Page 93 - Earth's Climate Past and Future

CHAPTER 4 • Plate Tectonics and Long-Term Climate 69

This pervasive aridity reflects two factors: (1) the
great expanses of land at subtropical latitudes beneath
the dry, downward-moving limb of the Hadley cell
and (2) the large amount of land in the tropics, causing
trade winds to lose most of their water vapor before
reaching the continental interior (companion Web site,
pp. 14–22). The ocean around Pangaea received far
more rainfall than the land and more than it does today.
Geologic evidence supports the model simulation of
widespread Pangaean aridity. The clearest evidence is
the distribution of evaporite deposits, salts that precip-
itated out of water in lakes and in coastal margin basins
with limited connections to the ocean. Evaporite salts
form only in arid regions where evaporation far exceeds
precipitation. More evaporite salt was deposited during
the time of Pangaea than at any time in the last several
hundred million years (Figure 4-13). Evaporite deposits
occurred in the interior and along the tropical east coast
of Pangaea, regions the model simulates as arid.
Because the moderating effects of ocean moisture
FIGURE 4-11 Pangaean trees Modern gingko trees are failed to reach much of Pangaea’s interior, the continent
descended from similar forms that first evolved some 200 Myr was left vulnerable to seasonal extremes of heating by
ago. (Courtesy of Mike Bowers, Blandy Farm, Boyce, VA.) the Sun in summer and cooling during winter. As a

assumed a level of 1650 parts per million, almost six
2
times the natural (preindustrial) value of 280 parts per
million. As we will see, this choice not only produced
temperature distributions consistent with the evidence 4 4
from ice and from frost-sensitive vegetation but also 4 4
simulated other climatic features that match indepen- 6
6
dent evidence from the Pangaean geologic record.
With the critical boundary conditions specified, the 4 4
model simulation is ready to run. After 15 years of
simulated time to allow the model climate to come to a
2
state of equilibrium, the results shown are based on the
last 5 years of simulated seasonal changes. A Annual precipitation (mm/day) > 4 2–4 < 2

4-5 Output from the Model Simulation of Climate
on Pangaea
Because of its huge size, we might anticipate that the
interior of Pangaea would have had an extremely dry
continental climate, in the absence of the moderating
influence of oceanic moisture. The climate model simu-
lation confirms this expectation.
The model simulates widespread aridity at lower
latitudes, especially in the Pangaean interior. Mean
annual precipitation and soil moisture levels are low
B Annual soil moisture (cm) > 8 2–8 < 2
across large expanses of interior and western Pangaea
between 40°S and 40°N (Figure 4-12). Precipitation FIGURE 4-12 Precipitation on Pangaea Climate models
values of 1–2 mm per day in these regions are equiva- simulate patterns of (A) annual mean precipitation and (B)
lent to annual totals of 15–25 inches (35-70 cm) per annual soil moisture on Pangaea. Broad areas of the tropics
year, comparable to those in semiarid grassland areas and subtropics were very dry. (Adapted from J. E. Kutzbach,
such as the western plains of the United States today “Idealized Pangean Climates: Sensitivity to Orbital Change,”
(Figure 4-12A). Geological Society of America Special Paper 288 [1994]: 41–55.)