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148 PART III • Orbital-Scale Climate Change

could be the source of the “true” forcing of summer mon-
IN SUMMARY, evidence from several continents fully
supports John Kutzbach’s orbital monsoon hypothesis. soons at the 23,000-year cycle. The problem is to provide
At this point, the hypothesis has passed so many tests a specific physical justification for the one chosen.
that it merits the higher status of a theory (Chapter 1). One view is that peak monsoon responses should
have the same phase as the peak in annual insolation
forcing at the June 21 summer solstice. Additions to this
already strong solstice forcing at the 23,000-year cycle
8-6 The Phasing of Summer Monsoons
could then drive the strongest monsoon response. In
The evidence examined in this chapter has shown that this view, the later (July) phase indicated by a range of
peak development of past summer monsoons at the evidence noted earlier results from lags in the climate
23,000-year cycle occurred in midsummer (July north of system that retard the peak monsoon development. The
the equator, February to the south). This evidence has size of this lag can be calculated as slightly less than 2000
been interpreted in two ways. To understand the differ- years, the difference between a phase of June 21 and a
ence in these interpretations, we need to return to the phase of middle or late July at the 23,000-year cycle.
effects of precession on insolation. Recall that Earth’s pre- Ice sheets have been proposed as one possible source
cessional motion produces a family of monthly insolation of such a retarding effect. Strong monsoons developed
curves, each offset from the preceding month by one- during intervals when northern hemisphere ice sheets
twelfth of a 23,000-year cycle, or slightly less than 2000 were present but melting rapidly. In this view, the large-
years. For example, the insolation signal for the month of scale cooling caused by the lingering ice sheets could
July lags that for the month of June by this amount (see have retarded full summer monsoon heating for 1000 to
Figure 7–18). Because an entire family of insolation 2000 years until the ice sheets became too small to have
curves is available as possible monsoon drivers, any month a major impact on tropical climate. Another possibility is

BOX 8-1 LOOKING DEEPER INTO CLIMATE SCIENCE

Insolation-Driven Monsoon Responses: Chronometer for Tuning

he clearly demonstrated link between summer insola- African Summer
Ttion forcing and monsoon responses at low latitudes lake levels insolation
Low High Low High
has become part of the basis for a new way of dating sedi-
Basalt
mentary records on land and in the oceans. This method, (Ar dating)
orbital tuning, can provide even better time resolution
than radiometric methods.
The tuning method is based on the relationship
between the insolation signal (the forcing) and the sum-
mer monsoon changes (the response). The timing of
orbital insolation changes is known with great accuracy
from astronomical calculations, and a range of monsoon Ages from
Depth in astronomical
responses can be measured in sediments. By making the dating
lake
simple assumption that the insolation driver and the
sediment
monsoon responses have kept the same relationship in
the past, the monsoon responses in the sediments can be
dated with nearly the same accuracy as the orbital forcing.
This method is most easily applied in ocean sediments
because deposition in the ocean tends to be continuous.

Tuning sediment sequences to orbital variations If lavas or
magnetic reversal boundaries provide radiometrically dated
levels in terrestrial or marine sediments, the age of
intervening sediment intervals can be determined by tuning
Basalt
monsoon-driven sediment responses to insolation changes at
(Ar dating)
the 23,000-year precession cycle.