Page 182 - Earth's Climate Past and Future

158 PART III • Orbital-Scale Climate Change

Surface temperature (˚C) FIGURE 9-3 Milankovitch theory According to the
-30 -20 -10 0 10 Milankovitch theory, (top) high summer insolation heats the
+ land and results in greater ice ablation, while (bottom) low
Ice mass balance (m/yr ) -1 melting form. (Modified from J. Oerlemans, “The Role of Ice Sheets in the
summer insolation allows the land to cool and ice sheets to
0
Pleistocene Climate,” Norsk Geologisk Tidsskrift 71 [1991]: 155–61.)
Net

the Arctic Ocean. These models portray the changes in
ice sheets across a single (idealized) high-latitude conti-
-2 (simplified) representation of the ice sheets surrounding
Maximum nent near the Arctic, but they ignore possible differences
between various sectors of the Arctic (Europe versus
North America). This simplification is reasonably well
Summer justified because ice sheets seem to have grown on all the
insolation continents around the Arctic, at least during the last
Time
glacial maximum 20,000 years ago (Figure 9–4).

Minimum 9-1 Insolation Control of Ice Sheet Size
+ The mechanism by which changes in summer insolation
Net accumulation control the size of ice sheets on northern landmasses in
Ice mass balance (m/yr ) -1 perature responses that alter both melting rates and ice
these models follows directly from Milankovitch’s the-
0
ory. Changes in summer insolation drive regional tem-
mass balance.
One kind of model represents this relationship as
changes in mass balance along a north-south line (Figure

vertical direction (altitude) and the other in a north-
-2 9–5). These transects have just two dimensions, one in a
-30 -20 -10 0 10 south direction (latitude). Changes in the other horizon-
Surface temperature (˚C) tal dimension (longitude) are ignored to allow the models
to simulate changes over longer intervals of time than
would be possible with full three-dimensional models,
which are computationally more demanding.

Arctic
Ocean Kara (?)
Cordilleran Barents Eurasia

Greenland

Laurentide Scandinavian

North FIGURE 9-4 Ice sheets around the
America Arctic Ocean At the last glacial
maximum, 20,000 years ago, ice
sheets surrounded much of the Arctic
Ocean. (Modified from G. Denton and
North Atlantic
T. Hughes, The Last Great Ice Sheets [New
York: Wiley, 1981].)