Geoscience Reference
In-Depth Information
Reconstructing Past Climates Using Oxygen Isotopes
The sorting of oxygen isotopes across space by their size is a pro-
cess that needs to be viewed in animated form to fully appreciate
and understand. To do so, select the animation
Reconstructing
Past Climates Using Oxygen Isotopes
. Once you complete the
animation, be sure to answer the questions at the end to ensure
you understand this concept.
Causes of Past Climate Change
as best explaining Quaternary climate fluctuations. Let's take
a closer look at this important theory by discussing its three
components:
As you have seen, a variety of proxy climate data indicate that
Earth's climate has fluctuated greatly during the Quaternary Peri-
od. Why did these fluctuations occur? One factor directly related
to global cooling is extensive volcanic activity because increased
levels of volcanic ash in the atmosphere block solar radiation. This
effect was observed most recently in 1991 when Mount Pinatubo
in the Philippines erupted catastrophically, resulting in a notice-
ably cool year globally in a sequence of otherwise warm years.
Another factor that can cause climate change is asteroid impacts
that dramatically increase levels of atmospheric dust, which also
block sunlight. The best known example of such an impact oc-
curred 65 million years ago when an asteroid about 10 km (6 mi)
wide struck Earth near the modern Yucatan Peninsula in Mexico.
Most geologists believe this impact, and subsequent extensive
global cooling, caused the extinction of the dinosaurs. Still anoth-
er factor that can cause global climate change is the movement
of landmasses. A good example of how this variable influenced
climate is the way North and South America became connected
when the Isthmus of Panama formed. This joining of landmasses
disrupted ocean circulation in the tropics, which, in turn, changed
the circulation of the atmosphere.
Although the factors just discussed are linked with known
periods of climate change on Earth, they are not considered to
be responsible for the numerous glacial cycles that have oc-
curred during the Quaternary Period. These cycles have been
linked to rhythmic changes in the Earth-Sun geometric rela-
tionship. Milutin Milankovich, a civil engineer and geophysi-
cist, first proposed this theory in the 1920s and argued that
fluctuations in the Earth's tilt, rotation, and orbit best explained
the glacial cycles. This theory was largely ignored until the
mid-1970s when, as discussed earlier, isotopic analyses of ice
cores revealed that many more glacial periods occurred than
was previously believed. Most importantly, scientists discov-
ered a close correlation between the orbital fluctuations pro-
posed by Milankovitch and the timing of glacial and interglacial
periods. Thus, the
Milankovitch theory
is now widely accepted
1.
Orbital Eccentricity
. The
eccentricity
of Earth's orbit
refers to the variation in the shape of the orbit over
time (Figure 9.21). Recall from Chapter 3 that Earth's
orbit is not perfectly circular in its current configura-
tion, but is instead slightly elliptical. The elliptical na-
ture of the orbit results in Earth being about 5 million
km (about 3.1 million mi) closer to the Sun at perihe-
lion on or about January 3 than it is during aphelion on
or about July 4. This difference amounts to an approxi-
mate 6% increase in incoming solar radiation from July
to January. Over the course of cycles that last approxi-
mately 90,000 to 100,000 years, however, the orbit
becomes even more elliptical. When the orbit is more
elliptical, the amount of insolation received at peri-
helion is about 20% to 30% greater than at aphelion.
Circular
Earth orbit
Aphelion
Perihelion
Sun
Elliptical
Earth orbit
Today
200 100 0 +100
Time (thousands of years)
Figure 9.21 Orbital eccentricity.
When Earth's orbit is highly
elliptical, we receive more radiation at perihelion than at aphelion.
This orbital variation occurs on cycles that last about 100,000
years and causes Earth to be approximately 18.27 million km
(11.35 million mi) closer to the Sun during orbital perihelion than
at aphelion.
Milankovitch theory
The theory that best explains Quater-
nary glacial/interglacial cycles through long-term variations in
Earth's orbital eccentricity, tilt, and axial precession.
