Page 155 - Earth's Climate Past and Future

CHAPTER 7 • Astronomical Control of Solar Radiation 131

result, an insolation maximum at June 21 is a summer
insolation maximum in the northern hemisphere, but it
Summer Winter is a winter insolation maximum in the southern hemi-
N N
(Max.) (Min.) sphere, where June 21 is the winter solstice. As a result
of the seasonal reversal at the equator, insolation signals
considered in terms of the season of the year are out of
(Max.) phase between the hemispheres for precession. This pat-
(Min.)
S S tern is exactly opposite in sense to the in-phase pattern
Winter Summer
for tilt at high latitudes of both hemispheres.
A Tilt Another way of looking at the relative phasing of
precessional insolation is to track changes between sea-
Summer Winter sons within a single hemisphere. The orbital position
N (Max.) N
(Min.) on the left in Figure 7-17B, which produces minimum
aphelion perihelion summer (June 21) insolation in the northern hemi-
sphere because it occurs at a distant position from the
(Max.)
(Min.) Sun (aphelion), must six months later cause maximum
S S winter (December 21) insolation in the same hemi-
Winter Summer
sphere when Earth revolves around to the perihelion
B Precession position (see Figure 7-17B right). As a result, preces-
sional variations in insolation at any one location always
FIGURE 7-17 Phasing of insolation maxima and minima move in opposite directions for the summer versus win-
(A) Tilt causes in-phase changes for polar regions of both ter seasons.
hemispheres in their respective summer and winter seasons. Precessional changes in insolation have an addi-
(B) Precession causes out-of-phase changes between
hemispheres for their summer and winter seasons. tional characteristic not found in changes caused by tilt:
an entire family of insolation curves exists for each sea-
son and month (and even day) of the year. As a matter of
because both poles are turned more directly toward the convention, insolation changes are typically shown only
Sun. For the same reason, more pronounced insolation for the extreme solstice months of June and December,
minima also occur at both winter poles for a higher tilt: but in fact every season and month precesses into parts
the two winter poles are tilted away from the Sun during of the eccentric orbit that are alternately farther from
the same orbit. the Sun and closer to the Sun at the same 23,000-year
If we compare the North Pole with the South Pole at cycle.
a particular month in the orbit, however, the two hemi- As a result, each season and month experiences the
spheres are exactly out of phase (see Figure 7-17A). The same 23,000-year cycle of increasing and decreasing
increased tilt angle that turns north polar regions more insolation values relative to the long-term mean, but the
directly toward the Sun in northern hemisphere summer anomalies (departures from the mean) are offset in time
also tilts the southern polar regions farther away from from the preceding month or season. These offsets pro-
the Sun at that same place in the orbit (southern hemi- duce an entire family of monthly (and seasonal) insola-
sphere winter). As a result, tilt causes opposite insolation tion curves (Figure 7-18). Each successive month passes
effects at the North and South poles for a given point in through perihelion (or aphelion) roughly 1916 years later
the orbit. than the previous month did (1/12 × 23,000 = 1916).
For precession, the relative sense of phasing
between seasons and hemispheres is exactly reversed 7-7 Insolation Changes by Caloric Seasons
from that of tilt (Figure 7-17B). Because Earth-Sun
distance is the major control on these changes in insola- Calculations of monthly insolation are complicated by
tion, a position close to the Sun (at perihelion) produces an additional factor related to the eccentricity of Earth’s
higher insolation than normal over Earth’s entire sur- orbit. Earth gradually moves through a 360° arc in its
face. A precessional-cycle insolation maximum occur- orbit around the Sun, but this angular motion does not
ring at June 21 (or December 21) will be simultaneous result in a constant rate of motion in space. Instead,
everywhere on Earth. Distant-pass positions (at aphe- Earth speeds up as it nears the extreme perihelion posi-
lion) will simultaneously diminish insolation every- tion and slows down near aphelion. As a result, as the
where on Earth. solstices move slowly around the eccentric orbit, they
An important fact to remember about precession is gradually pass through regions of faster or slower
that the seasons are reversed across the equator. As a movement in space.