Page 165 - Earth's Climate Past and Future

CHAPTER 8 • Insolation Control of Monsoons 141

occur at the same times in the past as stronger down- We can use the June 30°N insolation curve in Figure 8-5
and-out monsoon flows in winter. to construct a simple conceptual model that predicts how
At first it might seem that the climatic effects of the summer monsoon should have varied with time if the
these opposed insolation trends in the two seasons might orbital monsoon hypothesis is valid. The predicted mon-
cancel each other, but this is not the case for the annual soon response is based on three assumptions.
precipitation produced by the monsoons. Monsoonal First, we assume that a threshold level of insolation
winters are always dry, regardless of the amount of inso- exists, below which the monsoon response will be so
lation, because the air descending from higher in the weak that it will leave little or no evidence in the geo-
atmosphere holds very little moisture (see Figure 8-4C). logic record (see Figure 8-5). A good example is the level
As a result, orbital-scale changes in winter insolation of lakes in arid regions. The orbital monsoon hypothesis
have essentially no effect on annual rainfall. In contrast, predicts that the higher the level of summer insolation,
because summer monsoon winds coming in from the the stronger the summer monsoon rainfall and the
ocean carry abundant moisture, orbital-scale changes in higher the lake levels. But if the insolation value falls
summer insolation have a large affect and in fact the below a certain level, the weakened summer monsoon
dominant impact on annual rainfall. may produce so little rain that the lakes dry up com-
This imbalance is an example of a nonlinear pletely. A dried-out lake can no longer register changes
response of the climate system to insolation: the toward even drier climates because it has reached the
amount of rainfall is highly sensitive to insolation threshold limit of its ability to record climate.
change in one season (summer) but not sensitive to
changes in the other (winter). As a result, the system has
a strong annual response even though the equal-and- June insolation (30˚N)
opposite insolation trends in the two seasons might Weaker Stronger
have been expected to cancel each other. 0
Modern
Orbital-Scale Changes in North African insolation
Summer Monsoons 20,000

The orbital monsoon hypothesis can be tested against a
wide range of evidence, including changes in lake levels
across arid regions like North Africa. At low and middle 40,000
latitudes, changes in the amount of incoming solar inso-
lation follow the 23,000-year rhythm of orbital preces-
sion (Chapter 7). A June insolation curve from latitude
30°N covering the last 140,000 years clearly shows this
23,000-year tempo (Figure 8-5). Note that today’s June Years ago 60,000
insolation level is well below the longer-term average.
The orbital monsoon hypothesis predicts that stronger 80,000
summer monsoons should have occurred at those times Monsoon
in the past when summer insolation values were signifi- response
cantly larger than the modern value. 100,000 (lake levels)
The most recent instance when summer insolation
values were substantially higher than today’s occurred
near 11,000 years ago. Evidence we will examine in Part
IV of this book shows that lakes across tropical and 120,000
subtropical North Africa were at much higher levels Insolation
curve
during this insolation maximum and for some time Threshold
afterward. In fact, many lakes that were filled to high 140,000 value
levels at that time are completely dry today. This evi-
dence indicates that a strengthened summer monsoon FIGURE 8-5 Conceptual model of monsoon response
circulation reached much farther northward into North to summer insolation Increases in summer insolation
Africa 11,000 years ago than it does today, in agreement heating above a critical threshold value drive a strong
with the orbital monsoon hypothesis. monsoon response at the 23,000-year tempo of orbital
But this interval was only one brief period in a long precession. The amplitude of this strong monsoon response
history of rainfall changes in North Africa. What about is related to the size of the increase in summer insolation
the much longer-term behavior of the summer monsoon? forcing.