Biology Reference
In-Depth Information
in relatively recent times were compelling, however, and they implied the
monumental changes in climate and ecosystems now recognized for the
past 2.6 million years.
During the 1960s, conventional wisdom was that the Quaternary encom-
passed approximately the last million years, and that there had been four
major glacial advances—starting with the Nebraskan, through the Kansan,
Illinoian, and Wisconsinian, and four glacial retreats or interglacials, from
the Aftonian through the Yarmouth, Sangamon, and the present Holocene
or Recent—each lasting about 175,000 years. One implication was that the
present interglacial, beginning 11,000 to 12,000 years ago, would continue
for another 165,000 years or so. Another was that the ecosystems that char-
acterize each interval last for about 175,000 years, and that their present
arrangement would also continue for the next 165,000 years. Then the Ser-
bian astronomer and mathematician Milutin Milankovitch (1920) discov-
ered that the pace of long-term climatic change had been quite different.
Milankovitch documented that cycles occur in the position of the Earth
relative to the sun (fi g. 8.4). Any alteration in distance or orientation would
affect the amount of heat reaching the Earth's surface, and if there was a pe-
riodicity in the fl uctuations, this could be a factor in determining patterned
Figure 8.4
The three Milankovitch variations: eccentricity, precession, and obliquity.
