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CHAPTER 11 • Orbital-Scale Interactions, Feedbacks, and Unsolved Problems 193

N some cooling even as far east as the interior of Eurasia
90° (Figure 11–3B). Precipitation in these chilled regions
falls significantly because the colder ocean gives off
70° less water vapor to the atmosphere. This experiment
indicates that an ice sheet signal initially transferred
50° to the North Atlantic would then be transferred to
Europe and even Asia. Both signal transfers would have
30° occurred with lags of no more than a few centuries.
How far east the ice sheet signal might penetrate into
Asia is not entirely clear. The Siberian high-pressure
10°
center, which even now is the winter season center of
W 80° 40° 0° 40° 80° 120°E cooling and powerful winds in northern Asia (companion
A Input to model: Colder sea surface temperatures (°C) Web site, p. 21), plays an important role. Model experi-
ments indicate that the Siberian High would have been
0 to –4 –4 to –8 > –8
N strengthened in winter and would have lasted for a
90° longer part of the year in a world with ice sheets present.
These model experiments indicate that ice sheets
70° have a large downstream climatic impact across a broad
area of the northern hemisphere at high and middle lat-
50° itudes (Figure 11–4). The effects reach farthest south
during winter because of very strong wind flow during
that season. In summer, wind strength drops and local
30°
heating of the land by the Sun becomes more important.
In the northern tropics and subtropics, the second
10° fundamental tempo of global climate change exists—the
W 80° 40° 0° 40° 80° 120°E orbitally driven monsoon circulations. These changes
B Output from model: Colder air temperatures (°C) are dominated by the 23,000-year precession cycle,
compared to the predominant ice volume responses
FIGURE 11-3 Surface-ocean sensitivity test Sensitivity tests at 41,000 and ~100,000 years. In summer, these local
with a GCM show that (A) inserting cold ocean temperatures changes in solar radiation heating overwhelm the weaker
into an interglacial world (B) produces colder air temperatures message sent out by the ice sheets. Monsoon-related
over Europe and Asia. (Adapted from D. Rind et al., “The variations can also be found in the southern subtropics
Impact of Cold North Atlantic Sea-Surface Temperatures on (Chapter 8).
Climate: Implications for the Younger Dryas,” Climate Dynamics 1 The southern hemisphere lies entirely outside the
[1986]: 3–33.)
region under the direct influence of northern ice sheets
on atmospheric circulation. Tests with general circula-
Atlantic Ocean to near-glacial values north of 20°N tion models show that the presence of large masses of
(Figure 11–3). The rest of Earth’s surface was left in its ice in high northern latitudes has no significant effect
current state, with no ice sheets except the small one on temperatures anywhere in the southern hemisphere.
now on Greenland. This conclusion pertains only to transfers that occur
The model simulation shows that the cold North through the physical circulation of the atmosphere by
Atlantic sea surface projects very cold air temperatures winds and pressure fields. We will see later that other
downwind to the western maritime parts of Europe and means of transferring northern signals southward exist.

FIGURE 11-4 Regions of
ice-driven responses High and
middle latitudes of the northern
hemisphere show evidence of climate
responses controlled by changes in
the sizes of ice sheets.