The Solar System

26 PART 1^ |^ EXPLORING THE SKY

A second factor is also at work. Precession causes Earth’s axis

to sweep around a cone with a period of about 26,000 years, and

that gradually changes the points in Earth’s orbit where a given

hemisphere experiences the seasons. Northern hemisphere sum-

mers now occur when Earth is 1.7 percent farther from the sun,

but in 13,000 years northern summers will occur on the other side

of Earth’s orbit where Earth is 1.7 percent closer to the sun.

Northern summers will be warmer, which could melt all of the

previous winter’s snow and ice and prevent the growth of glaciers.

Th e third factor is the inclination of Earth’s equator to its

orbit, currently at 23.4°. Because of gravitational tugs of the

moon, sun, and even the other planets, this angle varies from 22°

to 24°, with a period of roughly 41,000 years. When the inclina-

tion is greater, seasons are more severe.

In 1920, Milankovitch proposed that these three factors

cycle against each other to produce complex periodic variations

in Earth’s climate and the advance and retreat of glaciers (■ Figure

2-11a). No evidence was available to test the theory in 1920,

however, and scientists treated it with skepticism. Many thought

it was laughable.

The Hypothesis

Sometimes a theory or hypothesis is proposed long before scien-
tists can fi nd the critical evidence to test it. Th at happened in
1920 when the Yugoslavian meteorologist Milutin Milankovitch
proposed what became known as the Milankovitch hypothe-
sis—that small changes in Earth’s orbit, precession, and inclina-
tion aff ect Earth’s climate and can cause ice ages. You should
examine each of these motions separately.
First, Earth’s orbit is only very slightly elliptical, but astrono-
mers know that, because of gravitational interactions with the
other planets, the elliptical shape varies slightly over a period of
about 100,000 years. As you learned earlier in this chapter,
Earth’s orbit at present carries it 1.7 percent closer than average
to the sun during northern hemisphere winters and 1.7 percent
farther away in northern hemisphere summers. Th is makes
northern winters very slightly warmer, and that is critical—most
of the landmass where ice can accumulate is in the northern
hemisphere. When Earth’s orbit becomes more elliptical, north-
ern summers might be too cool to melt all of the snow and ice
from the previous winter. Th at would make glaciers grow larger.

■ Figure 2-11

(a) Mathematical models of the Milankovitch effect can be used to predict temperatures on Earth over time. In these Earth globes, cool temperatures are rep-
resented by violet and blue and warm temperatures by yellow and red. These globes show the warming that occurred beginning 25,000 years ago, which ended
the last ice age about 12,000 years ago. (Courtesy Arizona State University, Computer Science and Geography Departments) (b) Over the last 400,000 years,
changes in ocean temperatures measured from fossils found in sediment layers from the seabed approximately match calculated changes in solar heating.
(Adapted from Cesare Emiliani)

0

20

30

70 °

60 °

100,000

Ocean temperature (

°C)

Solar heating

Observed ocean temperature

Predicted solar heating

200,000

Time (years ago)

a

b

300,000 400,000

Earth temperatures predicted

from the Milankovitch effect

25,000 years ago 10,000 years ago