Page 140 - Earth's Climate Past and Future

Equinox
March 20
Northern hemisphere spring
Southern hemisphere autumn
Solstice
June 21
Northern hemisphere summer
Southern hemisphere winter
23 ˚
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Solstice
December 21
Northern hemisphere winter
Southern hemisphere summer

Equinox
September 22
Northern hemisphere autumn
Southern hemisphere spring

Orbital changes All aspects of Earth’s present orbit have changed with
time: the tilt of its axis, the shape of its path around the Sun, and the
positions of the seasons on this path. These changes in orbit have driven n Part III we move from tectonic-scale
climatic changes on Earth. (Adapted from F. K. Lutgens and E. J. Tarbuck, climate changes to orbital-scale changes
The Atmosphere [Englewood Cliffs, NJ: Prentice-Hall, 1992].)
Iduring the last several million years, a

time when the continents and oceans were
reaching their present positions. During this interval,
changes in Earth’s orbit have been the major driver of
climate by altering the amount of solar radiation
received by season and by latitude (Chapter 7). Three
aspects of Earth’s orbit have varied over cycles ranging
in length from roughly 20,000 to 400,000 years: the tilt
of its axis, the shape of its yearly path of revolution
around the Sun, and the changing positions of the

seasons along that path.
Orbital-scale changes have occurred throughout
Earth’s history, but our focus here is on the last 3 Myr
because well-dated climate records are available from
far more sites than in earlier times. The resulting
increase in regional coverage provides greater insight
into the operation of the climate system. Most climate