Page 94 - Earth's Climate Past and Future

70 PART II • Tectonic-Scale Climate Change

Evaporite deposition extent with the vegetation evidence. Despite the high
4
2
(10 km /Myr) CO values used as input to the simulation, freezing still
2
0 20 40 60 occurs farther south in the model than the evidence
from past vegetation indicates.
Another characteristic of the climate of Pangaea was
the strong reversal between summer and winter mon-
soon circulations. Monsoon circulations are driven by
100 ?
the different rates of response of the land and the
oceans to solar heating in summer and radiative heat
loss in winter (companion Web site, pp. 15–18). The
large seasonal swings in land temperature and small
200 seasonal changes in ocean temperature reflect these
Myr ago Pangaea contrasting responses of land and ocean.
Strong solar heating over the part of Pangaea situ-
ated in the summer hemisphere caused heated air to rise
300 over the land and a strong low-pressure cell to develop
at the surface (Figure 4–15A). The rising of heated air
caused a net inflow of moisture-bearing winds from the
ocean, especially in the subtropics, bringing heavy rains
to the subtropical east coast (Figure 4–15B).
400
The situation in the winter hemisphere was exactly
the reverse. The weak seasonal heating from the Sun
and strong heat loss by longwave back radiation caused
cooling over the interior of Pangaea. The cooling
500 caused air to sink toward the land surface, built up high
pressures over the continent, and pushed cold, dry air
FIGURE 4-13 Pangaean evaporites The volumes of rock out over the ocean. As a result, precipitation over the
salt deposits (evaporites) formed on Pangaea about 200 Myr land was reduced.
ago were larger than those formed at any other time in the last Note that the winds on the eastern margins of Pan-
500 Myr and indicate very dry conditions. (Adapted from W. A.
gaea from 0° to 45° latitude reversed direction between
Gordon, “Distribution by Latitude of Phanerozoic Evaporites,”
Journal of Geology 83 [1975]: 671–84.) the seasons: warm summer monsoon winds blew from
the sea onto the land, but cold winter monsoon winds

result, the model simulates a huge seasonal temperature
response (Figure 4-14). In some mid-latitude regions,
Winter hemisphere
summer daily mean temperatures of +25°C (77°F)
alternated with winter daily mean temperatures of
–15°C (+5°F).
The occurrence of extremely continental climates
on Pangaea may help to explain the absence of ice
sheets at high latitudes. The simulated winter tempera-
tures were cold enough to provide the snowfall needed
for ice sheets to grow. But hot summers on Pangaea
even on the poleward margins of the landmass caused
rapid melting of snow and thereby prevented glaciation.
Ice sheets form more readily on smaller continents
Summer hemisphere
where summer temperatures are cooled by moist winds
Seasonal temperatures (°C) > 30 0–30 < 0
off the ocean.
The model simulation also indicates that average FIGURE 4-14 Temperature on Pangaea Climate model
daily land temperatures in winter would have reached simulations show extreme seasonal temperature contrasts on
the freezing point as far equatorward as 40° latitude Pangaea between the summer hemisphere, which was warmed
(see Figure 4-14), closely matching the low-latitude by solar radiation, and the winter hemisphere, which lost heat
limit of frost-sensitive vegetation on Pangaea. But with by longwave back radiation. (Adapted from J. E. Kutzbach,
winter nights likely to have been colder than the daily “Idealized Pangean Climates: Sensitivity to Orbital Change,”
mean, the model’s results actually disagree to some Geological Society of America Special Paper 288 [1994]: 41–55.)