Biology Reference
In-Depth Information
climatic events such as glaciations. There is such a pattern, and the spatial
relationships between the Earth and sun vary in three ways. One is in the
shape of the Earth's orbit, or eccentricity, which changes from nearly circu-
lar to elliptical. At present, the Earth receives about 6 percent more heat at
its closest point (the perihelion) than at its farthest point (the aphelion), but
at maximum eccentricity the difference is about 27 percent. The change in
orbit from most circular to most elliptical takes around 100,000 years, with
a longer eccentricity supercycle of approximately 405,000 years.
The second alteration is in the obliquity or tilt of the Earth's axis. At
present, the tilt is 23.5° from the vertical, but it varies from 22.1° to 24.5°.
The period of the tilt variation is around 41,000 years, and the effect is to
alternately minimize and maximize seasonal differences, especially toward
the poles, which also affects the intensity of ocean circulation (Lisiecki
et al. 2008).
The third alteration is precession. As the Earth spins, the direction to
which the polar axes point changes slightly, much like that of a spinning
top as it slows down. If this movement were projected onto a distant back-
ground it would inscribe a circle. At present the north polar axis points
toward Polaris (the end star in the handle of the Little Dipper), and in about
12,000 it will point toward Vega. The time it takes to inscribe a full imagi-
nary circle is about 23,000. These three variations are commonly called
stretch, tilt, and wobble, and the causes derive from differences in the gravi-
tational pull on the Earth's equatorial bulge by the moon, sun, and planets,
notably Jupiter and Saturn.
After the Milankovitch variations were proposed, a wealth of support-
ing evidence was forthcoming, including the pattern of varves (layers) in
ice from the Greenland (Alley 2000; Landais et al. 2006) and Antarctic ice
core projects, layering in ocean sediments, and the movement of tree line
along mountain slopes where the vegetation is sharply zoned and highly re-
sponsive to climate change. There is no reason to believe that the Milanko-
vitch variations, based on orbital and gravitational forces, should have oper-
ated only since 2.6 Ma, and indeed, evidence exists for much older cycles in
the Triassic of the Newark Basin, from Germany (Vollmer et al. 2008), and
from the Paleocene of the northwestern Pacifi c Ocean (Westerhold et al.
2008). It is likely the cycles are less evident prior to about 2.6 Ma because
of altered sediments, poorer resolution, and because high CO
2
concentra-
tion exerted an overriding infl uence on temperatures. With the waning of
CO
2
concentration in the later part of the Neogene, the Milankovitch varia-
tions become more evident. Thus, among the principal causes for large-scale
changes in past climate are (1) the Milankovitch variations, (2) fl uctuations
