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226 PART IV • Deglacial Climate Change

resulting in subfreezing temperatures at higher eleva-
+2 tions (companion Web site, pp. 15, 27–28). This rela-
Glacial temperature change (°C) –2 to explain the lowering of tropical mountain glaciers by
tionship has been used to estimate a cooling of 4°–6°C
0
600–1000 m during the glacial maximum.
Additional evidence for larger glacial cooling comes
from the descent of the upper tree limit and other kinds
–4
conditions on the upper flanks of mountains, tempera-
Alkenones
ture limits the growth of many kinds of vegetation, and
–6 CLIMAP of vegetation high on tropical mountains. In the harsh
S 20° 10° 0° 10° 20° N the vertical drop in high-altitude vegetation limits dur-
Latitude
(Tropical Indian Ocean) ing the last glaciation equals or even exceeds that of the
mountain glaciers (see Figure 12–21).
FIGURE 12-20 Confirmation of small Indian Ocean
cooling A biochemical method of estimating past sea surface 12-12 Actual Cooling Was Medium-Small
temperatures indicates a small cooling of the tropical Indian After years of disagreement, a resolution of this problem
Ocean, similar to the values found by CLIMAP. (Adapted from seems to be emerging—the tropical cooling was neither
E. Bard et al., “Interhemispheric Synchrony of the Last as small as CLIMAP claimed nor as large as the critics
Deglaciation Inferred from Alkenone Paleothermometry,”
Nature 385 [1997]: 707–10.) initially thought but near the middle of the two esti-
mates. One reason the CLIMAP estimates were too
small is that plankton are less sensitive at low latitudes to
alkenones that constitute small fractions of tiny plant changes in temperature than to changes in the availability
plankton (coccolithophores). The past abundances of of food. The low-latitude surface ocean is depleted in
these molecules can be measured in small CaCO plates nutrients, and plankton are forced to adopt strategies for
3
(coccoliths) deposited in ocean sediments (see Figure surviving where food is scarce. This requirement over-
2–14). The relative amounts of two types of alkenone whelms the relatively small dependence on temperature.
molecules are sensitive to temperature in the modern In addition, the CLIMAP reconstruction for the
ocean and can be used to reconstruct past temperatures. Pacific Ocean was based on samples in which CaCO
3
In a north-south transect of cores across the western had been extensively dissolved on the seafloor, thereby
Indian Ocean, the cooling indicated by both methods is altering the assemblages of foraminifera and coccoliths.
generally less than 2°C, with a larger cooling registered Similarly, the remnants of siliceous organisms (radiolaria
by the alkenone method only above 15°N (Figure and diatoms) left in the sediments are very different
12–20). Temperature estimates based on magnesium/ from those that originally lived in the surface waters.
calcium ratios (Chapter 6) have also been made for Estimates of temperatures at the Pacific Ocean surface
many other regions. These methods indicate sea- derived from these different types of plankton often
surface temperatures cooler than those of CLIMAP by disagree, an indication that some or all are unreliable.
as much as 1°–2°C in some regions, but the agreement
is closer in other regions.

12-11 Evidence for a Large Tropical Cooling 5 Grasslands Ice 600–1000
4
meters
A different view emerges from other indicators, most of Elevation (km) 3
which come from continental records. The most com- 2
pelling evidence is the descent of the lower limit of 1 Forest
mountain glaciers by 600–1000 m throughout the trop-
Andes today Last glaciation
ics and middle latitudes (Figure 12–21). This drop in
the elevation of the ice line has been interpreted as
FIGURE 12-21 Descent of tropical mountain glaciers and
requiring a cooling of 4°–6°C over tropical mountains.
forests The limits of mountain glaciers in the Andes were
The lower limit of mountain glaciers today is deter- 600–1000 m lower during the last glaciation than they are
mined mainly by temperature and secondarily by fac- today, and the upper limits of forests were similarly lower.
tors such as the amount of precipitation and the degree These major shifts indicate a tropical cooling of at least 5°C,
to which local mountain topography shelters the glaci- much larger than the 1°–2°C suggested by CLIMAP. (Adapted
ers from direct sunlight. Glaciers exist today on tropical from T. van der Hammen, “The Pleistocene Changes of
mountains higher than 5 km because the atmosphere Vegetation and Climate in Tropical South America,” Journal of
cools by 6.5°C or more per kilometer of elevation, Biogeography 1 [1974]: 3–26.)