Page 184 - Earth's Climate Past and Future

160 PART III • Orbital-Scale Climate Change

more positive and increases the accumulation of snow
Equilibrium line and ice. In effect, once ice sheets start to grow, they con-
Accumulation Ablation tribute to their own positive mass balance by growing
upward into a net accumulation regime. Eventually the
tops of the ice sheets reach elevations at which snowfall
is lower because the frigid air contains very little water
vapor and this positive feedback effect weakens.

North South
9-2 Ice Sheets Lag Behind Summer Insolation
Equilibrium line Ablation
Accumulation Forcing
The response of ice sheets to changes in summer insola-
tion is far from immediate. Imagine what would happen
if climate suddenly cooled enough to permit snow to
fall throughout the year and accumulate rapidly over all
of Canada. Based on the mass balance values plotted in
Figure 9–1, about 0.3 m of ice might accumulate each
year. But even under this unrealistically favorable con-
FIGURE 9-7 Ice elevation feedback As ice sheets grow higher,
more of their surface lies above the equilibrium line in a regime dition, a full-sized ice sheet 3000 m thick would take
of net accumulation, even if the equilibrium line does not move. 10,000 years to form. In reality, the ice would take
much longer to accumulate because the initial cooling
would not be instantaneous.
reach altitudes where temperatures are much colder. The geologist John Imbrie and his colleagues have
The tops of the ice sheets can reach elevations of 2 to led the exploration of the link between insolation and ice
3 km, where temperatures are 12° to 19°C cooler than volume. They use as an analogy a conceptual model
those at sea level (using an average lapse-rate cooling of based on variations through time in the intensity of the
6.5°C per kilometer of altitude (companion Web site, flame in a Bunsen burner shown in Figure 9–8 (top); also
p. 15). This cooling makes the ice mass balance still see Chapter 1. Because water has a high heat capacity

Maximum
Bunsen
burner
heating Water
Water temperature
temperature

Lag
Minimum
Time (in minutes)

Minimum
Solar
radiation Figure 9-8 Ice volume lags insolation (Top) As the
flame of a Bunsen burner is alternately turned higher
Ice volume and lower, the water heats and cools but with a short
Ice volume
time lag behind the changes in heating. (Bottom)
Similarly, long-term increases and decreases in
Lag summer insolation heating cause ice sheets to melt
and grow but with lags of thousands of years.
Maximum
Ice Time (in thousands of years) (Adapted from J. Imbrie, “A Theoretical Framework for
the Pleistocene Ice Ages,” Journal of the Geological Society
(London) 142 [1985]: 417–32.)