Page 186 - Earth's Climate Past and Future

162 PART III • Orbital-Scale Climate Change

(18,000 years), only a tiny fraction of the eventual
Insolation 10,000-year lag
Ice volume bedrock depression remains unrealized.
Bedrock behavior would work in the same sense but
in the opposite direction if the ice load were abruptly
removed. The rock surface would rebound toward the
level that is in equilibrium with the absence of an ice
A Orbital tilt (41,000 years) load. The initial rapid elastic rebound would be followed
by a slow viscous rebound lasting thousands of years.
Insolation Today parts of Canada (in the Hudson Bay region) and
Ice volume Scandinavia (around the Baltic Sea) are still undergoing
6,000-year lag
a delayed slow viscous rebound in response to ice melt-
ing that occurred many thousands of years ago.
Actual ice sheets in nature grow and melt much more
slowly than these idealized (instantaneous) examples.
B Orbital precession (23,000 years)
Time
FIGURE 9-9 Ice volume lags tilt and precession
(A) At the 41,000-year cycle of orbital tilt, the lag of ice
sheet size behind changes in summer insolation approaches
Ice sheet
one-quarter wavelength, or 10,000 years. (B) At the
23,000-year cycle of orbital precession, ice sheets lag roughly
one-quarter wavelength (6,000 years) behind changes in
Undeformed land surface
summer insolation and show the same modulation of
amplitude.
20,000 years later

loaded onto bedrock (Figure 9–10A). In time, the 3.3-km
ice sheet would eventually depress the underlying bed- 3 km
rock by 1 km. To put this bedrock change in a climatic
context, 1 km of elevation change is equivalent to a 6.5°C
1 km
change in temperature at Earth’s prevailing lapse rate. For Depressed land surface
this reason, these large changes in bedrock elevation can
translate into significant effects on temperature and mass A
balance at the surface of the overlying ice sheet.
Bedrock responds to the ice load in two phases 0 Ice load added
(Figure 9–10B). The initial reaction is a quick sagging
Immediate
beneath the weight of the ice. This elastic response (elastic) sinking
Depression
represents about 30% of the total vertical change in the
of bedrock
bedrock. Over the next several thousand years, the by ice sheet
bedrock continues to sink in a much slower (and larger) (km)
viscous response caused by the extremely slow flow of Gradual
rock in a relatively “soft” layer of the upper mantle (viscous)
between 100 and 350 km depth (see Chapter 4). sinking Full bedrock
This viscous response slows progressively as the depression
bedrock adjustment moves toward a final state of equi- 1
librium. Viscous behavior has a response time (see 10,000 20,000
B Elapsed time (years)
Chapter 1) of about 3000 years: that is, about half of the
remaining response needed to reach final equilibrium is FIGURE 9-10 Bedrock sinking (A) If an ice sheet 3.3 km
achieved every 3000 years. The rate of change of the thick were suddenly placed on the land, the bedrock would
curve gradually slows through time because each suc- sink almost 1 km under the load. (B) The initial sinking would
cessive 3000-year response time eliminates half of the be elastic and immediate, but the later response would be
remaining (unrealized) response (1 > 1/2 > 1/4 > 1/8, viscous and slower, with about half of the remaining sinking
and so on). After six response times of 3000 years each occurring every 3000 years.