Geoscience Reference
In-Depth Information
Earth's axis
precesses in
a circle
Solar
radiation
Axial tilt points
N. Hemisphere
away from Sun
Axial tilt points
N. Hemisphere
to Sun
Today
200
100
0
+100
Time
(thousands of years)
Summer at
perihelion
Winter at
aphelion
Sun
Growth of glaciers in north
Summer at
aphelion
Winter at
perihelion
Sun
Melting of glaciers in north
Figure 9.23 Orbital precession.
Over the course of a 23,000-year cycle, the Earth's axis slowly
wobbles. The impact of this wobble is that a gradual change occurs with respect to the hemisphere that
is tilted toward or away from the Sun at aphelion and perihelion.
15,000 years ago, tilt had increased to 24°, and the high lati-
tudes received much more summer-time radiation. In the ensuing
10,000 years, the great mass of ice melted almost entirely.
data derived from ice cores and marine sediments. Understanding
these relationships is especially important in the context of ongoing
and predicted future climate change on Earth. Although you may
have heard many points of view about this issue, the fact is that the
vast majority
of climate scientists around the world agree that some
kind of global climate change is occurring and will continue for the
foreseeable future. They further believe that the present warming
trend is mainly due to
anthropogenic
(caused by humans) influ-
ences associated with greenhouse gas production. The remainder of
this chapter is devoted to the scientific basis for this belief.
Human Interactions and Future
Climate Change
As we have now documented, climate conditions on Earth have
fluctuated dramatically in the recent past. We have also seen how
scientists can confidently know that these changes occurred by in-
vestigating ancient pollen records, tree rings, and oxygen isotope
Anthropogenic
Caused by humans.
