Page 166 - Earth's Climate Past and Future

142 PART III • Orbital-Scale Climate Change

This assumption has a good basis in fact. Many lakes value. Insolation levels below the threshold value leave
in North Africa that existed 11,000 years ago during the no monsoon evidence in the geologic record.
strong monsoon interval are dry today, even though a The strongest predicted monsoon peaks in Figure
weak summer monsoon still occurs at present in 8-5 occur between 85,000 and 130,000 years ago, when
response to today’s low levels of summer radiation. the summer insolation curve reached its largest maxima
Apparently it takes a threshold insolation value well because of modulation of the precession signal by
above the present level to bring most North African orbital eccentricity (Chapter 7). In contrast, the weaker
lakes into existence. insolation maxima near 35,000 and 60,000 years ago
Second, we assume that the strength of the mon- should have produced less powerful monsoons. All
soon response (such as the water level of the North long-term summer insolation minima (such as the one
African lakes) is directly proportional to the amount by we are in today) fall below the critical threshold.
which summer insolation exceeds the threshold value. We can examine actual climate records for evidence
This assumption has a reasonable physical basis: of this predicted monsoon response. Because most of
stronger insolation should drive stronger monsoons and North Africa is arid and because erosion of sediment is
fill lakes to higher levels. much more prevalent than deposition, its climate his-
Third, we assume that the strength of the monsoon tory is sparse and difficult to date. Fortunately, the
in the past as recorded in lake level records is a compos- nearby seas and oceans contain continuous and well-
ite of the average monsoon strength over many individ- dated records.
ual summers. Actually, this is more fact than assumption:
the wet monsoon circulations that develop every sum- 8-2 “Stinky Muds” in the Mediterranean
mer inevitably cease during the following winter. The
lakes fill because of the integrated effect of many wet The water that fills the Mediterranean Sea today has a
summers. When scientists sample records of these relatively high oxygen content. Near-surface waters are
changes, they are looking at year-by-year responses well oxygenated because they exchange oxygen-rich air
blended into a longer-term average over hundreds or with the atmosphere and because photosynthesis by
even thousands of summers. marine organisms produces O (companion Web site,
2
As a result of these three assumptions, we arrive at p. 32). The high oxygen content of deep waters results
the predicted monsoon response shown by the green from sinking of oxygen-rich surface water during win-
shading in Figure 8-5. Insolation maxima above the ter (Figure 8-6A).
threshold value produce a series of pulselike monsoon This sinking motion results from two factors (com-
(and lake) maxima at regular 23,000-year intervals. panion Web site, pp. 24–25): (1) the high salt content of
These pulses vary in strength according to the amount the Mediterranean Sea, caused by the excess of summer
by which summer insolation exceeds the threshold evaporation over precipitation, and (2) winter chilling

Strong
Weak
Nile runoff Nile runoff

Low-salinity lid
Organic debris
Oxygen-rich
deep water
Oxygen-depleted
deep water

Normal Organic-rich
deep-ocean black muds
sediments
A Weak summer monsoon B Strong summer monsoon
FIGURE 8-6 Mediterranean circulation and monsoons (A) In today’s Mediterranean
circulation, salty surface water chilled by cold air in winter sinks and carries dissolved oxygen
to deeper layers. (B) At intervals in the past, strong summer monsoons in tropical Africa
caused an increased discharge of Nile River freshwater into the eastern Mediterranean, creating
a low-density surface-water lid that inhibited sinking of surface water and caused the deep
ocean to lose its oxygen and deposit organic-rich black muds.