changing gravity and magnetic fields. As with tidal processes,
two disturbing phases appear to exist for each full cycle—so that
every 150 million years a very slight change takes place in the solar
system's galactic environment, possibly altering the earth's climate.
addition, earth-based processes are also involved. According to Plate
Tectonics theory, because of continental drift periodic changes take
place in the earth's geography, the effects of which can be understood
by considering the changes that preceded the present ice age. These
changes occurred about 60 million years ago, when a warm equatorial
seaway called the Tethys sea separated the northern landmass (Laurasia)
from the southern one (Gondwanaland), bringing warm swirling currents to
all the oceans. The old southern lands began drifting northwards,
however, so that Africa, Arabia, and India successively collided with
Eurasia. Finally, Australia separated from Antarctica, allowing a cold
current to circle the globe. One by one the former equatorial seaways
were blocked by land. Each ocean was now isolated and connected with
polar latitudes by great swirls of cold current.
then, is the very speculative ice age scenario: a slight external
cooling because of the galaxy's rotation, as well as a favourable
geographic-oceanographic setting. A chain reaction of cooling may then
be initiated by minor variations in the earth's orbit.
each ice age are remarkable fluctuations known as glacials and
interglacials: these are cold and warm phases that correspond to a cycle
of about 100,000 years. Recognition of this glacial cycle required
complex mathematical calculations, first worked out by a Yugoslav
scientist, Milutin Milankovitch (1879-1958), who showed that the cycle
has additional modulations that make it fluctuate considerably. They
correspond to three variables in the earth's orbit.
important of these variations is the eccentricity cycle of 93,408
years—the variation of the orbit from its almost circular path. This
affects the spin rate of the earth-moon system, which increases when the
earth and moon are closer to the sun. The slower the spin rate, the
stronger is the earth's magnetic field, which, in turn, tends to screen
off the incoming particles of high energy from the sun, thereby cooling
second of the orbital cycles is the change in the tilt of the earth's
equatorial plane in relation to its orbital plane over a period
averaging 41,000 years. About 25 per cent of the glacial and
interglacial temperature differences are due to this change, which
varies from about 22° to 25°.
third orbital phenomenon is the 25,920-year precession cycle, which is
similar to the wobble of a spinning top. At present the earth's axis
points to the Pole Star, and the northern hemisphere is closest to the
sun in the winter, resulting in relatively mild summers and winters.
About 11,000 years ago, however, the axis pointed so as to give the
northern hemisphere colder winters and warmer summers. Because the
winter ice in high latitudes remains far into the summer, this
arrangement 11,000 years ago resulted in a secondary glacial episode,
with intense droughts in the subtropics.
geographic element is also involved in the precession cycle. Besides the
blocked seaways of the present ice age, most of the northern hemisphere
is land, which generates a continental climate, whereas the southern
hemisphere is encircled by a continuous seaway that provides far more
maritime climates. If the land and sea were uniform in both hemispheres,
the precession effect would be cancelled out.
indicate that Milankovitch's cycles do not fully account for the timing
of events in the recent glacial/interglacial cycle. Some researchers
propose that other large-scale influences, including feedback from
changes in ocean currents, are equally important.
longest of the ancient ice ages was probably the Permo-Carboniferous,
which began about 300 million years ago and affected all southern
hemisphere lands. Still earlier, about 435 million years ago, another
giant ice sheet extended from Brazil to North Africa and all the way
across to Yemen and Saudi Arabia. Palaeomagnetic measurements indicate
that the South Pole then lay in West Africa. About 600 million years
ago, yet another great glacial age occurred. Layers of tillite, a rock
consisting of hardened glacial drift, provide evidence of these ancient