The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 284

relatively low eccentricity (0.017), the eccentricity will
decrease during the next 10,000 years before increasing
again. In 50,000 years, the Earth will again be in an orbit
of high eccentricity. During orbits of high eccentric-
ity, the Earth-sun distance is about 10 percent greater
in June than in December, resulting in incident solar
radiation being about 21 percent lower in June than in
December. Today, the Earth-sun distance is about 3.4
percent greater in June than in December, resulting in
incident solar radiation being about 6.9 percent lower
in June than in December.
Because the yearly averaged distance between the
Earth and the sun is less in a period of low eccentricity
than in a period of high eccentricity, yearly averaged
temperatures are higher in periods of low eccentricity
than in periods of high eccentricity. This can be seen in
Figure 12.20, which shows that natural interglacial tem-
perature maxima occurred 122,000, 230,000, 315,000,
and 403,000 years ago, all times of low eccentricity.

in Figure 12.20 are smaller variations, some resulting
from changes in obliquity.

12.3.2.9. Precession of the Earth's Axis
of Rotation
Precession is the angular motion (wobble) of the Earth's
axis of rotation about an axis fixed in space (Figure
12.22b). It is caused by the gravitational attraction
between the Earth and other bodies in the solar system.
Currently, the Northern Hemisphere is farther from the
sun in summer than in winter. In 11,000 years, angu-
lar motion of the Earth's axis of rotation will cause
the Northern Hemisphere to be closer to the sun in
summer than in winter. The complete cycle of the pre-
cession of the Earth's axis is 22,000 years. Precession
of the Earth's axis does not change the yearly or glob-
ally averaged incident solar radiation at the top of the
Earth's atmosphere. Instead, it changes the quantity of
incident radiation at each latitude during a season. In
11,000 years, for example, Northern Hemisphere sum-
mers will be shorter but possibly warmer than today
because the Northern Hemisphere will be closer to the
sun in summer. Southern Hemisphere summers will be
longer but possibly cooler than they are today because
the Southern Hemisphere will be farther from the sun
during its summer. Because seasonal changes in temper-
ature result in yearly averaged changes in temperature
at a given latitude, some of the cyclical changes in tem-
peratures seen in the Antarctic data shown in Figure
12.20 are due to changes in precession.

12.3.2.8. Changes in the Obliquity
of the Earth's Axis
The obliquity of the Earth's axis of rotation is the
angle of the axis relative to a line perpendicular to
the plane of the Earth's orbit around the sun. Figure
12.22a shows the range in the Earth's obliquity. Every
41,000 years, the Earth goes through a complete cycle
where the obliquity changes from 22 to 24.5 degrees
and back to 22 degrees. Currently, the obliquity is
23.5 degrees and decreasing. When the obliquity is low
(22 degrees), more sunlight hits the Equator, increasing
the south-north temperature contrast. When the obliq-
uity is high (24.5 degrees), more sunlight reaches higher
latitudes, reducing the temperature contrast. Thus, the
obliquity affects seasons and temperatures at each lati-
tude. Superimposed on the large temperature variations

12.3.2.10. Effects of the Milankovitch Cycles
The Milankovitch cycles are responsible for most of
the cyclical changes in the Earth's temperature in Figure
12.20, thereby causing advances and retreats of glaciers

24.5 o

Line perpendicular to
plane of earth's orbit
around the sun

22 o

Axis of
rotation Current N.

Hemisphere
winter

Current N.
Hemisphere
summer

N. Hemisphere
summer in
11,000 years

N. Hemisphere
winter in
11,000 years

Earth

(a) (b)

Figure 12.22. Changes in the (a) obliquity of the Earth's axis of rotation and (b) precession of the Earth's axis
relative to a point in space.