Page 289 - Earth's Climate Past and Future

CHAPTER 14 • Millennial Oscillations of Climate 265

IN SUMMARY, the evidence argues against a strong
cyclical effect of solar variability on climate. A link
between random (noncyclic) variations in Sun
strength and climate at shorter (decadal to century)
time scales remains a possibility.

14-7 Natural Instabilities in Ice Sheets Initial
bedrock
Another possible explanation of millennial oscillations pinning point
in climate is that they resulted from natural internal
A Initial ice margin
variations in the behavior of northern hemisphere ice
sheets. Considered as a whole, the great masses of ice
lying on the continents have very slow response times of
many thousands of years (Chapter 9). In contrast, the M elting
ocean margins of the ice sheets are capable of faster
changes because they slide on soft sediments and release
large amounts of ice (icebergs) to the ocean. B Heat from below C Depression of bedrock
Along the marine margins of ice sheets, ice flows
over bedrock with irregular bumps and depressions
(Figure 14–16A). The bottom layers of ice scrape
against higher-standing areas called bedrock pinning Ice retreat
points, and the resulting friction slows the flow of ice.
The bottom layers of ice can also freeze to the bedrock
and slow the flow even more. Ocean water can produce
the opposite effect: because the ice margins float in sea- New bedrock Initial bedrock
water, changes in sea level can lift the ice off its pinning pinning point pinning point
points. D New ice margin
One idea is that the slow natural release of small
amounts of heat from Earth’s interior can melt the FIGURE 14-16 Natural oscillations of ice margins (A) Marine
lower ice layers along ice margins (Figure 14–16B). The margins of ice sheets end in thin ice shelves flowing across
melting produces meltwater that trickles into the soft upward-protruding bedrock knobs. This ice can be dislodged
from these pinning points either by (B) Earth’s heat escaping
underlying sediments and makes them unstable, causing
from below and melting ice or by (C) the gradual weighing
the ice margins to surge forward into the ocean. The
down of bedrock under the heavy load of growing ice. Either way,
surges release icebergs, which float away elsewhere and
(D) the ice margin retreats inland and stabilizes over another
melt, and the thinned ice sheet margins retreat well
bedrock pinning point.
inland from their previous positions. The new margins
then slowly thicken and advance until the buildup of
heat from below again destabilizes them. times when large ice sheets existed. In addition, the
A second idea focuses on a different kind of interac- largest episodes occurred when the air and ocean were
tion between ice margins and the bedrock (Figure cold, rather than during the warmer intervals that
14–16C). Over time, as the ice margins thicken, their might be expected to cause ice margins to collapse
weight depresses the underlying bedrock, which gradu- because of faster melting. This evidence argues against
ally sinks over thousands years. At some point, depres- local warming as the driver of the iceberg pulses but
sion of the bedrock causes the ice to sink far enough allows for mechanisms based on internal ice sheet
with respect to sea level that it can be lifted and floated instabilities.
by ocean water. Because the ice margin is no longer The ice instability hypotheses also have limitations.
anchored to the bedrock, it flows faster, and the ice The fact that the composition of the debris deposited
streams release icebergs to the ocean. Once this out- by the icebergs came from many distinct source regions
ward flow of ice is exhausted, the ice stream retreats (North America, Europe, Iceland) indicates that the
to another bedrock pinning point farther upstream margins of most of the ice sheets were involved in most
(Figure 14–16D). ice-rafting events. But why would so many ice sheet
Both hypotheses are consistent with the observed margins be simultaneously involved?
episodes of accelerated ice rafting to the North Atlantic One possible link is sea level (see Figure 14–16C). If
Ocean. First, ice-rafting pulses occurred only during one ice margin surged and sent icebergs into the ocean,