Page 159 - Earth's Climate Past and Future

CHAPTER 7 • Astronomical Control of Solar Radiation 135

a minimum, the record would show only successive rate and number of days decreased to their current lev-
minima. These sampling attempts give completely dif- els. This gradual slowing in Earth’s rate of rotation was
ferent results because they are persistently biased caused by the frictional effect of the tides.
toward different sides of a highly modulated cycle. If Other changes in Earth’s orbit that can be inferred
the third scientist happened to start sampling exactly at from this kind of information, such as changes in
a crossover point between minima and maxima, the sci- Earth-Moon distance, are thought to have affected the
entist might extract a record suggesting that no signal wavelengths of tilt and precession over tectonic-scale
exists at all. intervals. One estimate of the slow, long-term increases
These differences show the danger of aliasing. in the periods of tilt and precession toward their pre-
Although this example is obviously chosen to show the sent values is shown in Figure 7-23.
worst possible effect of aliasing, undersampling is a
problem in most climate records.
Key Terms
7-10 Tectonic-Scale Changes in Earth’s Orbit
plane of the ecliptic precession of the ellipse
Over time scales of hundreds of millions of years, some (p. 120) (p. 124)
of Earth’s orbital characteristics slowly evolved, as tilt (p. 120) precession of the
shown by evidence in ancient corals. Corals are made of equinoxes (p. 124)
banded CaCO layers caused by changes in environ- solstices (p. 120)
3
mental conditions. The primary annual banding reflects equinoxes (p. 120) precessional index
seasonal changes in sunlight and water temperature perihelion (p. 120) (p. 128)
(Chapter 2). A secondary banding follows the tidal aphelion (p. 120) insolation (p. 129)
cycles created by the Moon and Sun. The tidal cycles wavelength (p. 122) caloric insolation seasons
also affect water depth and other factors in the reef period (p. 122) (p. 132)
environment that influence coral growth. time series analysis
Corals from 440 Myr ago show 11% more tidal frequency (p. 122) (p. 133)
cycles per year than modern corals do, implying that amplitude (p. 122) spectral analysis (p. 133)
Earth spun on its rotational axis 11% more times per modulation (p. 122) power spectrum (p. 133)
year than at present. As a result, each year had 11% sine waves (p. 122)
more days. Gradually over the last 440 Myr, the spin eccentricity (p. 123) filtering (p. 134)
aliasing (p. 134)
axial precession (p. 124)
0
Tilt
Precession (41,000)
(23,000) Review Questions
100 1. Why does Earth have seasons?

2. When is Earth closest to the Sun in its present
Myr ago 200 orbit? How does this “near-pass” position affect
the amount of radiation received on Earth?
3. Describe in your own words the concept of
modulation of a cycle.
300
4. Earth’s tilt is slowly decreasing today. As it does so,
are the polar regions receiving more or less solar
radiation in summer? In winter?
400
5. How is axial precession different from precession
of the ellipse?
20,000 30,000 40,000
Period (years) 6. How does eccentricity combine with precession to
control a key aspect of the amount of insolation
FIGURE 7-23 Tectonic-scale orbital changes Gradual
Earth receives?
changes in Earth’s orbit over long tectonic time scales have
caused a slow increase in the periods of the tilt and precession 7. Do insolation changes during summer and winter
cycles. (Adapted from A. Berger et al., “Pre-Quaternary have the same or opposite timing (sign) at any
Milankovitch Frequencies,” Nature 342 [1989]: 133–34.) single location on Earth? Why or why not?