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CHAPTER 11 • Orbital-Scale Interactions, Feedbacks, and Unsolved Problems 203

mechanism does not depend on (but also does not con- and hastened the melting of the slower-responding ice
tradict) the requirement of a long-term cooling to sheets within the northern continental interiors.
explain the transition from the 41,000-year to the One problem with explanations that focus only on
~100,000-year glacial world. It requires only a change the rapid terminations is that they ignore the rest of the
in the nature of the material on which the ice sits. ~100,000-year cycles—the longer intervals of episodic
ice growth that occurred during the other ~90,000
years. The accumulation of large ice sheets over these
11-6 Ice Interactions with the Local Environment
intervals cannot be explained as a linear one-for-one
Another group of explanations of the oscillations at response to changes in summer insolation. Ice accumu-
~100,000 years looks to interactions between the ice lation requires some kind of non-linear process just as
sheets and the changes they impose on the nearby climate much as rapid melting on deglaciations.
system. These ideas particularly focus on ways to explain
the speed of deglacial terminations. Large insolation max- 11-7 Ice Interactions with Greenhouse Gases
ima are assumed to initiate and pace deglacial melting (see
Figure 11–15), but processes within the climate system A final possibility is that greenhouse gases (particularly
are invoked as the mechanism of accelerated melting. CO ) play a key role in the ~100,000-year ice sheet
2
One idea is that the cooling produced by northern ice oscillations. An early proposal was that a CO response
2
sheets caused large amounts of sea ice to expand across at the ~100,000-year period emerged independently
nearby high-latitude oceans. This increased ice cover from somewhere in the climate system and drove an ice
would reduce the extraction of moisture that could be sheet response at that period. As shown in Chapter 10,
delivered to the ice sheets, and the ice sheets would melt however, the CO lead relative to ice volume is too
2
faster because of moisture starvation. Glacial geologists small (~2,000 years) for CO forcing to have been the
2
have criticized this idea because ice accumulation is a rel- primary relationship with the ice sheets (see Box 11–1).
atively weak factor in the mass balance of ice sheets com- The other possibility is that CO is primarily a posi-
2
pared with ablation (see Figure 9–1). As a result, moisture tive feedback on ice volume. In the 41,000-year glacial
starvation should have had relatively little effect on the world, according to this hypothesis, ice sheets formed
rapid reductions in ice volume during terminations. mainly at the 41,000-year period because of positive
Another proposal is that the windy, dusty glacial feedback from CO , but the ice melted during the next
2
world produced a thin coating of dust on the lower summer insolation maximum. In the new ~100,000-
southern margins of the ice sheets. Because dust has a year glacial world that developed after 0.9 Myr ago, ice
lower albedo than ice, it would have absorbed more sheet growth again occurred mainly at the 41,000-year
incoming solar radiation and warmed the surface of the cycle, but complete ice melting did not.
ice. The absorbed heat would have promoted greater The last interglacial-glacial oscillation (Figure 11–17)
18
melting and more rapid deglaciation. One problem with shows five intervals of ice growth marked by δ O
this idea is that a slightly thicker coating of dust can have increases. Neither of the two oscillations—those near
the opposite effect of insulating the ice surface from 95,000 and 45,000 years ago—that were a response only
solar heating and actually reducing the rate of ablation. to summer insolation changes at the 23,000-year cycle
Still another idea is that the large ~100,000-year ice resulted in ice volumes larger than had been attained ear-
sheets developed extensive margins that fronted on the lier in this climatic oscillation. All the net ice growth
ocean or rested on bedrock lying below sea level (simi- occurred during the three other oscillations—those near
lar to the modern West Antarctic Ice Sheet). These 115,000, 72,000, and 30,000 years ago. These ice growth
marine margins would have been vulnerable to rises in episodes were separated by approximately 41,000 years,
sea level that would have “lifted” them off the bedrock occurred during insolation minima at the tilt cycle, and
bases that otherwise stabilized their flow. As a result, were accompanied by large decreases in atmospheric
they could have responded more quickly to climate CO concentrations. Based on this evidence, net ice
2
changes (initiated by rising insolation levels) than the growth during this ~100,000-year climatic oscillation
more sluggish land-based ice sheets deep in the conti- occurred during 41,000-year episodes and was aided by
nental interiors. Evidence from marine sediments sug- positive CO feedback, much like the ice growth episodes
2
gests that ice sheets vulnerable to rising sea level first in the earlier 41,000-year glacial world.
appeared along the Arctic margin north of Norway near Unlike the earlier 41,000-year regime, however, only
0.9 Myr ago. One of these ice sheets—the Barents Ice a fraction of the ice that grew during the intervals
Sheet just north of Scandinavia (see Figure 9–4)—was centered on 115,000 and 72,000 years ago melted during
also among the first northern ice to melt during the the following insolation maxima. Instead, most of the ice
most recent deglaciation. The early disappearance of remained in place as a base for the next interval of growth
this ice sheet might have helped warm polar latitudes to even larger volumes. Because climate had cooled by