Geoscience Reference
In-Depth Information
the subsolar point is confined to a narrower latitude range.
In the context of continental glaciation, more tilt means
warmer summers, which makes it less likely that glaciers
can become established. This variable has been linked to
the warm Climatic Optimum in the early Holocene (Fig-
ure 9.20). On the other hand, conditions are more favor-
able for glacier growth when tilt is less because summer
temperatures are cooler at high latitudes.
3.
Orbital Precession
. The
precession
of Earth's orbit
refers to the way Earth slowly wobbles on its axis as
it orbits the Sun. Over the course of an approximately
23,000-year cycle, the axis slowly migrates, making it
appear as if the planet is wobbling, as when a spinning
top slows down (Figure 9.23). This change alters the
orientation of Earth with respect to perihelion and aph-
elion. If a particular hemisphere is pointed toward the
Sun at perihelion, that hemisphere will be pointing away
at aphelion, and the difference in seasons will be more
extreme. This seasonal effect is reversed for the opposite
hemisphere. Currently, northern summer occurs near
aphelion.
Axial tilt of 22.1
°
glacier growth
Axial tilt of 24.5
°
glacier melting
Solar
radiation
Today
200 100
0
+100
Time (thousands of years)
Figure 9.22 Tilt obliquity.
The amount of Earth's axial tilt varies
over 40,000-year cycles, ranging from 22.1° to 24.5°. When tilt is
less, glacial source areas experience cooler summers, allowing
glaciers to grow. Seasonality is greatest when axial tilt is high.
As discussed previously, isotopic data from ice cores indi-
cate a close correlation between orbital variations and periods
of glaciation. Expansion of glaciers seems to occur on 23,000-,
46,000-, and 100,000-year cycles, with one, two, or all of the
orbital variables playing a major role within any given glaciation.
The 23,000- and 46,000-year cycles are minor fluctuations and
are apparently most related to eccentricity of orbit and precession
of the equinoxes—in other words, the time of year when a par-
ticular hemisphere is tilted toward or away from the Sun. Major
glaciation seems to occur every 100,000 years, and although this
100,000-year cycle mirrors the length of the orbital eccentricity
cycle, there appears to be no distinct relationship between the
two. Instead, the 100,000-year rhythm seems to be most closely
related to fluctuations in axial tilt and to the difference in the long
time it takes a continental glacier to grow and the relatively short
time it takes for the glacier to melt. When ice masses began to
expand about 110,000 years ago, for example, axial tilt was 22°.
Subsequently, it took 80,000 or 90,000 years for the ice sheet to
reach its maximum size during the Wisconsin glaciation. About
This kind of variability would result in a substantially
different climate from what we experience today, with
cooler summers in the high latitudes of the Northern
Hemisphere than currently occur.
2.
Tilt Obliquity
. The term
obliquity
refers to the variations
that occur with respect to the axial tilt of Earth. As you
know, the axis of Earth is tilted 23.5° from the plane of
its orbit around the Sun (the ecliptic). This tilt fundamen-
tally explains the seasonal differences that we experience.
During a cycle that averages about 40,000 years, the tilt of
Earth's axis varies between 22.1° and 24.5° (Figure 9.22).
As tilt increases, the seasonal contrast becomes greater, so
that winters are colder and summers are warmer. A tilt of
24°, for example, results in about 8% more solar radiation
received at high latitudes in summer. This greater varia-
tion occurs because the subsolar point reaches higher lati-
tude when Earth is tilted more. Conversely, when tilt de-
creases, the difference in seasons is less extreme because
The Milankovitch Theory
To see how the Milankovitch theory looks when animated,
go to the
Geo Media Library
and select
The Milankovitch
Theory
. This animation shows the way that the Earth-Sun
geometric relationship changes through time and how it con-
tributes to glaciation. As you watch the animation, notice how
variations in orbital eccentricity, axial tilt, and orbital preces-
sion result in cooler summers at high latitudes where conti-
nental glaciers begin to grow. After you complete the anima-
tion, be sure to answer the questions at the end to test your
understanding of this concept.
