Page 147 - Earth's Climate Past and Future

CHAPTER 7 • Astronomical Control of Solar Radiation 123

today. Leverrier discovered these variations in the
1840s. The shape of an ellipse can be described by ref-
Tilt
erence to its two main axes: the “major” (or longer) axis
22˚ 23˚ 24˚ and the “minor” (or shorter) axis (Figure 7-6). The
0.0
degree of departure from a perfectly circular orbit can
One be described by
41,000–year
cycle 2 2
————
a – b
= ––––––
a
0.5
where is the eccentricity of the ellipse and a and b are
Myr ago half of the lengths of the major and minor axes (called
the “semimajor” and “semiminor” axes).
The eccentricity of the elliptical orbit increases as
these two axes become more unequal in length. At
1.0
the extreme where the two axes become exactly equal
(a = b), the eccentricity drops to zero because the orbit
2
is circular (a – b = 0). Eccentricity ( ) has varied over
2
time between values of 0.005 and 0.0607 (Figure 7-7).
The present value (0.0167) lies toward the lower (more
1.5 circular) end of the range.
Changes in orbital eccentricity are concentrated
mainly at two periods. One eccentricity cycle shows up
FIGURE 7-4 Long-term changes in tilt Changes in the tilt as variations at intervals near 100,000 years (see Figure
of Earth’s axis have occurred at a regular 41,000-year cycle. 7-7). This cycle actually consists of four cycles of nearly
equal strength and periods ranging between 95,000 and
131,000 years, but these cycles blend into a cycle near
Decreases in tilt have the opposite effect: they 100,000 years.
diminish the amplitude of seasonal differences. Smaller The second eccentricity cycle has a wavelength of
tilt angles put the Earth slightly closer to the configura- 413,000 years. This longer cycle is not as obvious, but it
tion shown in Figure 7-3A, which has no seasonal shows up as alternations of the 100,000-year cycles
differences at all.
between larger and smaller peak values. Larger ampli-
tudes can be seen near 200,000, 600,000, 1,000,000, and
IN SUMMARY, changes in tilt mainly amplify or 1,400,000 years ago (see Figure 7-7). A third eccentric-
suppress the seasons, particularly at the poles.
ity cycle also exists at a period of 2.1 Myr, but this cycle
is much weaker in amplitude.
7-4 Changes in Earth’s Eccentric Orbit through Time

The shape of Earth’s orbit around the Sun has also var-
ied in the past, becoming at times more circular and at
other times more elliptical (or “eccentric”) than it is

b
Summer Winter
June 21 December 21
N N
a

S S
2
2
Winter Summer (a – b ) 1 / 2
Eccentricity ε = a
FIGURE 7-5 Effects of increased tilt on polar regions FIGURE 7-6 Eccentricity of an ellipse The eccentricity of
Increased tilt brings more solar radiation to the two summer an ellipse is related to half of the lengths of its longer (major)
season poles and less radiation to the two winter season poles. and shorter (minor) axes.