Abrupt Climate Change: A Paleoclimate Perspective From the World’s Highest Mountains - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 218

coastal and highland cultures in Figure 4 argues that the
current trends are counter to the patterns of population con-
centrations over the last 1500 years.

ure 5c), much of which occurs in areas of highest SSTs and
500 mbar temperatures, especially over the western tropical
Paci c. Tropical thermal energy and humidity are the fuel for

the latent heat engine that controls so much of the planet s

weather systems. A satellite view of the Earth shows the
clouds in the intertropical convergence zone just north of the
equator, where warm air rises and releases moisture (Fig-
ure 6). As the warm, moist air rises over the equator and
cools, the maximum latent heat release occurs at the 500 mbar
level [Webster, 2004]. To the north and south of this band,
around the geographic boundaries of the tropics, dry air
descends as it cools, giving rise to deserts and arid regions.
This major circulation pattern, the Hadley Cell, undergoes

'

4. CLIMATE CHANGE AND ITS CURRENT EFFECTS
ON THE CRYOSPHERE

Figures 5a and 5b show that during the boreal winter
(December-January-February) the middle troposphere and
SSTs are highest in the tropical latitudes, although tropical
atmospheric temperatures tend to remain fairly uniform
throughout the year [Sobel, 2002]. This thermal energy is
instrumental in the production of tropical precipitation (Fig-

Figure 5. December-January-February global temperatures at (a) the middle troposphere (500 hPa) and (b) at the sea
surface. (c) Xie-Arkin global precipitation. Image courtesy of C. Bretherton and A. Sobel.