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194 PART III • Orbital-Scale Climate Change

Mystery of the 41,000-Year Glacial World
Insolation surplus
The evidence for 41,000-year variations in ice sheets
between 2.75 and 0.9 Myr ago is based on variations
18
in marine δ O values in benthic foraminifera (see
Figure 9–13). Temperature overprints at the 41,000-
year cycle are thought to account for somewhat less than
half of the range of δ O variation found during this
18
interval, but the remaining signal clearly shows that ice
volume varied mainly at the 41,000-year tempo. Three
explanations have been proposed for the mismatch
between the strong insolation forcing at 23,000 years
and the dominant ice volume response at 41,000 years. High eccentricity and precession
Low eccentricity and precession
11-1 Did Insolation Actually Vary Mainly at Insolation Insolation
41,000 Years? deficit deficit
The atmospheric scientist Peter Huybers proposed that F M A M J J A S O
climate scientists have been wrong in thinking that sum-
mer insolation changes in the northern hemisphere have Month
been dominated by the 23,000-year period of precession. FIGURE 11-5 Intensity versus length of season Variations
He acknowledged that large variations in the amplitude of in the intensity of summer insolation at the 23,000-year
insolation changes do occur at the 23,000-year period dur- precession cycle are balanced by changes in the duration
ing summer, but he claimed that these changes are can- of the summer season.
celled by reductions in the length of the summer season.
This explanation originates with work centuries ago season, which would be subject to Kepler’s second law. In
by the astronomer Johannes Kepler. Kepler’s second law addition, it seems likely that the 182 days of higher (“sum-
states that planetary bodies moving in an elliptical orbit mer”) insolation would span most of the part of the year
vary in angular speed with their distance from the Sun. when ice sheets lie in an ablation regime. If so, the fact
When Earth is close to the Sun (at perihelion), it moves that insolation variations at the 23,000-year precession
faster than it does at other times in the orbit. When the signal are still larger than those at the 41,000-year tilt sig-
eccentricity of Earth’s orbit is unusually high, Earth’s nal in the caloric half-year insolation index suggests that a
precessional motion brings it even closer to the Sun at mismatch still exists between the forcing and the response.
perihelion during the summer season, and it moves with 11-2 Interhemispheric Cancellation of
even greater speed. At such times, the length of the 23,000-Year Ice Volume Responses?
summer season is reduced by Earth’s greater speed.
Huybers proposed that the net effect is that higher- The marine geologist Maureen Raymo proposed a
than-normal levels of insolation caused by Earth being different explanation for the mismatch. She assumed
unusually close to the Sun at perihelion are offset by the that northern hemisphere ice sheets have responded
shorter-than-normal length of the summer season. The with a strong 23,000-year signal as expected from the
result is that no net insolation change occurs on Earth insolation forcing, but she suggested that these changes
at the 23,000-year cycle (Figure 11–5). Insolation varia- were cancelled out in the global signal by a 23,000-year
tions caused by changes in tilt are not affected by this fac- response of Antarctic ice with opposite timing (phasing).
tor. If this explanation is correct, no mismatch actually Changes in precession occur at precisely opposite
exists between the insolation forcing and the ice sheet times in the two hemispheres from a seasonal point of
responses, both of which vary mainly at 41,000 years. view (Chapter 7). For example, the northern hemisphere
One problem with this explanation is that the 23,000- summer currently has an insolation minimum because
year signal remains dominant even in the caloric summer Earth is in its aphelion (distant-pass) position at that
half-year index used by Milankovitch (Chapter 7). This time of year. But six months later in early January, which
half-year index integrates the total insolation over those is summer in the southern hemisphere, Earth has moved
182 days of the year for which insolation levels are higher into its perihelion (close-pass) position. As a result,
than the other 182 days of the year. The specific 182 the southern hemisphere currently has a 23,000-year
calendar days in each caloric season vary through time insolation maximum, exactly opposite the summer
because of the insolation “boosts” introduced at different minimum in the northern hemisphere. Through time,
times of the year by Earth’s precession. This shifting defi- the two hemispheres maintain these exactly opposite
nition of “summer” avoids the problem of a fixed summer seasonal insolation trajectories at the 23,000-year cycle.