CS_Ch12_ClimateChange
3/1/2005
4:57 PM
Page 770
Climate Change
Again, because of the varying pull of the other planets on the Earth–Sun
system, the Earth’s obliquity changes over a period of about 40,000 years.The
maximum angle of tilt is about 24 1/2°, and the minimum angle is about 22°.
At times of maximum tilt, seasonal differences in temperature are slightly
greater.At times of minimum tilt angle, seasonal differences are slightly less.
Geo Words
Milankovitch cycles: the
cyclical changes in the
geometric relationship
between the Earth and the
Sun that cause variations in
solar radiation received at the
Earth’s surface.
axial precession: the wobble
in the Earth’s polar axis.
orbital precession: rotation
about the Sun of the major
axis of the Earth’s elliptical
orbit.
Precession
Have you ever noticed how the axis of a spinning top sometimes wobbles slowly
as it is spinning? It happens when the axis of the top is not straight up and down,
so that gravity exerts a sideways force on the top.The same thing happens with
the Earth.The gravitational pull of the Sun, Moon, and other planets causes a
slow wobbling of the Earth’s axis.This is called the Earth’s axial precession, and
it has a period of about 26,000 years.That’s the time it takes the Earth’s axis to
make one complete revolution of its wobble.
There is also another important kind of precession related to the Earth. It is
the precession of the Earth’s orbit, called orbital precession.As the Earth
moves around the Sun in its elliptical orbit, the major axis of the Earth’s
orbital ellipse is rotating about the Sun. In other words, the orbit itself rotates
around the Sun! The importance of the two precession cycles for the Earth’s
climate lies in how they interact with the eccentricity of the Earth’s orbit.This
interaction controls how far the Earth is from the Sun during the different
seasons. Nowadays, the Northern Hemisphere winter solstice is at almost the
same time as perihelion. In about 11,000 years, however, the winter solstice
will be at about the same time as aphelion.That will make Northern
Hemisphere winters even colder, and summers even hotter, than today.
Milankovitch Cycles
eccentricity
tilt
(%)
(degrees)
time (thousands of years ago)
0
0
2
precession combined signal average marine
oxygen-isotope
index
(radiation
ratio
received)
6 22.0 23.0 24.0 0.04 -0.02-0.07
-2.7 0.0 2.7
2α
0 -2α
100
200
300
400
+
+
=
500
600
700
800
Figure 4 Interpretation of Milankovitch cycles
over the last 800,000 years.
770
EarthComm
Early in the 20th century a Serbian
scientist named Milutin Milankovitch
hypothesized that variations in the
Earth’s climate are caused by how
insolation varies with time and with
latitude. He used what is known
about the Earth’s orbital parameters
(eccentricity, obliquity, and
precession) to compute the
variations in insolation. Later
scientists have refined the
computations.These insolation cycles
are now called Milankovitch
cycles. (See Figure 4.)