Page 143 - Earth's Climate Past and Future

CHAPTER 7

Astronomical Control

of Solar Radiation

Each year we feel the effects of Earth’s orbit around the Sun through seasonal
changes in the angle of the Sun’s rays and their effects on temperature and
other responses. We experience seasonal changes because Earth is tilted as
it orbits the Sun—toward the Sun in summer and away from it in winter. The
seasonal cycle is by far the largest climate-related signal humans experience in
a lifetime.
In this chapter we examine much longer-term changes in Earth’s orbit that
are equally important to the climate system: changes in the angle of tilt of
Earth’s axis of rotation, in the shape of its orbit as it revolves around the Sun,
and in the timing of the seasons in relation to its noncircular orbit. These
longer-term variations in Earth’s orbit occur at cycles ranging from 20,000 to
400,000 years in length, and they cause cyclic variations in the amount of
solar radiation received at the top of the atmosphere by latitude and by season.
These changes in incoming radiation drive the climatic changes explored in
subsequent chapters.